The Long Patience

Here is the fact that most surprises people when they first meet the real shape of life's history, and it surprised the scientists too. Having arrived early and fast, life then did almost nothing new for an immense stretch of time. For something like two billion years, more than a third of the entire history of the Earth, life remained single-celled, microscopic, and, to our eyes, monotonous. No plants, no animals, no anything you could see with the naked eye. Just microbes, ocean after ocean and age after age of them. Biologists sometimes call this vast dull middle stretch the "boring billion," and it is easily the longest chapter in the story of life, dwarfing everything we usually think of as history. If the whole four-billion-year saga of life were compressed into a single calendar year, complex visible creatures would not appear until well into November. Almost everything before that is microbes, waiting.

But "waiting" is exactly the wrong word, because in that long quiet the microbes were not idling. They were solving the deepest chemical problems of being alive, and one of their solutions would go on to remake the entire planet from the air down. That solution was photosynthesis: the trick of catching sunlight and using its raw energy to build food out of thin air and water. Many microbes found versions of this, but one group, the cyanobacteria, hit upon the most powerful and most dangerous version of all. They learned to do it by prying apart the water molecule itself, splitting H₂O to get at the hydrogen they wanted, and throwing away the other component as worthless waste. That discarded waste product was oxygen.

We call oxygen the breath of life, so it takes a real effort of imagination to see it as the ancient microbial world first met it: as poison, as pollution, as a corrosive gas that rusted and killed. To the anaerobic life of that early Earth, life that had evolved in a world with no free oxygen and could not survive contact with it, the cyanobacteria's exhalation was a slow-motion catastrophe. And it built up with terrible patience. For hundreds of millions of years the oxygen did not even reach the air, because the oceans were full of dissolved iron that greedily soaked it up, rusting and sinking to the seafloor in immense red layers, band upon band of them, which we still mine today as the world's great iron-ore deposits. They are, quite literally, the rust of a poisoned ancient ocean, turned to stone. Only after the seas' capacity to absorb it was finally exhausted did free oxygen begin to accumulate in the atmosphere. That turning point, around two and a half billion years ago, is what geologists call the Great Oxidation Event, and it was very probably the first true mass extinction in the history of life: a mass poisoning of the many by the few, an entire atmosphere transformed, a planet remade by its own inhabitants as an accidental byproduct of how they made their living. It is worth holding that thought carefully, because it is the first time in our story that life reshaped the whole Earth without meaning to, blind to what it was doing. It will not be the last time. Much later in this book the culprit will be us, and the gas will be different, but the shape of the thing will be the same.

The oxygen crisis reached even the climate. By poisoning the microbes that had been pumping out a powerful heat-trapping gas, the rising oxygen appears to have stripped the young Earth of much of its natural greenhouse, and the planet plunged into one of the most severe cold spells it has ever known. Life had not merely changed the air. It had, without the faintest intention, reached up and altered the temperature of the whole world.

Out of that transformed, oxygen-rich world came the second great innovation of the long quiet, and it is one of the strangest and most beautiful events in all of biology. Until then, all life had been simple cells, small bags of chemistry with almost no internal structure. Then, once, a cell swallowed another cell, and, instead of digesting it, kept it alive inside. The captive happened to be a bacterium especially good at the new, powerful trick of using oxygen to release energy, and inside its host it simply went on doing exactly that, now for the benefit of both. The two became one. And here is the part that turns a fact into something uncanny: that captured bacterium is still inside you, right now. It is the mitochondrion, the power plant humming in every one of your cells, burning your food with oxygen to keep you alive, and it still carries its own separate loop of DNA, distinct from the DNA in the cell's nucleus, a memento of the day, well over a billion years ago, when it was a free-living creature that got swallowed and stayed. Every breath you take feeds a domesticated bacterium in each of your trillions of cells.

This idea, that the complex cell is a merger, a cooperative built from formerly independent creatures, was assembled into its modern form by the biologist Lynn Margulis in 1967, and, in a pattern we are beginning to recognize as almost a law of science, she was met not with applause but with ridicule. Her founding paper was rejected by fifteen journals before it found a home. The notion seemed too strange, too much like a fairy story of creatures crawling inside one another. And, exactly as with Alfred Wegener and his drifting continents in the last Part, the strange idea turned out to be simply, provably true. The evidence became overwhelming once biologists could read the molecules: the mitochondrion's own genome, its distinctive double membrane, its bacterial-style inner machinery, all of it betrays its origin as a swallowed microbe. Margulis had not even been the first to suspect it; others had floated the idea decades earlier and been ignored just as thoroughly. But she marshaled the case and withstood the mockery until the field caught up to her. The lesson lands twice. First, that complex life, the entire visible living world including you, is founded on an act of cooperation so total that the partners fused into a single body. And second, quietly, that the history of knowing is itself full of Margulises and Wegeners, right ideas dismissed for a generation because they did not fit, a pattern this book will follow all the way to the present and find still running. Later, a second such merger, the capture of a photosynthesizing cyanobacterium, would produce the green cell that is the ancestor of every plant and every blade of grass on Earth. The living world runs, at its very root, not only on competition red in tooth and claw, but on ancient, permanent, intimate alliances.

Even armed with the complex cell, life took its time. Complex cells learned, slowly, to band together into simple cooperatives, the first hesitant experiments with being a body rather than a loner. But the planet kept throwing obstacles in the way that beggar belief. At least twice, somewhere between about seven hundred and six hundred million years ago, the entire Earth appears to have frozen over almost from pole to pole, ice sheets reaching down nearly to the equator, the oceans sealed beneath a white lid, the whole planet a pale ball hanging in space and reflecting the sunlight uselessly back. We call it Snowball Earth. That life came through these deep freezes at all is a testament to how tenacious it had become, huddled around the warmth of volcanic vents and clinging on in pockets of open water until the planet's own volcanoes slowly rebuilt the greenhouse and the ice retreated. And each freeze seems to have acted as both a brutal filter and a strange spur, because it was in the aftermath of the last great thaw, in a world newly warmed and newly oxygenated, that life finally did something it had not managed in three billion years of trying.

That is the quiet paradox of this chapter, and the reason it is named for patience. Life did not race toward complexity. It arrived early and then idled for eons, inventing, across that long slow interval, the chemistry of catching light, the cooperative architecture of the complex cell, the alliances and the sheer resilience it would need for everything to come. Recall the idea from Part I, that a living thing is an island of order paid for by spending energy into a running-down universe. For the overwhelming majority of Earth's history, that is all life was: countless microscopic islands of order, holding on, refining the trick, quietly salting the seas and the sky with oxygen and possibility, dressing a stage for a play that would not begin for billions of years. And then, in a burst so sudden that it stunned the geologists who first read it written in the rocks, the long patience ended and the curtain went up.