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The modern world gives us such ready access to nachos and ice cream that it’s easy to forget: Humans bodies require a ridiculous and—for most of Earth’s history—improbable amount of energy to stay alive.
Consider a human dropped into primordial soup 3.8 billions years ago, when life first began. They would have nothing to eat. Earth then had no plants, no animals, no oxygen even. Good luck scrounging up 1600 calories a day drinking pond- or sea water. So how did we get sources of concentrated energy (i.e. food) growing on trees and lumbering through grass? How did we end up with a planet that can support billions of energy-hungry, big-brained, warm-blooded, upright-walking humans?
In “The Energy Expansions of Evolution,” an extraordinary new essay in Nature Ecology and Evolution, Olivia Judson sets out a theory of successive energy revolutions that purports to explain how our planet came to have such a diversity of environments that support such a rich array of life, from the cyanobacteria to daisies to humans.
Judson divides the history of the life on Earth into five energetic epochs, a novel schema that you will not find in geology or biology textbooks. In order, the energetic epochs are: geochemical energy, sunlight, oxygen, flesh, and fire. Each epoch represents the unlocking of a new source of energy, coinciding with new organisms able to exploit that source and alter their planet. The previous sources of energy stay around, so environments and life on Earth become ever more diverse. Judson calls it a “step-wise construction of a life-planet system.”
In the epoch of geochemical energy 3.7 billion years ago, the first living organisms “fed” on molecules like hydrogen and methane that formed in reaction between water and rocks. They wrung energy out of chemical bonds. It was not very efficient—the biosphere’s productivity then was an estimated a thousand to a million times less than it is today.
Sunlight, of course, was shining on Earth all along. When microbes that can harness sunlight finally evolve, the productivity and diversity of the biosphere leveled up. One particular type of bacteria, called cyanobacteria, hits upon a way of harnessing the sun’s energy that makes oxygen (O2) as a byproduct, and with profound consequences: The planet gets an ozone (O3) layer that blocks UV radiation, new minerals through oxygen reactions, and an atmosphere full of highly reactive O2.
Which brings us to the epoch of oxygen. Given an opportunity, oxygen will steal electrons from anything it finds. New oxygen-resistant organisms evolve with enzymes to protect them from oxygen. They have advantages too: Because oxygen is so reactive, it makes the metabolism of these organisms much more efficient. In some conditions, organisms can get 16 times as much energy out of a glucose molecule with the presence of oxygen than without.
With more energy, you can have motion and so in the epoch of flesh, highly mobile animals become abundant. They can fly, swim, ran to catch prey. “Flesh” is source of concentrated energy, rich in fats and protein and carbon.
Then one particular type of animal—those of the genus Homo—figure out fire. Fire lets us cook, which may have allowed us to get more nutrition out of the same food. It lets us forge labor-saving metal tools. It lets us create fertilizer through the Haber-Bosch process to grow food on industrial scales. It lets us burn fossils fuels for energy.
This is only a short summary, but I encourage you to read the essay in full; it’s highly readable despite being published in an academic journal. Judson is a writer by profession; she’s the author of the best-selling Dr. Tatiana's Sex Advice to All Creation, and she recently reviewed a book on the octopus for The Atlantic.
Aside from the big thematic framework, the essay is packed with small insights that will make you sit up a bit straighter and think a bit harder. (My favorite is her description of how viruses operate as “agents of death,” and play a significant role in the evolution of early microbes.) “I think any paper that can elicit that response regardless of the field is cool, especially us for jaded scientists who are often like ah yeah yeah” says Noah Fierer, a microbiologist at the University of Colorado, who also called the paper a “must read” for microbiology students.
The essay is a condensed and crystallized version of a book Judson has been writing for a decade. It reads like the synthesis of research over many years and in many disciplines because it is. When I asked Judson about her book, she replied with this email describing the writing process:
For several years, I thrashed. I wrote fragments. I read more papers, collected more examples. I took trips to look at rock formations, or at colonies of bacteria. I pestered people with questions. (Many of these were total strangers; their generosity has been prodigious.) I bored my friends. I thrashed. I hired a coach. I wrote more fragments. Until, one day, I had a kaleidescope moment: the material suddenly rearranged itself in my mind, making a new picture. It happened after I had given a talk at an institute in France; later that day, I was speaking to a friend...and suddenly this pattern of energy expansions leapt out at me. I knew how to organise the book.
“Buoyed up by this ‘eureka’ feeling,” Judson said, she decided to put her ideas out in the scientific literature. The peer review process also connected her with other people thinking about the same ideas. “It was pleasant surprise that we found another kindred spirit,” Timothy Lenton, an earth system scientist at the University of Exeter, told me. Lenton reviewed her essay for the journal and has also written about energy revolutions. The two have since corresponded.
Lynn Rothschild, an astrobiologist at NASA Ames, told me “It was one those papers where damn, I wish I thought of writing it.’” At the very end, Judson speculates that other life-planet systems in the universe may have also evolved through a series of energy expansions. If we want to look for life, we shouldn't only look for planets look like present-day Earth—a point Rothschild has been making for years. “When people talk about looking for an Earth-like planet, they say it’s got to have oxygen and I go, ‘Are you crazy?,’” she says. “If you were looking at Earth billions of years ago you wouldn’t have seen it.”
So Earth’s evolution over billions of years might give us a blueprint for finding life less complex than ours. But what might a planet that has been through more energy expansions than Earth look like? Put another way, what’s next for the Earth?
One way to ask that question is to ask what innovation will launch us into the next energetic epoch and leave it’s mark on the environment. Another is to ask what life will look like in that epoch—both what lifeforms could become extinct and what could eventually become possible. After all, it took billions of years and several energy expansions to make oxygen-breathing, flesh-eating, fire-wielding humans possible on Earth.
Sarah Zhang is a staff writer at The Atlantic. Follow on Twitter.