The Gaia hypothesis revisited: Introducing an Organic Theory of Gaia

de Castro, Carlos; Lauer, Arthur

doi:https://doi.org/10.5194/egusphere-2025-1532

CC1: 'Comment on egusphere-2025-1532', Scott Gilbert, 16 Apr 2025

Thank you, Carlos, for sending me the link to your paper. I think you are perfectly correct that (1) the dissipative framework of the Second Law of Thermodynamics makes a metabolic Gaia highly probable, and (2) that the holobiont is an excellent metaphor/exemplar for such a metabolic Gaia. I don't know if Prigogine's Trinomials need to be invoked, as I suspect that autotrophic metabolism will work. It's great that you come to these conclusions from Earth Science Studies. I've made similar claims for a thermodynamic metabolic Gaia coming from the biological side of the frame. Here, I use "sympoiesis" (qua Haraway) rather than "symbiogenesis" (qua Margulis), since my emphasis has been on how symbionts get together to effect development. You can find my analyses in A Book of the Body Politic edited by Bruno Latour, Simon Schaffer, and Pasquale Gagliardi (https://www.cini.it/wp-content/uploads/2020/03/BODY-POLITICS-complete-Online.pdf), and in an article that is presently in online preprint stage (< https://www.preprints.org/manuscript/202309.1072/v1>). In this latter essay, I try to link the holobiont and Gaia to Hans Jonas' notions of metabolism. It attempts to show that if one understands that (1) organisms are holobionts that are constructed through symbiotic interactions (sympoiesis) and (2) that metabolism acts to form cells, organisms, and ecosystems through the same thermodynamic principles, then (3) Gaia becomes a logical biological outcome. In fact, Gaia can be modeled as a holobiont organism that is the integration of its numerous enmeshed metabolic systems. (The ideas in this paper have been revised, expanded, and submitted as part of a book manuscript). You might find some interesting confirmations of your ideas in these papers. I think that our ideas, coming from different perspectives, mutually support each other.

Citation: https://doi.org/10.5194/egusphere-2025-1532-CC1

AC1: 'Reply on CC1', Carlos de Castro, 20 Apr 2025

Thanks, Scott.

Yes, the more processual and dynamic concept of sympoiesis could be useful to our discussion. We believe that the generalization we are trying to make about symbiogenesis is also more dynamic than the one Margulis drew. In fact, we understand the formation of structures as a scale where dichotomies become blurred. Thus, in our article (see lines 182 to 226 of the preprint) we observe that it is the degree of irreversibility (mutual dependence) that allows us to move from individual to holobiont (sensu Margulis) to organism. We recognize, like you, that interactions can also be "external" (not necessarily endosymbiotic). Especially if Gaia is admitted, the difference between, for example, symbiotic bacteria that metabolize vitamin B within our body, and other Gaia “cell” external to our body that synthesize vitamin C, which "we" then ingest, rather than absolute, is one of degree.

We admit that the degree of symbiotic integration and irreversibility is very high in what traditional biology has identified as organism-individual and has classified since Linnaeus, but we extend it through ecological metabolisms (sensu your sympoiesis?) to the largest scale. In our article, we aim to lay out the main hypotheses and how their interactions give rise to an organic and evolving Gaia. We certainly use the framework of the laws of thermodynamics, as do other authors you also cite in your work. However, we take two further steps:

First, as mentioned in Appendix A, the second law says nothing about whether or not there is a tendency to increase the rate or speed of entropy creation (the latter we assume and believe to be observable). Hence, we appeal to the TMaxPs as a reasonable hypothesis to apply precisely to systems such as those studied by biology or ecology. Nevertheless, we view thermodynamics and the TMaxPs as a facilitating rather than a directing force in the evolutionary processes of biology.

This is why, in a second step, we recur to Prigogine's trinomial (an analogous paradigm (Kauffmann, Maturana and Varela,...) would also be possible) that introduces two types of downward causation that we have called ‘organization/orchestration’ and ‘technique’. We believe this step to be important because it opens the door to biology as a discipline that cannot ultimately be reduced to (quantum) physics. The reason for this irreducibility lies in the properties that living beings in particular have, namely their capacity for expansion, which makes them collide with local limits or constraints, and which they are capable of breaking. This makes continuous evolution of life possible by the closure and the coupling of Prigogine trinomials through the process of generalized symbiogenesis (or sympoiesis?), which already belongs entirely to the biological-ecological realm and which we no longer observe in complex dissipative physical systems.

Citation: https://doi.org/10.5194/egusphere-2025-1532-AC1

CC2: 'Comment on egusphere-2025-1532', Alejandro Merlo Oteo, 23 May 2025

Thank you for sharing the preprint. I think it does a really good job of bringing together different problems that have been raised regarding the reception and understanding of some key aspects of the Gaia Hypothesis. The idea of the tendency to overcome limits through cooperation and symbiosis seems particularly insightful.

I have a comment regarding the material definition of Gaia itself - what constitutes Gaia, and what lies outside it. In the OGT, Gaia seems to be understood as the biosphere, that is, the totality of living beings and the organic system they constitute. However, it seems to me that in the original Gaia theory, after some initial clarifications, Gaia was understood more broadly as the system composed of the biosphere and all the other parts of the Earth that were affected by it in a way that sustained life. Thus, Gaia would include parts that are traditionally considered non-organic, such as the atmosphere, hydrosphere, and, following what you rightly point out in the paper, possibly even the mantle-lithosphere interactions, etc. It is in that sense that Lovelock uses the term "superorganism" to describe Gaia in analogy to the termite mound, which includes both traditional organisms (the termites) and the earthly mound itself, which exhibits thermostatic capacities. I think this view is not only well connected to modern Earth system science studies (which increasingly acknowledge the central role that organisms play in the Earth, while not necessarily being interested in the important question of wether the whole system is "alive" in some sense), but also to traditional or ancestral more encompassing worldviews of "mother nature".

In your account, it seems those parts of the Earth that are not directly constituted by living organisms would remain outside Gaia and considered as its modified surroundings. It appears to me this might make the "teleology" problem more difficult: as the functionality of "non-organic" parts of the Earth system would need to arise, not as the result of their mutual systemic interaction (under conditions of great dynamism introduced by organisms), but rather as the result of an interested activity of Gaia acting as a whole on its surroundings in its own behalf, what you come to call technique in the paper.

These clarifications aside, I appreciate the rigor and originality of your approach and look forward to seeing how this work develops. Thank you again for sharing it.

Citation: https://doi.org/10.5194/egusphere-2025-1532-CC2

AC2: 'Reply on CC2', Carlos de Castro, 25 May 2025

Thank you, Alejandro.

What constitutes life, an organism, a species, or an individual (e.g., as opposed to a holobiont)? These are questions that have been debated for centuries. In our view, which is reflected in the manuscript, nature neither is dichotomic nor behaves dichotomicallyy, and, thus, the terms and categories just mentioned might be perhaps better thought of in terms different shades of grey, rather than as black and white.

That said, we acknowledge that Gaia represents a unique (super)organism and is therefore difficult to compare or reason about by analogies. Just as with FUCA (First Universal Common Ancestor), LECA (Last Eukaryotic Common Ancestor), or any primordial multicellular organism that emerged de novo—potentially with physical structures such as “the fur of a mink or the shell of a snail” (cf. Lovelock)—Gaia poses conceptual and definitional challenges.

Nonetheless, Gaia finds a useful analogy in the example you provided of the termite mound, which we also have used in our reference, taking as example the ‘organism’ Michaelensis. The Michaelensis organism is composed of the living “cells” of the termite mound—namely, the termites, fungi, and their bacterial symbionts—towards which its functions and objectives are directed. The physico-chemical structure that they construct and maintain, visible to us as the termite mound, would be analogous to Gaia’s Biosphere—the “house” Gaia has built for herself.

Of course, as with a termite mound, we again encounter definitional ambiguities concerning where the organism and the termite mound ends and how it modulates or influences its surrounding ecosystem. We would speak of Gaia’s interactions with systems external to the biosphere (e.g., plate tectonics, mantle dynamics) in a similar way, although the latter are clearly physical systems. They might have “self-organizing” qualities but we do not attribute to them inherent purposes or emergent goals in the way we do with living beings—whether understood through a “weak” teleology (as if) or a “strong” teleology (as is).

Thus, Earth System Sciences should not only study the influence of life/Gaia on the Earth System, but within the framework of the OGT ecology would be more appropriately understood as a form of physiology, and the biosphere as Gaia’s ‘termite mound’.

