A central tenet of Earth system science is that Earth's macroscopic environmental conditions reflect the co-evolution of the biosphere and the geosphere. Nevertheless, a universal theory of the biosphere's role in the Earth system remains elusive. This paper reviews the state of knowledge and reflects on future directions from the perspective of scientifically understanding Earth as an integrated whole. Four biosphere dimensions need to be considered in developing a theoretical foundation for the biosphere’s role in the Earth system: physical, i.e., physics-based influences of life on planetary state, such as through albedo, roughness length, and latent energy fluxes; chemical, i.e., the material operations of the biosphere as part of the planetary biogeochemical cycles, chemically linking biotic and abiotic components of the Earth system; biological, i.e., the genetic programme (biodiversity) and its derived structures required to operate the organisms of a functional biosphere performing the Earth system functions; and ecological, i.e., the formations of organisms into ecosystems and their biotically altered larger environments as conduits of life for exerting macro-scale influence on Earth as a whole, by performing Earth system functions as a biosphere that is more than the sum of its parts. Today, it is primarily the physical and chemical dimensions that are included in Earth system models and their biogeochemical submodels, while empirical studies on the biological dimension often focus on limited geographical areas and species enumeration, and only to a lesser extent on systemic effects. For studying the biosphere’s role in the Earth system, a comprehensive approach across scales is required. Owing to its shared genes and evolutionary history, we argue that the biosphere can be regarded as a single global entity. However, biosphere-geosphere interactions and their Earth system consequences differ geographically and between the marine and terrestrial realms, with the terrestrial biosphere generally influencing Earth conditions more directly and over shorter time scales than the marine. A better understanding (working towards the development of a more universal theory) of the biosphere’s role in the Earth system is needed to advance Earth system science and, not least, for developing better capacity for prediction of Earth’s responses to anthropogenic perturbations.  More systematic collaboration than currently occurring is required between biogeophysics, biogeochemistry, biodiversity science (extending to traits and functions), ecosystem science, and land systems research.