caterpillarThere was a time on Earth when everything was red. Or rather reddish pink, as James Lovelock corrected me when I showed him an earlier version of this essay. Lovelock and Lynn Margulis formulated the Gaia hypothesis, and Margulis had a further extraordinary insight into the origins of complex life on the planet.

The Earth is about 4.5 billion years old. Although the exact time life first appeared is disputed, it took hold quickly—many geologists believe life’s first chemical signature at 3.8 billion years old. Where life came from we don’t yet know. But we know that the first forms of life must have withstood fierce, hot conditions on the early Earth. Today we find bacteria called extremophiles living at deep-sea vents where superheated water pours out at temperatures up to 300 degrees Centigrade. The earliest life must have also been tough like this.

However life got to Earth, whether spontaneously arising here or riding on comets’ tails to seed the surface, Earth looked very different from today. For the first 2 billion years that life colonized the land surface, life breathed a different atmosphere. Back then, oxygen was not freely available in the air. It was bound in iron compounds. That’s why everything was reddish pink. Iron gets reddish when it rusts, and rusting happens when oxygen combines with iron molecules.

For the first 2 billion years, oxygen was bound up in iron compounds, and there was none of it to breathe. Instead there was methane, the odiferous component of swamp gas and farts. The creatures that could breathe methane were bacteria, simple single celled organisms. They lived in scums and agglomerations and eventually built bacterial cities in shallow water called stromatolites, which look a bit like reefs.

During two billion years on the pinkish Earth, the only sounds were winds sighing and wave sloshing. The original bacteria lived by digesting chemical compounds they found lying around. Evolution was meanwhile playing a quiet trick. If the original bacteria were like sheep grazing on chemicals, wolves were in the making. Some of the methane breathing bacteria evolved into predators who ate the grazing bacteria.

This evolution of predation prepared life for a grand jump in complexity. It happened this way: sometimes when a predator bacteria engulfed its prey, the prey would successfully resist being digested and dissolved. The prey maintained its integrity within the predator and kept right on living. Inside the predator, the former prey got a free ride, and was happy to feed off what the predator consumed, a neat turnabout. Over time, the two bacteria became dependent on each other, or symbiotic. When one reproduced, they both reproduced. They had fused their destiny and their DNA.

This is how complex cells were born on Earth. Margulis called it the theory of endosymbiosis. Although she was ridiculed when she first voiced it, the theory has become a central tenet of biology.

The newly symbiotic cells continued to evolve. The original prey became the cell’s nucleus, directing much of the cellular game. It is just these kinds of symbiotic cells that evolved into us. Every cell in our body is one of these symbionts—a eukaryotic cell. You can say that when the first eukaryotic bacterial cells evolved into more complex creatures, it was bacteria taking ever more complex and differentiated shapes, creating ever more specialized sub-groupings of bacterial colonies to perform specific tasks.

We could thus say that our eyes are bacterial colonies specialized for sensitivity to light, and that our guts are collections of bacteria specialized to digest other colonial bacteria we eat as our meals. Our brain is a vast mat of densely connected bacteria engaged in weaving a loom of bacterial light we call thought. We communicate our thoughts as speech to the other bacterial colonies living in our friends’ heads. It’s a bracing vision.

But back to the reddish pink Earth. When the eukaryotic cells formed, they developed a talent for evolving. One of the things they figured out was photosynthesis, the process by which plants use atmospheric gas, water and light to produce energy.

At first, only a few cells would have had photosynthesis. At this precarious moment they might have been wiped out, but for a singular advantage that photosynthesis gave them: the byproduct of photosynthesis is free oxygen. The beauty of it is that while the photosynthetic bacteria were immune to the oxygen, it killed everyone else. For the methane breathing anaerobes who had ruled the surface for 2 billion years, oxygen was a poison gas.

As the photosynthesizers—the aerobes—kept pumping oxygen, they got ever greater control, until they dominated the planet. The anaerobes retreated to environments without oxygen, like the mud at the bottom of ponds.dragon_fly

Over a couple of hundred thousand years, the photosynthesizers liberated the bonded oxygen. The sky went from reddish pink to blue. The land from reddish pink to a palette of hues including green for the first time.

Life leaped forward into oxygen. You could say that the oxygen that poisoned the anaerobes was the Earth’s first great environmental crisis, and that we are here because it.

If we cause the next such global crisis, and bring life as we know it crashing down, there is no need to worry. The hardy anaerobes from the dawn of life are still with us. They don’t just live at the bottom of ponds. They now also colonize our guts in the billions. If things favor them again, they’d be happy to crawl out of our guts as we expire and take back the keys to the Earth.