In any case, in the current article we have deliberately chosen not to delve too deeply into these conceptual aspects, given the manuscript’s already considerable length. Rather, we aim to elaborate a physical-biological theory that demonstrates the processes through which a singular individuality of extremely high organicity emerged—composed of “cells” with equally high internal organicity. These are processes that are, in fact, analogous to those that produced the first super-individuals during the major transitions in evolution (MTEs), such as LECA or the first termite mound—but occurring at a planetary scale.

In light of this, the next step in our research would be to assess Gaia’s degree of organicity and to determine whether this degree of organicity is indistinguishable, greater or smaller than the degree of organicity found in what we typically define as ‘organisms’. Depending on the result, we might then classify Gaia accordingly. Otherwise, we risk falling into theoretical or possibly anthropocentric biases (e.g., we rarely view a beehive as a super-organism—perhaps simply because its “cells” are more spatially separated than what would be considered typical?).

In this sense, “we are cells of Gaia” is more than a metaphor—it is a scientific hypothesis, open to empirical testing. This is precisely because we now have a supporting scientific theory and because the physiological perspective that this theory implies, leads us to expect phenomena that differ from those predicted by cybernetic Gaia models, classical Earth System Science, or reductionist approaches in biology and ecology, which tend to focus on much smaller, less integrated organisms not organized at such a high, systemic level.

Citation: https://doi.org/10.5194/egusphere-2025-1532-AC2

RC1: 'Comment on egusphere-2025-1532', Anonymous Referee #1, 06 Aug 2025

This paper proposes a new variant of Gaia theory which the authors call ‘Organic Gaia Theory’. I am supportive of Gaia, and I believe that non-equilibrium thermodynamics, symbiosis etc. are important concepts which can be used to theorise about planetary scale life. However, this manuscript reads like a collection of disjointed ideas and definitions with no strong through-line. Key-concepts are not explained, it is not clear how conclusions follow from premises or how the ideas described here are different from what has been proposed in more detail and with more clarity by others. At best it is an idiosyncratic restatement of Gaia theory.

I provide some detailed feedback below which represents my best efforts to understand the paper and make suggestions from improvement. However I do not think that even a significant revision will suffice and encourage the authors to withdraw the paper and carry out the work they suggest in Section 4 of rigorously formulating their theory, constructing models, identifying what is new about it and making predictions.

Section 1. The introduction recounts a version of the intellectual history of Gaia theory.

While biologists were certainly opposed to a very naive version of Gaia, they largely ignored it, a point also made by Dutreuil [1]
Some well known biologists who continued to work on Gaia came to view it quite sympathetically in later work [2][3]
The framing of ‘strong’ and ‘weak’ Gaia was rejected by Lovelock who regarded this as a rhetorical trap by his opponents (in particular Kirchner). ‘By setting up these two straw hypotheses, it was easy for them to demolish the strong, which I had never claimed, and leave me with the weak Gaia hypothesis, doomed to ignominy by the adjective ‘‘weak.’’’[4]
The somewhat recent review of (Lenton et al., 2018) is cited, but there is a large body of work before and since about Gaia. None of which is cited, though several of the author’s own recent papers are.
Characterising the work of Doolittle and reviewed by Lenton as trying to ‘scale natural selection beyond the local interactions of organisms’ inaccurate.
The term ‘Organic Gaia Theory’ is confusing. Gaia is supposed to be a theory of how interactions between life and the environment cause emergent behaviour at planetary scales. ‘Organic’ Gaia theory to me implies a theory of only the living component - isn’t this just ecology? Alternatively it suggests somehow the original Gaia Theory is ‘inorganic’, but what would this mean? It has always been a theory of coupled organic and inorganic entities.

Section 2. Lists three ‘postulates’ of this new Gaia theory.

Postulate 1, I can accept that non-equilibrium thermodynamics has a lot to say about the appearance of complexity, but which thermodynamic maximisation principles exactly are of interest? The maximum entropy production principle associated with Kleidon is mentioned. The implication is that other principles are intended. What are they and why are they necessary? Can they be formulated mathematically? There are many other vaguely defined terms here e.g. in what sense in a candle flame ‘resilient’ or ‘self-organized’?

Even so, is this postulate even true? Do thermodynamically open, dissipative systems really tend to evolve towards stable and resilient structures of high complexity? The biological examples seem rather carefully chosen and in any case don’t prove that it is thermodynamics that causes biological complexity, rather than more standard evolutionary dynamics. Other examples e.g. a continuously stirred reaction vessel with an inflow and outflow, sometimes show interesting chemical dynamics, and sometimes reach a steady state.

Postulate 2, I am open to the idea of downward causation where interactions at the higher level affect the lower level components (which is well accepted in e.g. physiology or economics). However I do not see what exactly the ‘Prigogine trinomial’ concept is supposed to achieve. The reference to the author’s previous work does not help illuminate the matter. Figure 1 seems to state the obvious fact that wholes shape and are shaped by their parts. There is a self referential explanation, where the text refers to the Figure for explanation and the Figure caption to the text. The rest of the text describes processes which have, with more clarity and rigor, been described in cybernetic or complex systems language. If the ‘Prigogine trinomial’ were a well known concept, with some well developed theory around it this might be a useful conceptualisation, but it is not.

Regarding the truth of the postulate, again why should there be such a tendency? There are situations where fluctuations destroy the system, or are small and don’t affect it much. Under what conditions should they cause complexity to emerge?

Postulate 3, Symbiosis is certainly an important biological process but it is not obvious that ‘local limits to the growth and evolution of life tend to be resolved through symbiotic cooperation’. The extinction of one or more species, or the overexploitation of some crucial resource are other possibilities. So again, this postulate is not self evidently true and not enough convincing evidence has been provided.

Overall then, the truth of these postulates is dubious and, if anything, are what should be explained by a theory of Gaia. How Gaia emerges from, say, thermodynamic considerations and how symbiosis tends to emerge as an important force is interesting. There is much work on both of these questions, the former from Kleidon and the latter by a variety of authors e.g. [5,6,7]. The postulates seem to amount to assuming an unusual and very strong version of the Gaia hypothesis with the author’s own concept of the Prigogine trinomial replacing other more standard terminology.

To make this rigorous I think these postulates need to be broken down into much simpler statements where things are defined clearly with examples. If the authors are convinced that the ‘Prigogine trinomial’ is a useful concept they need to show exactly what it means, with examples, when it applies and ideally, give a mathematical formulation, in the same way one would define say ‘entropy’ or ‘complexity’.

As a note of style - naming postulates and concepts after authors who did not propose them is not good practice and I strongly suggest renaming them.

Seeks to derive consequences of the postulates of the previous section. Some simple inferences can be ‘left as an exercise to the reader’ but I find almost no support or argumentation in favour of the assertions made. Some examples

“The Prigogine trinomial can be applied to the ecological realm as ecosystems constitute complex dissipative systems and the relationships established among organisms belong functionally to the ecosystem that hosts them (De Castro and McShea, 2022) . Consequently, Postulates 1 – 3 (sect. 2), not only explain the increase and stability of hierarchical levels in MTEs but also suggest a tendency to form ecological macrostructures.” Consequently? What is the argument here, why do these postulates explain this? This seems to be assumed by postulate 2 not a consequence of it. Checking the reference does not illuminate.
Immediately following “This implies the formation of large ecosystems and the emergence of top-down causation that functionally integrates organisms, including their physical-chemical and biological processes” even if I accept the previous, why does that imply this? There is no argument given.

Figure 2 is just a series of photographs, it does not demonstrate anything or constitute an argument. This section continues on in this manner. Statements like “given the entropy-enhancing effect of complex nested Prigogine trinomials, in the presence of TMaxP, the emergence of Gaia is not only possible but probable” are in no way obvious consequences of the postulates. Entropy is a quantity that can be measured, to verify this statement one would need to mathematically formulate what a Prigogine trinomial is, how it arises from a particular thermodynamic law, why that enhances entropy and how that increases the probability of Gaia emerging, again, with Gaia precisely defined in terms of some measurable variables. This is not done.

The ten steps in 3.2 are the same, they are just a series of assertions, which aren’t derived from the postulates, and indeed seem to be quite simpler than the postulates themselves. There is no logical deduction or explanation of why any of them should occur or why one follows the next. 1. Why does diversity expand as well as number? 2. In what way do constraints ‘appear’? 4. Why are the limits broken rather than a steady state reached? 5. Why do new limits arise and exactly what is meant by this? 6. What causes more cooperation rather than, say, competition or neutrality? 7. How are functions transferred, what is the physical/biological meaning and mechanism of this. 8. Why does this process iterate? What is the meaning of ‘organicity’ and how can it be measured (which it must if it can be said to increase). 9. Why does it accelerate? 10. What does ‘exponential growth potential’ mean in this context, in what sense can Gaia ‘beget’ itself, exponentially?

