In a big-picture synthesis that draws on research from fields ranging from geochemistry and paleontology, to microbial ecology and anthropology, Judson argues that the development of the Earth has taken place in five stages. Each stage has been marked by the evolution of organisms that can exploit a new and more powerful source of energy, allowing life to become progressively more diverse and complex. Each stage has also caused large impacts on the planetary environment — which, in turn, have shaped the course of evolution. This framework shows the evolution of life and Earth to be highly inter-dependent.

The five energy sources are: geochemical energy, sunlight, oxygen, flesh, and fire. Geochemical energy and sunlight have been present since the Earth formed, roughly 4.5 billion years ago. The first lifeforms are thought to have been powered by geochemical energy, with the ability to harness sunlight evolving later. Oxygen, flesh, and fire, however, are all consequences of evolutionary events.

While we don’t (yet!) have examples of living planets or moons elsewhere, this framework of energy steps suggests that the development of a biosphere is underpinned by the types of energy available to it — and this, in turn, depends on both the planet and its cosmic situation. For example, geochemical energy may allow life to emerge — but if a planet or moon is far from a star, then energy from light will be unavailable. Without light, a life-planet system may get “stuck” at an early stage of its development.

Similarly, a world with a geology different from Earth’s might not be able to accumulate oxygen, even if life was producing it. Without oxygen, again, life would probably be rather limited. Conversely, some planets might have accumulated energy sources — and thus developed complex biospheres — much faster than Earth has.

“A complex biosphere is not an automatic consequence of evolution. It also depends on how life interacts with its world,” says Judson. “To flourish, a living world must make its own environment richer.”

Oxygen, which first accumulated in Earth’s atmosphere about 2.3 billion years ago, is a consequence of the activities of a group of organisms called cyanobacteria; these evolved to use the energy in sunlight to split molecules of water, a process that releases oxygen as a byproduct. New forms of life then emerged that could use the oxygen. This eventually led to flesh — the evolution of mobile animals that feed on other organisms — roughly 550 million years ago. Fire, which needs both oxygen and fuel, was not possible on Earth before the evolution of land plants, around 420 million years ago. (Earth is the only planet in the Solar System to have fire.)

Each of these developments produced significant changes in the life-Earth system. The appearance of oxygen created the ozone layer, and led to a massive increase in the number of different minerals at Earth’s surface. At the flesh stage, organisms began to get big, and ecosystems became far more complex. Flesh also had major geological impacts. For instance, by putting pressure on other lifeforms to protect themselves from being eaten, flesh led to the evolution of shells. These have sometimes accumulated in vast quantities — think of the chalk cliffs of Dover in the UK, or Rügen in Germany.

Fire, meanwhile, was first used as an energy source by human ancestors, perhaps 1 million years ago, for cooking — a process that makes it easier to extract energy from food. Over time, fire also became the foundation of many labor-saving human technologies. Early examples include the use of fire to make tools: Neanderthals used fire to heat birch resin to make glue, and used the glue to attach axe heads to handles. More recent examples of fire-based technologies include furnaces to smelt iron or make glass; the steam engine and the internal combustion engine; and the Haber-Bosch process for synthesizing nitrogen fertilizer, a technology that has contributed to an enormous increase in the human population over the past 100 years. The technology of fire is also having environmental impacts, from rising levels of carbon dioxide in the atmosphere, to increasing levels of nitrogen and plastic pollution, to a remaking of the landscape with mines, tunnels, dams, and cities.

“This framework shows how life and Earth have evolved together. With the exploitation of each new energy source, changes have fed back to alter the environment,” says Judson. “These environmental changes have, in turn, altered the evolution of life, and made the planet the amazing place that it is today.”

“We humans are well suited to this planet because we evolved here,” she adds. “But most people don’t realize that the planet itself has changed hugely over time — because of life.”