Imagine that a human being fell into the primordial soup 3.8 billion years ago when life began.He would have nothing to feed himself.The earth had no plants,no animals,and even less oxygen.He would need to be very lucky to survive on only 1600 calories a day and drink pond or sea water.So,how did we get concentrated energy sources (i.e. food) by growingplants and exploiting forests? How do we beat a planet that can support billions of humans with big brains, warm-bloodedness and upright posture?

In "The Energy Expansions of Evolution," an extraordinary new essay published in the journal Nature Ecology and Evolution , Olivia Judson shows a theory of successive energy shifts that purports to explain how our planet came to have such a diversity of environments that support such a rich variety of life, ranging from cyanobacteria to daisies to humans.

Judson divides the history of life on earth into five energy epochs, a new scheme you won't find in geology or biology books. in order, the epochs would be geochemical energy, sunlight, oxygen, flesh, and fire. each epoch represents an emergence of a new energy source, coinciding with new organisms able to exploit them and alter the environment around them. the sourcesJudson calls this "the gradual building of a system of life on a planet.

Key events during the evolution of energy expansions on earth. (i) Life emerges; the geochemical epoch begins. (ii) anoxygenic photosynthesis: beginning of the second energy epoch, sunlight. (iii) Emergence of cyanobacteria. (iv) Optimal oxidation event: third energy epoch, oxygen. (v) Fossils of probable eukaryotic appear. (vi) Fossils of red algae appear. (vii)Beginning of the fourth energy epoch, flesh. (viii) vascular plants colonize the land. fire appears on the land. Finally, wood stumps for burning indicate the beginning of the fifth energy epoch, fire. The dates of (i) - (iii) are highly uncertain. for (i) has the earliest date for which there is consistent evidence of life. (ii) has the earliest date for which there is evidence ofphotosynthesis. In (iii) is the date currently supported by fossil evidence for the presence of cyanobacteria. The scratch marks represent intervals of 25 million years. Figure drawn by F. Zsolnai.

At the time of geochemical energy 3.7 billion years ago, the first living organisms fed on molecules such as hydrogen and methane that formed from the reaction between water and rocks. They isolated energy from chemical bonds. That wasn't very efficient - since the productivity of the biosphere was roughly a thousand to a million times less than it is today.

A sunlight When microbes that can feed on sunlight finally evolved, the productivity and diversity of the biosphere stabilized. A particular type of bacteria, called cyanobacteria, found a way to transform solar energy into oxygen (O 2 ) as a byproduct, but with profound consequences: the planet acquired aozone (O 3 ) layer that blocks ultraviolet (UV) radiation, new minerals from oxygen reactions and an atmosphere full of highly reactive O 2 have appeared.

Now let's go to the time of oxygen New organisms have evolved enzymes that make them resistant to oxygen. They also have some advantages: because oxygen is very reactive, it makes their metabolism much more efficient. Under some conditions, these organisms can get 16 times more energy from just one molecule of glucose in the presence of oxygen than without.

With more energy, you can have much more movement and so, at the time of meat , animals that were highly mobile become abundant. They could fly, swim and run to catch their prey. "Meat" is a concentrated energy source, rich in fats, protein and carbon.

So a particular type of animal - those of the genus Homo - have discovered fire Fire allows us to cook, which may have allowed us to obtain more nutrients from food. In addition, it also allows us to forge metal tools, making us save on labor, create fertilizers through the Haber-Bosch synthesis, to produce food on large industrial scales, and also the burning of fossil fuels into energy.

This is only a brief summary, but I encourage you to read the full article. It is highly readable despite being published in a scientific journal. Judson is a professional writer; she is the author of the best-selling Dr. Tatiana's Sex Advice to All Creation and recently reviewed a book on octopuses for The Atlantic .

In addition to the great subject matter, the essay is full of little insights that will make you adjust your posture in your chair and think a little harder. (My favorite part is the description of how viruses function as "agents of death" and how they played a significant role in the evolution of the first microbes.) "I think any article that can provoke this reaction, regardless of the field ofstudy, is cool, says Noah Fierer , a microbiologist at the University of Colorado, who also called the article "required reading" for microbiology students.

The essay is a condensed and polished version of a book Judson had written a decade ago. When I asked Judson about her book, she responded with this email describing the process of creating the work:

"I tried for several years. I wrote fragments. I read several articles, collected more and more examples. I paid for trips to observe rock formations and bacterial colonies. I pestered people with questions. (Many of these were completely strange, yet very generous.) I pestered my friends. I tried and tried. I hired a technician. I wrote more fragments. Until one day,I had a kaleidoscope moment: The material suddenly organized itself in my mind, creating a new image. This happened after I had given a talk at an institute in France. Later that same day, I was talking to a friend... and suddenly this pattern of energy expansions came to my mind. And then I knew how to organize the book."

"Driven by the feeling of 'eureka,' " Judson said, she then decided to put her ideas into the scientific literature. The peer review process also connected her with other like-minded people. "It was a pleasant surprise when I heard we had found another scientist with similar ideas," said Timothy Lenton , an earth systems scientist atUniversity of Exeter. Lenton then revised his essay for the journal and also wrote about the energy revolutions.

Lynn Rothschild , an astrobiologist at NASA Ames , said, "It was one of those studies where, damn, I wish I had written it. "Ultimately, Judson speculates that other "life systems" may also have evolved on other planets through a series of energy expansions.If we want to look for life forms, we shouldn't just look on planets like the current earth - something Rothschild comesdoing for years. "When people talk about looking for life on an earth-like planet, they claim it has to have oxygen, and I say, 'Are you crazy?'" she says. "If you looked at the earth billions of years ago, you wouldn't see any sign of oxygen."

So, the evolution of life on earth over billions of years may give us an idea of how to find life forms less complex than ours. But, what would a planet that has gone through many more energy expansions than earth look like? In other words, what will earth's next energy expansion look like?

One way to ask this question is to ask what would usher in the next energy epoch and how that would leave its mark on the environment. Another way is what life will be like in that period - both the life forms that might be extinct and those that would eventually become possible. After all, it took billions of years and several energy expansions to make it possible for beings to existaerobically-breathing, meat-eating, fire-wielding humans. [ The Atlantic]