Like much of the previous and current work on Gaia - I suggest the authors formulate some sort of model (mathematical or otherwise) and show how these steps occur. Step 1 is performed by many models, e.g. [5]. So what is it about that model that this paper goes beyond? What is missing? Can it be added so we can see the process of hierarchy building play out in such a system?

Suggests that Gaia can be considered a living organism. The arguments for this are that, somehow, the process described in 3 makes this inevitable. The authors give a personal definition of ‘living’ which they admittedly do not provide any argument in favour of, compared to the many others which exist. Then they do not demonstrate how their Gaia fulfils this definition, nor why the ‘cybernetic and Darwinian’ versions (which are not defined) fail.

Some future work is suggested, which amounts to verifying these proposed properties hold for Gaia, developing models and making predictions. I quite agree, this kind of thing is necessary for any theory development and I suggest the authors try to do this.

[1] Dutreuil, S., 2014. Michael Ruse, The Gaïa hypothesis: science on a pagan planet: University of Chicago Press, Chicago, 2013, 272 pp, 26.00(e-bookversion: 18.00).

[2] Lenton, T.M., 2005. Hamilton and Gaia. Narrow Roads of Gene Land: The Collected Papers of WD Hamilton: Last Words, 3, pp.257-264.

[3] Doolittle, W.F., 2017. Darwinizing gaia. Journal of Theoretical Biology, 434, pp.11-19.

[4] Lovelock, J., 2004. Reflections on Gaia. https://doi.org/10.7551/mitpress/6100.003.0003

[5] Williams, H.T. and Lenton, T.M., 2007. Artificial selection of simulated microbial ecosystems. Proceedings of the National Academy of Sciences, 104(21), pp.8918-8923.

[6] Nicholson, A.E., Wilkinson, D.M., Williams, H.T. and Lenton, T.M., 2018. Gaian bottlenecks and planetary habitability maintained by evolving model biospheres: The ExoGaia model. Monthly Notices of the Royal Astronomical Society, 477(1), pp.727-740.

[7] Arthur, R. and Nicholson, A., 2022. Selection principles for Gaia. Journal of Theoretical Biology, 533, p.110940.

[8] Goldblatt, C., 2015. Habitability of waterworlds: runaway greenhouses, atmospheric expansion, and multiple climate states of pure water atmospheres. Astrobiology, 15(5), pp.362-370.

Citation: https://doi.org/10.5194/egusphere-2025-1532-RC1

AC3:
'Reply on RC1', Carlos de Castro, 21 Aug 2025

Thank you for taking the time to read the manuscript and providing your assessment. Unfortunately, as the issues raised are repetitive, we do not provide an extensive rebuttal to every point raised and will focus only on the most critical points, with the possibility of providing further details in subsequent replies.

Our impression is that the review of the manuscript is incomplete, biased and in some points erroneous, possibly due to a misunderstanding of the content. This will be explained in the following sections while constructive criticism given by the reviewer will be addressed at the end of our reply.

First, at no point does the anonymous reviewer make reference to the Appendices, where several of their concerns are discussed and addressed, in addition to being treated in the main body of the text. Examples:

• ‘Organic’ Gaia theory to me implies a theory of only the living component.” — In the appendix C, specially the C.2 devoted to the Gaian trinomial, it is discussed how the OGT is not solely about the living component; moreover, the more accurate analogy would be that the Gaia organism constructs the biosphere, which, in technological terms, could be considered its extended phenotype.

• “The maximum entropy production principle associated with Kleidon is mentioned. The implication is that other principles are intended. What are they and why are they necessary?” — All relevant principles are listed in the main text and the appendices. As for the MEP principle it appears several times in the main text and is discussed in the respective appendix section (A.2) dedicated to this topic.

• “Other examples, e.g. a continuously stirred reaction vessel with an inflow and outflow, sometimes show interesting chemical dynamics, and sometimes reach a steady state.”

• “There are situations where fluctuations destroy the system, or are small and don’t affect it much. Under what conditions should they cause complexity to emerge?”

These remarks do not contradict anything we propose here. On the contrary, in the appendix A devoted to this issue, we provide a more detailed and elaborated discussion, including a similar example concerning Bénard cells. Thus, contrary to what the reviewer seems to infer, our account is in fact consistent with Postulates 1 and 2.

• “If the authors are convinced that the ‘Prigogine trinomial’ is a useful concept they need to show exactly what it means, with examples, when it applies and ideally, give a mathematical formulation, in the same way one would define say ‘entropy’ or ‘complexity’.”

Even Prigogine himself, despite more than two decades of effort, never provided a formal mathematical formulation of his trinomial; he nevertheless advanced it as a hypothesis. The main base of the de Castro and McShea (2022) paper is the Prigogine trinomial (so it is a useful concept). Moreover, in the 1990s, the author de Castro (C.) pointed out to Prigogine that this trinomial renders Darwinian and later theories such as Neo-Darwinism at the very least incomplete or incoherent. Crucially, all evolutionary theories—except the Organic Gaia Theory—fail to satisfy his trinomial. Prigogine found these objections to Darwinian theories reasonable, not objections to the trinomial itself (personal communication) (see also appendix C).

What the reviewer requests (illustrating how the Prigogine trinomial can be applied) is precisely what we do, although it is correct that we do not rely on a mathematical formalization. In this context, we want to emphasize that there is, to date, no generally accepted mathematical formulation of “complexity”; the reviewer is asking far more of this paper than has ever been required of Darwin, Lovelock, Margulis, Prigogine, or indeed any major theorist in biology. None of these authors have offered a formulation of their theories in the same way that, for example, thermodynamics has formalized entropy. If there are formalizations, they are only realized through partial models.

Indeed, when empirical observations have contradicted predictions derived from Neo-Darwinism (see, e.g., de Castro and McShea 2022, or de Castro 2013 on apoptosis—an example we intend to develop further in a forthcoming publication as example for a successful prediction derived from the OGT), evolutionary theorists have not discarded the theory, as would typically happen in the physical sciences (see also Appendix C). At most, they have proposed extensions of natural selection beyond “classical” organic individuals (bacteria, animals, etc.)—and even then, without rigorous mathematical formulations.

In fact, de Castro and McShea argue that attempts to model the emergence of Major Evolutionary Transitions (METs) over geological time within the framework of classical biology produce, at best, a linear function. Gould’s well-known “drunkard’s walk” argument (lacking mathematical formalization) similarly implied no intrinsic “force” driving an increase in complexity, analogous to a diffusive process, which can be shown to yield a square-root time dependence—slower than linear. Moreover, mathematical simulations (by author C.) demonstrate that when more realistic assumptions are introduced under natural selection —acknowledging that increasing size and complexity associated with METs slow down the reproduction rate and the number of individuals to be the subject of selection (see de Castro and McShea article discussing it—the resulting dynamics are even less compatible with the observed acceleration).

Nevertheless, in de Castro and McShea article it is argued that thermodynamic hypotheses and Prigogine’s trinomial could accelerate this process, although without establishing sufficiency for the observed pattern: namely, the empirical acceleration of transitions consistent with an exponential function. Positive feedback and Gaia’s own top-down capacities over its “cells,” as described in Section 3, represent plausible mechanisms which, if mathematically formalized, would naturally yield exponential functions (as noted in our article, exponential and Verhulst-type functions are expected, see section 4.2). However, this kind of discussions and explorations, in our opinion, belongs to a subsequent article (already in preparation) following the presentation of the OGT we provide here, rather than simultaneously or beforehand, as the reviewer seems to suggest.

Second, we believe that many of the doubts and problems raised by the anonymous reviewer stem from a misunderstanding of the aims of this article and from treating “Gaia theory” as if it were a single, unique framework. In doing so, the reviewer has misidentified our proposal as merely an idiosyncratic restatement of that singular ‘Gaia theory’, rather than recognizing it as a new theory grounded in a hypothesis—the Organic Gaia hypothesis—from which no fully developed theory has previously been articulated in academic journals.

As we emphasize in both the Introduction and even in the Abstract, there have been several Gaia hypotheses and, consequently, several Gaia theories. In these parts of the manuscript, we already define the problem that we aim to address in this work.

Examples of the reviewer’s comments illustrate this misunderstanding:

• “This paper proposes a new variant of Gaia theory…”

• “It is not clear how the ideas described here are different from what has been proposed in more detail and with more clarity by others. At best it is an idiosyncratic restatement of Gaia theory.”

• “The introduction recounts a version of the intellectual history of Gaia theory.”

• “Gaia is supposed to be a theory of how interactions between life and the environment cause emergent behaviour at planetary scales. ‘Organic’ Gaia theory to me implies a theory of only the living component—isn’t this just ecology? Alternatively, it suggests somehow the original Gaia Theory is ‘inorganic’, but what would this mean? It has always been a theory of coupled organic and inorganic entities.”

