Different schemata for considering the history of life allow different types of insights. For example, de Duve136 identified a series of (mostly) biochemical events that happened just once, and discussed to what extent they would be likely to happen again were the tape of life to be replayed. Knoll and Bambach137 put forward six ‘megatrajectories’ in the history of life, where each megatrajectory corresponds to the ecological diversification of a new type of life form (prokaryotes, unicellular eukaryotes, land plants, etc), thus linking evolutionary change with ecological complexity. And famously, Maynard Smith and Szathmáry138,139 proposed a framework based on transitions between different replicating units (genes, chromosomes, individuals, and so on); this has been profoundly helpful in generating a deeper understanding of the levels at which natural selection operates140.

In recent work, Lenton and colleagues7 developed a schema for thinking about ‘revolutions’ in the history of life and Earth. As in the Perspective presented here, their focus is energy. But rather than considering expansions in the types of energy underpinning the biosphere, the authors examined a series of changes in free energy inputs and how these have altered global material cycles. On the basis of their analyses, they conclude that human sustainability will not only require a shift from fossil fuel to solar power, but also a far more active effort to recycle materials such as metals.

Here, I have taken a more bottom-up approach. In considering expansions in the types of energy underpinning the biosphere, I have sought to describe the step-wise construction of a life–planet system. Using energy expansions as the lens reveals a fundamental, recursive interplay between events in the evolution of life and the development of the planetary environment. From this viewpoint, a number of insights emerge.

First, increasing the types of energy sources available to life has led to a far more complex biosphere. Although only geochemical energy and sunlight can power the de novo transformation of inorganic carbon into living tissue, the complexity of the current biosphere rests on multiple levels of energy use. Cyanobacteria, for instance, often require the presence of non-light-using consort organisms in order to grow well141–143. Conversely, owing to the metabolic capacities of their prokaryotic symbionts and endosymbionts, eukaryotes are able to live in a far wider range of environments than they could otherwise access82. The step-wise diversification of the biosphere has, in turn, led to an expansion of possible niches, from more complex microbial mats to old shells and abandoned burrows. At the same time, the capacity of life to impact the planetary environment—and thereby the environment in which future life will evolve—has expanded dramatically with each epoch.

Because the construction of the biosphere has depended on these energy expansions, the vanishing of an energy source, even temporarily, could cause a corresponding contraction in the biosphere. In the context of the Phanerozoic, some authors have attributed large-scale patterns of both biospheric expansion and contraction to corresponding fluctuations in oxygen availability, with expanding ocean anoxia corresponding to mass extinction events (end-Permian144,145; end Triassic146). Likewise, Krin147 has suggested that one factor in the mass extinction at the end of the Cretaceous may have been dust ejected by the Chicxulub asteroid impact, which may have blocked out the sun long enough to cause a global collapse in photosynthesis. Quantifying this pattern further would be an interesting line for future research.

A related avenue for future research would be an examination of macroevolutionary trends of energy use. For example, Vermeij148 argued that the Phanerozoic has been characterized by the repeated replacement of low-energy life forms by those able to harness larger amounts of energy. Among the trends he identified were endotherms tending to replace ectotherms, and angiosperms tending to replace gymnosperms. (The lower-energy form does not always become extinct; sometimes its range is just restricted to a low-energy environment.) Investigating this trend for earlier epochs—or even applying it to human societies149—might be enlightening.

A second insight that emerges from this Perspective is that the two clear inflection points in the history of Earth—the Great Oxidation Event and the emergence of mobile animals—also coincide with expansions in the kinds of energy sources available to, and consumed by, living beings. The Great Oxidation shifted the prevailing chemistry of the atmosphere and upper ocean and made oxygen gas abundant. The emergence of life forms that eat one another transformed the nature of ecosystems, and introduced a powerful new set of evolutionary interactions, thus accelerating the pace of macroevolutionary change. From this point of view, the familiar observation that Earthly life is powered by the sun takes on a more nuanced aspect: the modern biosphere is powered not merely by sunshine but by the oxygen that results from using sunshine in a particular way.

This Perspective further suggests that, through the harnessing of fire as a source of energy, Earth has now arrived at a new inflection point. Considering life–Earth history through the lens of energy expansions supports the view that the Anthropocene is a genuinely novel phase of the planet's geological and biological development—a conclusion independently reached by Lenton and colleagues7. The technology of fire may also, perhaps, mark an inflection point for the Solar System and beyond. Spacecraft from Earth may, intentionally or not, take Earthly life to other celestial objects (though whether any Earthly life forms can thrive elsewhere remains unknown).

As this is the only life–planet system we currently know of, it is impossible to know how representative it is of life–planet systems in general. But if the development of other life–planet systems requires a similar series of energy expansions, the framework presented here suggests a way to anticipate the paths that such systems might take. For instance, if a planet has only geochemical energy—perhaps because it is far from its star, or because it is a nomad150,151 and has no star at all—any life present may have “a limited future in terms of the heights it could achieve”152. Or suppose a planet is unable to accumulate oxygen. This could happen if living organisms never evolve a way of splitting water to produce the gas in the first place6,153; but even if they do, the planet itself may have characteristics that prevent oxygen from ever building up6,66. Without oxygen, the geological, ecological and evolutionary potential of a life–planet system is likely to be constrained, even if life forms analogous to eukaryotes in their energy-harnessing power (Box 2) were to evolve. Conversely, some planets might be able to accumulate new forms of energy, and life forms able to take advantage of them, much faster than Earth has66.

In short, this Perspective of energy expansions suggests that the likely development of a life–planet system will depend on the interplay between the planet's cosmic situation, its intrinsic properties, and the paths that evolving life can potentially take. The example of this life–planet system suggests that the development of a flourishing, complex biosphere depends on a virtuous circle between evolving life forms and transformations of their planetary home.