• “The postulates seem to amount to assuming an unusual and very strong version of the Gaia hypothesis…”

This last remark hits the core of our manuscript: as we state explicitly in the Abstract and contextualize in the Introduction, the OGT indeed advances a stronger and more unusual version of the Gaia hypothesis. The fact that the reviewer perceives this as surprising reinforces our concern that the distinctiveness and novelty of the Organic Gaia Theory have not been fully appreciated.

Third, the reviewer appears to conflate scientific hypotheses or postulates with laws, models, or “truths,” and thus seems to demand from them standards that are inappropriate to their nature. To recall the Oxford Dictionary definitions:

• Postulate: “a thing suggested or assumed as true as the basis for reasoning, discussion, or belief” (a meaning the reviewer repeatedly overlooks).

• Hypothesis: “a supposition or proposed explanation made on the basis of limited evidence as a starting point for further investigation” (which is also insufficiently acknowledged by the reviewer).

While there are distinctions between a postulate and a hypothesis, the statements we present as postulates (highlighted in Section 2) can readily be framed as hypotheses: they are supported by references, by real-world examples that align with them, and by subsequent reasoning. At no point do we claim that they are scientific laws expressed mathematically, or self-evident truths, as the reviewer seems to expect. Indeed, even scientific laws do not necessarily require a formal mathematical formulation in order to be fully understood. As Wikipedia notes: “scientific laws are statements based on repeated experiments or observations, that describe or predict a range of natural phenomena.”

What can be demanded of a postulate or hypothesis is that it be testable and thereby capable of evolving into a natural law. A hypothesis and a theory, in order to be fully scientific, must be subject to empirical testing. The hypotheses derived from our postulates meet this requirement. In fact, as shown in de Castro (2013), the Organic Gaia hypothesis can be considered scientific, despite the reluctance of mechanistic paradigms and certain Neo-Darwinist theorists to admit it as such.

The purpose of the article, starting from these “minor” postulates and hypotheses, is to make the larger hypothesis—that Gaia can be characterized as an organism—more plausible, specifically by illustrating how Gaia might have emerged as such an organism.

Examples of the reviewer’s comments reveal this confusion:

• “Even so, is this postulate even true? Do thermodynamically open, dissipative systems really tend to evolve towards stable and resilient structures of high complexity?”

• “The biological examples … don’t prove that it is thermodynamics that causes biological complexity.” — The latter is a claim we never make. Rather, we state that thermodynamics facilitates the probability of complexity (see also Appendix A), and we provide examples to render the postulates more plausible and thereby allow their translation into hypotheses (see the definitions cited above).

• “If the ‘Prigogine trinomial’ were a well known concept, with some well developed theory around it this might be a useful conceptualisation, but it is not.” — Here again, the reviewer demands of a postulate or hypothesis what would only be expected of a fully developed theory, thereby conflating categories.

Fourth, the reviewer seems to be asking of our article a series of extensions that would make it considerably longer—well beyond what is already addressed, in greater detail, in the appendices that were overlooked—while at the same time demanding a level of development that has never been historically or epistemologically required in the initial presentation of new theories in biology, including Gaia, nor even in certain physical theories, which lend themselves more readily to mathematical formalization.

Author C. has in fact cited two of his books and two articles that already address several of the points raised by the reviewer, including connections with empirical observations and even predictions. Moreover, both in the main text and in the appendices, we indicate possible directions for future research, aimed at achieving greater formalization and consolidation of the OGT and of the Organic Gaia hypothesis as explanatory and predictive frameworks. However, this is only possible once there is a first publication that summarizes the fundamental premises of the OGT. Thus, although our manuscript is not sufficient to accept or reject the OGT, it nonetheless provides the necessary basis for an approval or rejection at a later time through future formal, theoretical, mathematical and/or empirical work.

Examples:

• “Section 1. The introduction recounts a version of the intellectual history of Gaia theory …” and subsequent paragraphs: The reviewer seems to expect a review of the intellectual history of Gaia, although the Introduction serves a different purpose. Including such a review would unnecessarily extend the text. In fact, in an earlier draft—judged excessively long for an interdisciplinary journal—we had included an extensive critical review of the intellectual history of Gaia. That section was removed because we considered it more suitable for a separate letter to a history or philosophy of science journal. Should the editors deem it necessary, we could reintroduce it as an appendix.

• “I … encourage the authors to … carry out the work they suggest in Section 4 of rigorously formulating their theory, constructing models, identifying what is new about it and making predictions.” (and similar remarks repeated elsewhere).
It should be noted that what is new in this article compared to other Gaia theories is, in fact, quite clear. Section 3 may already be interpreted as a phenomenological model (see also Apendix C). More generally, every theory is a model (a mental construct), which may or may not eventually be expressed mathematically. Darwin, for instance, included not a single equation in his long seminal text, nor in the formal presentation he gave jointly with Wallace (an effort comparable in scope to what we do here). It is also important to emphasize:

i) Including further formalization in the main body of the article would considerably lengthen an already long text, in which the appendices already supply additional depth.

ii) Such demands were not made of earlier biological theories (Lamarck, Darwin, Neo-Darwinism) nor of Gaia theories. Lovelock introduced his hypothesis and only later, in response to controversy, developed a simplified “toy model,” Daisyworld, which itself was criticized as a gross simplification of Gaia’s complexity. Subsequent partial models (e.g., those of Lenton et al.: Williams and Lenton 2008, Nicholson et al. 2017, Lenton et al. 2018) addressed specific issues, but not at the stage of the first proposal of the theory; neither did Doolittle provide formal models (e.g. Doolittle 2019, Doolittle and Inkpen 2018). All of them in the context of how some natural selection of “individuals” could explain some planetary homeostasis. Author C. has developed extensions of Daisyworld, including models inspired by the Organic Gaia hypothesis, which yield improved regulation. These are mentioned in the 2013 paper and in an accessible preprint (Rubin and de Castro 2021). There, it is shown that models adhering to classical Darwinian or Neo-Darwinian assumptions (e.g., with mutations, entrance of cheaters…) actually impair habitability through weaker temperature regulation, while explicitly non-Darwinian models achieve stronger regulation (e.g., “Butterflyworld”, “Pando” and “Gaian constraints” models in Rubin and de Castro 2021). Notably, Lovelock’s presentation of cybernetic Gaia was less systematic and formal than ours, and the same applies to the initial theoretical expositions of Lenton (e.g. Lenton 1998 based in the theory of Lovelock and the Daisyworld model, or Lenton and Wilkinson 2003) and Doolittle (from 1981 to 2014 and again 2024): their modeling work came later, in other several papers –as cited before-, and was limited to specific aspects (e.g., Lenton’s model of bacterial escalation does not constitute a mathematical description of a complete Gaia theory).

iii) These models are cybernetic, and therefore difficult—if not impossible—to generalize to the physiological scale and properties of Gaia proposed here. In physiology or medicine, for example, there are no complete models describing the emergence of an organism from a single ovum, or even of the first bacterium; only partial models of specific features exist. The Daisyworld-type approach does not pursue the same aim as the OGT. Author C. regards it as less relevant at this point (although he has developed in System Dynamics Daisyworld-like models for many years), since regulation models have already revealed the difficulties inherent in classical biological theories—particularly their vulnerability to “free riders,” which can destabilize the larger system (including in the model cited by the reviewer). It is therefore more important to first recognize the OGT; from this foundation, more adequate models can then be developed.

iv) Regarding mathematical formalization: based on the positive feedback mechanisms described in the 10 steps in section 3.2 (in terms of cybernetics and systems dynamics, methodologies in which the authors are trained), one expects Gaia’s evolution to follow exponential or Verhulst-type functions, with successive couplings producing an overall exponential trajectory of Gaia’s growth and internal complexity over geological time. This leads to a testable prediction: Gaia will tend to increase exponentially in size and complexity—as long as absolute physical limits are not reached. Specifically, the OGT, by positing a teleological organism with strong top-down causation and intrinsic positive feedback, yields the hypothesis that not only Major Evolutionary Transitions (MTEs) occurred in an accelerated manner, but also that Gaia’s size increased exponentially (or sigmoidally) over geological time. In the exponential case, Gaia has not yet encountered physical growth limits; in the sigmoidal case, an initial long exponential phase would eventually slow down, and identifying the limiting factors would become the focus. The Cambrian explosion and the later colonization of land suggest that Gaia may still be in an exponential phase. Internally, the recent emergence of colonial organisms and of properties as complex as the psyche further support the idea that Gaia’s internal complexity is also still in an exponential phase—though in the case of colonial organisms, few further leaps may be expected, given planetary constraints, indicating a possible shift towards a sigmoidal trajectory.

However, these considerations go beyond the scope of the present paper. They illustrate the many consequences of adopting the new paradigm introduced by the OGT, i.e. recognizing Gaia as an evolutionary organism. The authors are actively working on related issues, such as analyses and models of RAMO properties, quantitative comparisons with other cycling systems (non-organic), and predictions and observations of phenomena that contradict both classical biological theories and cybernetic Gaia. Current efforts include reinterpreting concrete cycles under the Organic Gaia framework—for example, the hydrological cycle and the functional role of transpiration for Gaia. All this ongoing work relies on the manuscript as presented here because it provides and explains the paradigmatic framework under which subsequent more detailed assessments of various parts of Gaia can be realized.

Last, some of the reviewer’s other remarks are difficult to understand and appear to stem more from a superficial and negatively predisposed reading than from a substantive critique. Thus, we will not comment them in detail. For instance, the reviewer’s comment on Figure 2 (“Figure 2 is just a series of photographs, it does not demonstrate anything or constitute an argument.”) in our perspective is inaccurate. A careful reading of the figure reveals that it is not just a series of photographs: beneath each image, and arranged along an ascending line of “size,” are the Prigogine trinomials, successively coupled in such a way that each “smaller” structure is simultaneously a function of a larger one. The purpose of the figure is thus to visualize the process of coupled trinomials and the transfer of functions to higher levels of organization, step by step, from bacteria to Gaia.

In addition, the figure directly emphasizes biofilms and stromatolites as subsystems that very likely gave rise to Gaia as early as the Archaean, given that stromatolites constitute the earliest recognizable ecosystems, leaving fossil traces more than 3.5 billion years ago. The figure is therefore neither irrelevant nor superfluous; it serves precisely as an aid to the comprehension of the text. Moreover, it is fully analogous—though applied in the direction leading toward Gaia—to the figure of trinomials applied to Major Evolutionary Transitions in the paper by de Castro and McShea, thereby reinforcing, implicitly, the connection with that earlier work.

Nevertheless, we regard some of the reviewer’s comments as constructive and more understandable:

On the use of names such as Prigogine and Margulis in relation to Postulates 2 and 3: we hold that normally it is the authors’ prerogative to decide on nomenclature, provided it is not offensive or misleading. We acknowledge that in the case of Margulis the nomenclature could create confusion because the generalization of her ideas on symbiogenesis belongs to the OGT, not to her academic work. In contrast, in the case of Prigogine, careful reading of his writings, in which he repeatedly discussed the trinomial, as well as personal exchanges with him three decades ago, confirm that our formulation is essentially aligned with his, although he was somewhat more restrictive in his use of “fluctuations.” Nonetheless, we are willing to accept the reviewer’s suggestion and to remove both names from the main text and the appendices. We would, however, retain the didactic expressions “Prigogine dynamics” and “Margulis dynamics” in specific contexts (for instance, in Figure 3), since these are not meant to imply authorship but to facilitate comprehension.

On the placement of appendices: should the editors or another reviewer deem it appropriate, parts of the appendices could be incorporated into the main body of the article. We could also expand the appendices to include a more extended historical-critical review of the issues raised in the introduction and highlighted by the reviewer. Furthermore, some of the ongoing work on the cycling of different systems could be introduced, as evidence that Gaia is at least as complex as other organic systems, given that such cycling is only observed in organic contexts. However, this material is too extensive—because of the numerous calculations involved—to be treated in detail here, and our intention is to publish it separately.

On the reviewer’s comment starting with “consequently? What is…”: We acknowledge that the phrasing may have been unclear. We therefore propose revising it as follows: “The three postulates can be applied to the ecological realm, as already indicated in the Final Note of the paper by de Castro and McShea. There, the cascading trinomial was applied, which implicitly assumes cooperation/coordination in organisms; that is, both Postulate 2 and Postulate 3 are relevant.” In fact, the three postulates are already implicitly present (recall that the OGT inspired that paper, not only the Prigogine trinomial). While the theory was not developed there as it is here in relation to Gaia, what applies to Gaia also applies to its internal functioning and therefore to the Major Evolutionary Transitions. This is why the Final Note in that paper implicitly points to all three postulates, including Postulate 1, at ecological and macro-ecological scales.

On the reviewer’s comment starting with “Immediately following…”: here we agree with the reviewer that the wording may create confusion, as we were anticipating material explained in Section 3. We accept either its deletion or the addition of a clarifying remark explicitly pointing to Section 3.

References:

- De Castro Carranza, 2013. En defensa de una teoría Gaia orgánica. ECOS 22, 113–118. https://doi.org/10.7818/ECOS.2013.22-2.17

- De Castro and, McShea, 2022. Applying the Prigogine view of dissipative systems to the major transitions in evolution. Paleobiology 48, 711–728.

- Doolittle, 1981: It’s Earth really motherly? The CoEvolution Quaterly 1981(spring)58-63

- Doolittle, 2014: Natural selection through survival alone, and the possibility of Gaia. Biol. Philos, 29:415-423

- Doolittle, 2017: Darwinizing Gaia. J. Theor. Biol 434:11-19

- Doolittle, 2019: Making evolutionary sense of Gaia. Trends in Ecology and Evolution. 34(10):889-894

- Doolittle, 2024: Darwinizing gaia: natural selection and multispecies community evolution. MIT Press.

- Doolittle and Inkpen 2018: Processes and patterns of interaction as units of selection: an introduction to ITSNTS thinking. PNAS 111:4006-4014

- Lenton et al. 2018: Selection for Gaia across multiple scales. Trends Ecol. Evol. 33:633-645

- Lenton, 1998. Gaia and natural selection. Nature 394:439-447

- Lenton and Wilkinson, 2003. Developing the Gaia Theory. Climatic Change 58:1-12.

- Nicholson et al., 2017: Multiple states of environmental regulation in well mixed model biospheres. J. Theor Biol. 414:17-34.

- Rubin and de Castro, 2021: Gaianizing Darwin: Natural selection impairs the effectiveness of planetary temperature self-regulation: eartharxiv.org/repository/view/2673

- Williams and Lenton, 2008: Enviromental regulation in a network of simulated microbial ecosystems. PNAS 105(30): 10432-10437

Citation: https://doi.org/10.5194/egusphere-2025-1532-AC3
- RC2:
  'Reply on AC3', Anonymous Referee #1, 22 Aug 2025
  
  I am not surprised the authors object to my unfavourable assessment, nevertheless I made a good faith attempt to read their work, including reading the appendices and the author's previous paper. I completely reject claims of bias, and I am not even sure what is implied by this. As stated, I am in favour of Gaia generally and believe the work of Kleidon, Doolittle, Lenton and much other Gaia theory is of high quality.
  The authors in their reply simply assert that their work is clear and argue largely on the basis of self-citations and rather grandiose claims e.g. "Moreover, in the 1990s, the author de Castro (C.) pointed out to Prigogine that this trinomial renders Darwinian and later theories such as Neo-Darwinism at the very least incomplete or incoherent. Crucially, all evolutionary theories—except the Organic Gaia Theory—fail to satisfy his trinomial." I do not find this type of argumentation convincing.
  The main problem I have with this work is the lack of clarity and rigour. The authors kindly remind me of the meaning of the word 'postulate'. I remind them that postulates ought to be true, and that conclusions have to be derived from them via logical (not necessarily mathematical) argumentation. A system of postulates should also generally be minimal. The authors have not provided convincing evidence that their postulates are true or necessary. Even assuming they are, they have not provided clear, logical arguments that derive conclusions from these postulates.
  My best faith interpretation of the 'Organic Gaia Theory' is that cells are made of chemicals but are more than the sum of their parts and develop their own 'individuality'. The actions of the cell may affect the chemical processes composing it and vice versa. In a similar way multi-cellular organisms are made of cells, ecosystems are made of organisms and so on up to Gaia. If this is what is intended I do not find it compelling. As mentioned, related ideas have been discussed for a long time in complex systems science. Secondly, as pointed out by Margulis, the key property of Gaian regulation is largely carried out by microbes, connecting the smallest scales directly to the largest, rather than in a hierarchy as implied here. Thirdly the 'individuals' at each level are very different - a cell is not like a monkey is not like a forest except in the most general or metaphorical terms.
  If the above is not what is intended then it is the perogative of the authors to make their theory clear. Their paper will likely be seen by readers with such, apparently poor, reading ability as myself.
  To summarise my suggestions in this regard:
  Section 1 discusses briefly the intellectual history of Gaia. For reasons stated, I do not believe this is an accurate history.
  Section 2 discusses the postulates. I think these need to be simplified and convincing evidence given for when and where they apply. As discussed previously, I am not convinced by the argument provided.
  Section 3 proposes to develop organic Gaia theory. There should be a clear, logical connection from the postulates to this development. In particular, explaining how the 'algorithm' of 3.2 follows from the postulates, with all terms clearly defined.
  Section 4 suggests that Gaia is 'alive' according to the definition of life provided but does not demonstrate this via evidence or argumentation and makes vague suggestions for future work. Actually carrying out at least some of this work seems necessary, otherwise what are we to make of a theory without any applications or predictions?
  The key concept of the 'trinomial' needs to be explained in much more depth. Figure 1 is a drawing not an explanation. Although not under review, I do not think it is clearly explained in the prior work cited by the author which simply asserts that it 'can be applied to any sufficiently complex dissipative system', calling this 'Prigogine’s conjecture'. Why should we believe this is true? Even if it is, how can 'sufficient complexity' be determined? What does it mean to 'apply' the trinomial? How are the elements in the boxes identified? What do we do once we identify them? How does describing a system in this way allow us to make new conclusions or give us new insight? And so on. To reiterate, this is not a well known concept and should be explained here, since it is supposed to be crucial.
  On the subject of names, if the authors intend their theory to be something other than a version of Gaia particularly concerned with life, then I suggest a different title than 'Organic Gaia Theory'.
  
  I am happy to re-review a manuscript revised in such a way. Should the editor deem my requests unreasonable, so be it. I do not think there is any productive purpose in continued back and forth in these comments.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1532-RC2
  - AC4: 'Reply on RC2', Carlos de Castro, 01 Oct 2025
    
    We make a detailed reply to the points raised by the reviewers, marking in italics the proposed changes of the revised article. We will add new appendices to the revised manuscript that address several of the suggestions of the reviewers.
    
    Reviewer anonymous 1, second reply:
    Reviewer:
    “I am not surprised the authors object to my unfavourable assessment, nevertheless I made a good faith attempt to read their work, including reading the appendices and the author's previous paper. I completely reject claims of bias, and I am not even sure what is implied by this. As stated, I am in favour of Gaia generally and believe the work of Kleidon, Doolittle, Lenton and much other Gaia theory is of high quality.
    The authors in their reply simply assert that their work is clear and argue largely on the basis of self-citations and rather grandiose claims e.g. "Moreover, in the 1990s, the author de Castro (C.) pointed out to Prigogine that this trinomial renders Darwinian and later theories such as Neo-Darwinism at the very least incomplete or incoherent. Crucially, all evolutionary theories—except the Organic Gaia Theory—fail to satisfy his trinomial." I do not find this type of argumentation convincing.
    The main problem I have with this work is the lack of clarity and rigour. The authors kindly remind me of the meaning of the word 'postulate'. I remind them that postulates ought to be true, and that conclusions have to be derived from them via logical (not necessarily mathematical) argumentation. A system of postulates should also generally be minimal. The authors have not provided convincing evidence that their postulates are true or necessary. Even assuming they are, they have not provided clear, logical arguments that derive conclusions from these postulates.”
    Authors response:
    The reviewer acknowledges the works of Kleidon but in their replies the reviewer criticizes the first postulate, when much of the work of Kleidon relates with it, and Kleidon in fact is one leading authors proposing TMaxP at Gaia level.
    In our perspective, as well also of the reviewer, the fundamental point of disagreement between the reviewer and us, the authors, is the meaning of a ‘postulate’. The reviewer requires us to provide evidence for their truth. Conversely, we hold that the validity of the postulates should be put to a test through predictions generated through the theory we present in the article in a series of subsequent works made by the scientific community.
    Our whole manuscript is written on the premise that, by assuming the postulates, a new perspective on Gaia can be developed that opens up new research questions and advances our understanding of certain phenomena that cannot be explained convincingly by other paradigms in biology and Earth System Sciences, for example the phenomenon of programmed cell death or the necessary coordination of high recycling rates of materials used by life forms at planetary scale. In fact, any theory is built on postulates that are assumed as true, and theories are judged primarily by their explanatory potential, not by their postulates. Therefore, on the definition, meaning and purpose of ‘postulate’, we restate that our use of this term reflects both scientific practice and the role of postulates in theory-building. A postulate is not required to be self-evident or “true” as the history of science illustrates: Ptolemy assumed the Earth as the fixed centre of the Universe, while Copernicus assumed the Sun in this role. Both assumptions were ultimately incorrect, but they were nonetheless fruitful starting points for scientific progress. Similarly, Newton’s mechanics rests on assumptions later superseded by Einstein’s postulate that nothing can exceed the speed of light in a vacuum (a postulate not self-evident nor intuitive with very few observations according with it in 1905). Newton’s postulate (the velocity of light is added to the speed of the vehicle for a relative stationary observer) was eventually shown to be false, yet it remains useful in engineering and many discipline sciences in low-energy/low-speed contexts. These examples show that postulates need not be self-evident truths but rather operational assumptions guiding theoretical development.
    Reviewer:
    “My best faith interpretation of the 'Organic Gaia Theory' is that cells are made of chemicals but are more than the sum of their parts and develop their own 'individuality'. The actions of the cell may affect the chemical processes composing it and vice versa. In a similar way multi-cellular organisms are made of cells, ecosystems are made of organisms and so on up to Gaia. If this is what is intended I do not find it compelling. As mentioned, related ideas have been discussed for a long time in complex systems science. Secondly, as pointed out by Margulis, the key property of Gaian regulation is largely carried out by microbes, connecting the smallest scales directly to the largest, rather than in a hierarchy as implied here. Thirdly the 'individuals' at each level are very different - a cell is not like a monkey is not like a forest except in the most general or metaphorical terms.
    If the above is not what is intended then it is the perogative of the authors to make their theory clear. Their paper will likely be seen by readers with such, apparently poor, reading ability as myself.”
    A bacteria and a monkey are different but nevertheless they are both organisms. Gaia is an organism in the same way, i.e. different from a monkey, but of the same class, not only metaphorically. We aim to apply not only “complex system science”, but biology and physiology to Gaia.
    Reviewer:
    “To summarise my suggestions in this regard:
    Section 1 discusses briefly the intellectual history of Gaia. For reasons stated, I do not believe this is an accurate history.”
    Authors response:
    We would like to stress that a comprehensive review of the intellectual history of Gaia was not the objective of our introductory text but rather to shortly explain how and why the notion of Gaia as a superorganism beyond a metaphor became dismissed. Nevertheless, for the revised manuscript we have prepared an additional appendix which explains in greater detail the ambivalence of the two co-authors of the Gaia hypothesis – Lovelock & Margulis – regarding the question whether a cybernetic/mechanistic or an organic/purposeful view of Gaia was more appropriate to approach the phenomenon of planetary life. Accordingly, we think that the Organic Gaia Theory is a correct name for our theory.
    
    Reviewer:
    “Section 2 discusses the postulates. I think these need to be simplified and convincing evidence given for when and where they apply. As discussed previously, I am not convinced by the argument provided.”
    Authors response:
    The reviewer uses the term “postulate” differently from the way it has been used in the history of science, as explained before. First, we define the postulate in a sentence (we will rename it according with the reviewer’s suggestion to not use Prigogine and Margulis names). Later, we give many arguments and observational examples that are coherent with the postulate, so that each paragraph becomes a scientific hypothesis related with the postulate.
    To clarify more this and other questions raised by the reviewer we will add in the revised manuscript in the introduction of section 2 the following:
    “Each postulate statement is followed by the hypotheses we argue for, supported through theoretical developments and observational evidence up to the planetary scale that make them plausible in our view. We believe that the interplay among these hypotheses makes the emergence of Gaia as an organism possible.
    Before presenting these postulates and hypotheses, we want to emphasize that we implicitly start from widely recognized postulates, namely that living beings tend to expand (normally via reproduction or replication, but also, at least at some stage of their life, via growth) and that this tendency will necessarily encounter some local limiting factor, since (finite) flows of energy and matter are required for the dynamic processes that "define" them as living beings.
    Also, we have made more explicit connections with appendices in this section to improve our arguments.”
    Reviewer:
    “Section 3 proposes to develop organic Gaia theory. There should be a clear, logical connection from the postulates to this development. In particular, explaining how the 'algorithm' of 3.2 follows from the postulates, with all terms clearly defined. “
    Authors response:
    We agree with the reviewer that Section 2 could be more explicitly connected with Section 3. Thus, we propose modifying it and adding sentences in parentheses around the points of the "algorithm", connecting those points with postulates or observations. We also add a new appendix about modelling Gaia that also connects with this section.
    The proposed change of the “algorithm” reads as follows:
    “1. Expansion of prokaryotes in number and diversity (obvious historical property of life)
    Appearance of partial and local constraints and limits to their growth, diversity and complexity (Darwin's postulate taken from Malthus (Darwin 1859) who applies it to humans and economic science. Malthus could have taken it from Buffon and Linnaeus who formulated it earlier in the natural sciences (Vernadsky 1929). ‘Steps’ 1 and 2 are common to the OGT’s perspective as well as Darwin’s theory on evolution, as well as their later versions or extensions. In fact, we think that they are common to all theories since Lamarck in biology).
    
    Probabilistic tendency, (observed in the universe) to form complex structures that increase the rate of entropy creation of the structure + the universe compared to the time before the structure was formed (postulate 1).
    
    The structures are the result of some type of force or "cooperation" between parts (otherwise we could not speak of "structure"). At least dynamical complexity is a ratchet point: the structure is stable, once emerged, if the energy gradient and the contour conditions remain (postulate 1 pointing toward postulate 2).
    
    Symbiosis and the formation of genetic and ecological nets (in the case of living beings, this is an observation: As living population grow, interactions inevitably occur no longer only with the physical material environment but with other living beings that also influence the physical environment - from which they take matter and energy and eliminate matter and disperse energy - (see appendix C), while it is consistent with Margulis' dynamics, which will become relevant, because it gives rise to the symbiogenesis of new structural forms, closing Prigogine trinomials in cascade later on. As this dynamic continues, the possibilities of figure 2 of appendix C begin to appear).
    
    Breaking of partial and local limits and new expansion of the living beings (the new structure has the possibility of breaking some limits, examples are described with the MTE – e.g., a multicellular organism can regulate its internal temperature, bacteria cannot and therefore bacteria are more structurally limited to the external temperature or its fluctuations: a bacterium freezes quickly if the external temperature is less than 0ºC, a polar bear does not. This “step” is also coherent with the figure of the appendix C.1. referring to the leap made by postulates 2 and 3 to remove, with recycling, a possible limiting material factor.)
    
    New partial limits (any growth that may occur will sooner or later encounter a limit, things do not grow forever, therefore, we return to points 1 and 2 and enter a positive feedback cycle).
    
    More cooperation-symbiosis leading to emerging macro-structures facilitated by Prigogine and Margulis Dynamics appears (Postulates 2 and 3 are repeated, forming increasingly closed trinomials, i.e. with top-down causation and transfer of functions to the superior structure that emerges with properties of individuality)
    
    Transfer of functions and processes to the emerging structure and causality from the emerging structure back to its parts that increasingly become functions of the structure, as well as to exchanges with the environment which are modified in benefit of the structure (complete closure of Prigogine's trinomial, postulate 2, see appendix C.2. at the Gaian level.)
    
    After repeated iterations from 1 to 7: Qualities of high organicity emerge via organic symbiogenesis (postulate 3); the structure becomes a superorganism that limits and selects the internal expansion of former ‘cells’, i.e. ‘orchestrates’ or ‘coordinates’ its functional parts.
    
    Corollary 1: When Gaia acquires a very high level of organicity, such that we can classify it as an organism, its complexity and purposes may be sufficient for Gaia to accelerate the steps 1-10 internally (she becomes increasingly capable of organizing her internal processes, her organicity increases, and hers RAMO properties increase over time; this, in fact, is already beginning to be part of the predictions that follow from the OGT and its processes).
    Corollary 2: Gaia becomes an evolutionary organism with exponential growth potential, which also applies to her internal complexity (also a testable prediction of the OGT).”
    Reviewer:
    “Section 4 suggests that Gaia is 'alive' according to the definition of life provided but does not demonstrate this via evidence or argumentation and makes vague suggestions for future work. Actually carrying out at least some of this work seems necessary, otherwise what are we to make of a theory without any applications or predictions?”
    Authors response:
    The reviewer here and in the first review suggested that Gaia should be made ‘measurable’ or represented mathematically. Nested trinomials, as well as the OGT itself, are difficult to represent mathematically because of the complexity linked to biological phenomena (see for more details our previous reply to rev.1). In our view, the RAMO properties provide means to measure the organicity of Gaia. This work is already underway. For the revised manuscript, we have prepared an additional appendix on ‘Modelling Gaian dynamics’ where we provide a simple but concrete example how an evolutionary process in the light of the OGT can be represented through a set of differentiated equations. In this appendix, we also explain some weaknesses of existing models on Gaia and discuss the difficulties different types of models face when trying to represent emergent properties. Furthermore, we add another appendix on the cycling ratios of Gaia, which can be shown to be comparable to that of a complex organism, and far higher than in non-organic complex systems. This finding strengthens the notion of Gaia as super-organism / organic macrostructure.
    It was suggested by both reviewers that we give examples of concrete predictions stemming from the theory and discuss potential difficulties to verify/falsify the theory and/or the predictions made from it. As already pointed out, the RAMO properties can be applied to Gaia in future work. To illustrate this, we will introduce this work in a new appendix to the main text where we compare the water cycling of Gaia (which is part of the R property) with the water cycling of other complex systems, including organic and inorganic ones. Additionally, we have prepared an appendix on programmed cell-death (or apoptosis) of unicellular organisms as a concrete prediction resulting from the OGT. The appendix on modelling Gaian dynamics also contains a concrete prediction regarding the dynamics of the evolution of planetary life. In the appendix and the discussion, we show that the behaviour and predictions made are testable, i.e. the falsifiability of the theory is granted.
    Reviewer:
    “The key concept of the 'trinomial' needs to be explained in much more depth. Figure 1 is a drawing not an explanation. Although not under review, I do not think it is clearly explained in the prior work cited by the author which simply asserts that it 'can be applied to any sufficiently complex dissipative system', calling this 'Prigogine’s conjecture'. Why should we believe this is true? Even if it is, how can 'sufficient complexity' be determined? What does it mean to 'apply' the trinomial? How are the elements in the boxes identified? What do we do once we identify them? How does describing a system in this way allow us to make new conclusions or give us new insight? And so on. To reiterate, this is not a well known concept and should be explained here, since it is supposed to be crucial.”
    Authors response:
    The trinomial is explained with sufficient detail in the published and cited article as it is essential for the entire argument raised there. We also insist that the questions raised by the review are explained in the appendices of the manuscript, mainly in the appendix C2, where the trinomial is applied in detail at the Gaia level, point by point, and causal relation by causal relation, and with examples and comparisons with the scientific literature.
    Figure 1 is not just a drawing; it represents the concept as it has been used many times by Prigogine (including in his Nobel price lecture). In case that the reviewer is referring to Figure 2 (as in the first review), we want to stress that it is not just a drawing but related with the analogous figure in the paper of de Castro and McShea. In any case, we have changed Figure 2’s caption to explain better and to clarify the other question raised by the reviewer about the necessity of hierarchies or not:
    “Figure 2: Succession of Prigogine trinomials nested within each other in the ecological realm. Gaia emerges as the functional structure from “inferior” hierarchical levels. Bacteria cells usually live in complex communities (biofilms), being functional for biofilms as cells are functional for tissues in animals (Chai et al. 2024). Equally, we speak of “systems” like the nervous or the immune system in a hierarchical superior structure like in our body. The hierarchy is also blurred in the sense that causal relations between cells and the organism as a whole occur, or, like here, between bacteria and other “ladders” and Gaia. See text for more explanations. Photographs are of public domain.”
    Also, we think that Figure 1 and 2 facilitate the understanding of the Figures in the appendices that show more details about the trinomials applied to Gaia.
    
    On the subject of names, if the authors intend their theory to be something other than a version of Gaia particularly concerned with life, then I suggest a different title than 'Organic Gaia Theory'.
    Our objective is not to develop “a version of Gaia particularly concerned with life”. The OGT is a new theory concerned with the first Gaia hypothesis of Lovelock, in which Gaia is interpreted as organism. Therefore, for clarity and for historical reasons, we speak of an ‘organic Gaia’ (theory).
    
    Reviewer:
    “I am happy to re-review a manuscript revised in such a way. Should the editor deem my requests unreasonable, so be it. I do not think there is any productive purpose in continued back and forth in these comments.”
    
    Citation: https://doi.org/10.5194/egusphere-2025-1532-AC4

RC3: 'Comment on egusphere-2025-1532', Praful Gagrani, 04 Sep 2025

In my understanding, the logic of the paper is as follows. The authors want to put forth an organic theory of Gaia where they argue that the Earth as a whole (Gaia) is a biophysical entity. The authors define a biophysical organism through three postulates and then posit that these postulates are satisfied by the Earth as a whole. The postulates for biophysical organisms are: 1. they evolve towards stable structures of high complexity until they reach a limit 2. they form emergent structures which exercise downward causation (Prigogine trinomials) 3. the trinomials can symbiotically cooperate to form a nested trinomials with higher limits. Gaia is then defined as the topmost limit of the nested trinomials.

While the idea is appealing and well-communicated, there are a few shortcomings to the narrative in my opinion.

Philosophical and scientific shortcomings:

While the authors acknowledge the "teleology problem" with downward causation when the Prigogine trinomial is introduced, they don't address it in anyway. Furthermore, the language in the rest of the article does not offer any nuanced interpretation through which the problem can be sidestepped. Due to this, their description of biophysical organisms gives a lot more agency to them than is warranted, and perhaps even required.

I will illustrate this problem with the RAMO properties. Recyclability: Do organisms have the ability to recycle elements that they "use" the most, or are there some elements that are recycled easily and thus using them is an optimal solution? Multifunctionality: Are the different functions performed by parts of the organisms coupled by the organism, or the coupling of these functions is what produces the concept of an organism? And so on...

Shortcomings in the future proposal (Sec 4.2):

There is no proposed way of experimenting with Gaia, the super-organism, without experimenting with its parts. If indeed Gaia is posited to have an ontological existence, there must be a concrete description of what is meant by "a being able to modify her contour conditions..." (395). The authors should also consider and expound on scenarios where the hypothesis would be false or discuss whether it is unfalsifiable because of the spatial and time-scales involved.

Citation: https://doi.org/10.5194/egusphere-2025-1532-RC3

AC5: 'Reply on RC3', Carlos de Castro, 01 Oct 2025

Reviewer 2, Prauful Gagrani:
Reviewer:
“In my understanding, the logic of the paper is as follows. The authors want to put forth an organic theory of Gaia where they argue that the Earth as a whole (Gaia) is a biophysical entity. The authors define a biophysical organism through three postulates and then posit that these postulates are satisfied by the Earth as a whole. The postulates for biophysical organisms are: 1. they evolve towards stable structures of high complexity until they reach a limit 2. they form emergent structures which exercise downward causation (Prigogine trinomials) 3. the trinomials can symbiotically cooperate to form nested trinomials with higher limits. Gaia is then defined as the topmost limit of the nested trinomials.”
Authors response:
We are arguing that Gaia is a biological entity, a superorganism, that lives in her biosphere (which is not the entire Earth). It is Gaia that constrains, as a cell does, her biosphere, analogous to how the lipid membrane is constrained and time-bound by the cell. The biosphere would be precisely analogous to the snail's shell, as Lovelock wrote, thinking of Gaia at the beginning of his exposition of Gaia (1972). With the snail, it is difficult to determine precisely where the non-living ends and the living begins; yet, we clearly see that its boundaries and constraints have been partly defined by it from within, starting with the creation of its “house”.

To be more explicit in the distinction between biophysical and organic systems, in the new historical appendix we use the analogy brought up by Lovelock himself, that we just pointed out.

Reviewer:
“While the idea is appealing and well-communicated, there are a few shortcomings to the narrative in my opinion.
While the authors acknowledge the "teleology problem" with downward causation when the Prigogine trinomial is introduced, they don't address it in anyway. Furthermore, the language in the rest of the article does not offer any nuanced interpretation through which the problem can be sidestepped. Due to this, their description of biophysical organisms gives a lot more agency to them than is warranted, and perhaps even required.”

Authors response:
We have expanded the appendix on teleology and added a new appendix on the issue already addressed by Lovelock and Margulis and other authors, explicitly adding this sentence there:
“We assume that Gaia has at least the same teleological properties as any organism, if not more, since the transfer of functions (speaking of function is actually speaking of teleology, since the "by and for" is used) is toward Gaia. The cells of my body lose functionality within themselves, and an "external" agent coordinates them functionally for that external agent, my body. Thus, they lose their intrinsic teleological character. In the appendix (and also in de Castro 2019), we hold that intrinsic teleology is a defining characteristic of life, perhaps even equivalent to what is being alive.”
In the new appendix about the programmed cell death in unicellular organisms, the logic of the transfer of functions toward Gaia and the downward causation from Gaia toward the unicellular organisms give raise to the formulation of the predictions made there.

Reviewer:
“I will illustrate this problem with the RAMO properties. Recyclability: Do organisms have the ability to recycle elements that they "use" the most, or are there some elements that are recycled easily and thus using them is an optimal solution ?”

Authors response:
In theory, at the atomic level, the easiest elements to recycle are noble gases and metals, because they do not mix with other elements and are therefore easier to "separate." Also, metals are relatively abundant in the Earth’s crust. Thus, human civilization recycles Fe much more (its magnetic property makes it easy to separate) or Al (where the initial separation from bauxite, already oxidized Al, is energetically very expensive, but once in the form of "pure" Al it is easy to recycle). On the other hand, although Gaia and many organisms effectively recycled easy and useful elements such as iron internally, they will also be dedicated to recycling those they use the most, which are neither the easiest to recycle (because they mix easily, a requirement to make complex molecules, cycling carbon from CO2 costs a good part of all photosynthetic energy!) nor the most abundant in the Earth's crust. Of course, life will use optimal solutions at each "level" whenever possible, and the path is also contingent and historical, but above all it will do so in its "interest." In any case, both seeking optimal and novel solutions given the restrictions she encounters gives Gaia more, not less, agency. The fact that Gaia exceeds the R capacities of water of the most complex organisms, is both an organic necessity and an organic property. We do not know of non-organic systems with the R capacity of complex organisms. Gaia, therefore, is not less organic but more organic than other organisms; this is an ongoing work but we have added a preliminary result in a new appendix. The internal cycling already begins to define an autopoiesis or autocatalytic cycles, an interiority. This can be measured with a first parameter: the number of times it is “used or cycled” internally with respect to the needs for “food” or “waste” generated from and emitted to the outside. The rest of the RAMO properties are also in some way more quantifiable than properties such as autopoiesis, or homeostasis.
Other variables such as growth and replication are easier to quantify. We have already mentioned that what we expect based on the OGT is a growth of Gaia (see also the new appendix about modelling Gaia), of its size and of its internal complexity measured through the MTEs for example, with an exponential tendency and ultimately, when absolute limits emerge, a logistic one.

Reviewer:
“Shortcomings in the future proposal (Sec 4.2):
There is no proposed way of experimenting with Gaia, the super-organism, without experimenting with its parts.”
Authors response:
Yes, it can be done, and we have laid the basis for it, although it will need to be expanded: RAMO properties, such as recyclability, are measured as a whole, not by its parts, although of course we can measure how much iron is recycled in her oceans or in her cells. Precisely, our proposals allow for a comparison of a ‘whole’ with other ‘wholes’. It is true that to measure water cycling in the human body, we have used certain ‘gates’ (e.g., how much water passes through the kidneys per day versus how much water enters through the mouth), but that does not mean it is different from the fact that we can view it in both cases as an ontological existence (Gaia; the human body).
Reviewer:
“If indeed Gaia is posited to have an ontological existence, there must be a concrete description of what is meant by "a being able to modify her contour conditions..." (395 ). The authors should also consider and expound on scenarios where the hypothesis would be false or discuss whether it is unfalsifiable because of the spatial and time-scales involved.”

Authors response
The ability to modify boundary conditions is equivalent to having ‘technological capacity‘, represented by the causal arrow of the Gaian trinomial that runs from Gaia, the structure, to the fluctuations (the boundary conditions). We can use your own example (Adams et al. 2024), where oxygen is mediated by the whole of life, by Gaia. Here, it is not accurate to say that only one part of the system comes into play to explain it, even though there are very important parts, such as the cells that carry out photosynthesis, and "external" restrictions, such as the fact that excess oxygen “burns” life. Being able to modify one's contour conditions is precisely the example of both oxygen and the ozone layer—analogous to a protective membrane against an external restriction. Precisely because R is very high, it also defines how one interacts with the outside world. The RAMO properties that are found in organisms and not in non-organic systems, at least not to the degree of the former, are “falsifiable”: if quantitative evidence was found for a low level of R, A, M, O at the scale of the biosphere compared to a complex organism, and similarly low levels of R,A,M,O in purely physical systems, then, the larger hypothesis that Gaia is an organism would be falsified.
Of course, if Gaia has ontological existence as a living being, with physiology, physiological properties that determine how its parts function are expected. An example is programmed cell death in unicellular cells (prokaryotes and unicellular protists), which can be explained much more directly, once we consider Gaia a multicellular organism to which unicellular organisms belong and for which they perform functions.

In order to address these questions in the revised manuscript we have added new appendices.

Citation: https://doi.org/10.5194/egusphere-2025-1532-AC5


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The Gaia hypothesis revisited: Introducing an Organic Theory of Gaia

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