3.1 — Four Billion Years in One Chapter

Nearly every organism alive uses the same molecule to carry information — DNA — read in the same three-letter code, with almost the same letters standing for the same amino acids. A bacterium in a hot spring, an oak tree, a jellyfish and you all use the codon GGC to mean glycine.

There is no chemical reason it had to be that code. Other assignments would work fine. The shared code is evidence that everything living descends from one population that already had it — the last universal common ancestor, usually shortened to LUCA. It was not the first life, and it was not alone; it is simply the most recent point from which everything surviving today descends.

This chapter compresses the four billion years from that ancestor to the first humans. Volume V does the biology properly; here we need only the sequence, because a few of its events set the conditions for everything humans later did.

The clock, once more

Chapter 1.1 gave the one-year calendar. Hold onto it, because the proportions are the point.

Event	Years ago	On the one-year calendar
Earth forms	4,540 million	1 January
Oldest evidence of life	~3,700 million	late February
Oxygen begins to accumulate	~2,400 million	end of March
Complex cells	~1,800 million	late July
Multicellular life	~800 million	mid-October
Animals with hard parts	539 million	18 November
Land plants	~470 million	24 November
The Great Dying	252 million	12 December
Dinosaurs end	66 million	26 December
First stone tools	3.3 million	31 December, 7:30 p.m.
Our species	~300,000	31 December, 11:37 p.m.
Farming	~11,000	31 December, 11:58:40 p.m.

Life starts early and then does almost nothing for two billion years

The oldest widely accepted evidence for life is around 3.5 to 3.7 billion years old — carbon with a biological isotope signature in Greenland rocks, and structures called stromatolites in Australia that are layered mats built by microbial communities, of a kind still forming today in Shark Bay.

That is early. Recall from Chapter 1.2 that the planet had oceans by 4.4 billion years ago. So life appeared within a few hundred million years of the surface becoming habitable, which is either evidence that the chemistry is not hard once conditions allow, or evidence that we have one sample and should not generalise. Both are fair readings.

Then, for roughly two billion years, life is entirely single-celled and mostly unremarkable to look at. This is the fact that most surprises people. More than half of the history of life is bacteria and archaea. There is a real lesson in it: the hard steps in evolution are not the ones that look dramatic.

But that invisible world invented all the important chemistry, and everything since has been variations. Fermentation, photosynthesis, respiration, nitrogen fixation, and the electrical machinery that runs every cell were all worked out by microbes in this period.

The oxygen catastrophe

Around 2.4 billion years ago, oxygen produced by photosynthesising bacteria began to accumulate in the atmosphere. This is the Great Oxidation Event.

For most of the life then existing, oxygen was poison. It is chemically aggressive and it destroys the delicate molecular machinery of organisms that never evolved defences against it. This was, by a reasonable measure, the largest extinction in Earth's history — and it was caused by life itself.

Three consequences run forward into everything else.

The banded iron formations of Chapter 1.12, which are the world's iron ore, were laid down as the new oxygen reacted with iron dissolved in the ancient oceans.

Oxygen made large, active life possible. Respiration using oxygen releases roughly fifteen to eighteen times as much usable energy from the same food as fermentation does. Nothing large, fast or warm-blooded is affordable without it.

And oxygen built the ozone layer (Chapter 2.1), which is what eventually made dry land habitable, since without it ultraviolet radiation sterilises exposed surfaces.

One cell swallows another, and everything changes

Around 1.8 billion years ago — possibly earlier — one single-celled organism engulfed another and did not digest it. The swallowed cell was a bacterium good at using oxygen. It survived inside, kept working, and both cells did better together than apart.

That relationship became permanent, and the descendant of the swallowed bacterium is the mitochondrion, the structure that supplies energy in every cell of your body right now.

The evidence is unusually clean. Mitochondria have their own small circular DNA, of bacterial type. They have double membranes, as you would expect from being engulfed. They divide independently of the cell. And their gene sequences place them squarely among a specific group of bacteria. A later, separate engulfment of a photosynthesising bacterium produced the chloroplast, and that is where plants come from.

This is endosymbiosis, and the biologist Lynn Margulis argued for it through the 1960s against sustained rejection before it became textbook orthodoxy. Complex cells — everything from an amoeba to a whale — exist because of two ancient mergers.

Multicellularity, and then an explosion

Multicellular life appears around 800 million years ago and has evolved independently many times, which suggests it is not a hard step once you have complex cells.

Then, beginning about 539 million years ago, comes the Cambrian explosion: within perhaps 20 to 25 million years, nearly every major body plan of animals alive today appears in the fossil record — arthropods, molluscs, chordates, echinoderms.

Why then is genuinely argued, and the honest answer is several contributing causes rather than one: oxygen finally reached levels supporting active predatory lifestyles; the evolution of predation itself started an arms race in which armour, speed and eyes all paid off; and the evolution of hard parts made animals both more capable and vastly more likely to fossilise, so part of the "explosion" is an improvement in the record rather than in the life.

The Burgess Shale in Canada and Chengjiang in China preserve these animals in extraordinary detail, including soft tissue.

Life moves onto land, and then keeps being knocked down

Plants colonise land around 470 million years ago, followed by arthropods and then, around 375 million years ago, by the first four-limbed vertebrates hauling themselves out of shallow water. Every land vertebrate since — amphibian, reptile, bird, mammal, you — is descended from those fish, which is why your arm has one bone, then two, then a cluster, then digits: the same layout as their fins.

Then the five mass extinctions of Chapter 1.10 knock the whole system over five times. Each one changed the direction of the story. The end-Permian nearly ended complex life. The end-Triassic handed the world to dinosaurs. The end-Cretaceous took it away from them and handed it to mammals.

The 66 million years that made us possible

With the large dinosaurs gone, mammals — which had existed for 150 million years as small nocturnal animals — expanded into every vacated role within about 10 million years. Whales, bats, horses, elephants and primates all appear in this window.

The climate then cooled, for the reasons in Chapters 1.8 and 1.10 — the Himalaya rising and weathering, ocean gateways opening and closing, carbon dioxide declining. Antarctica froze around 34 million years ago. Cooling and drying shrank the forests and spread grasslands, and grasslands are hard to live in if you are built for trees.

Grass itself is a significant invention. It grows from the base rather than the tip, so it survives being eaten and burnt, and its tissues contain abrasive silica. Animals eating it needed continuously growing teeth, longer guts, and the speed to survive in open country. The great grazing herds — and the predators following them — are a product of this cooling.

Primates split off as tree-dwelling mammals with grasping hands, forward-facing eyes for judging distance between branches, colour vision for finding fruit, and larger brains. Apes appear around 25 million years ago in Africa, and around 7 million years ago one African ape lineage began doing something unusual: standing up.

What this leaves you with

Four points carry forward into every later Part.

Life reshapes the planet. Oxygen, the ozone layer, most sedimentary rock, the coal and oil of Chapters 1.10 and 1.12, and the composition of the air are all biological products. Humans changing the planet is not a new category of event; it is a new speed.

Contingency is real. No principle required mammals to inherit the Earth. An asteroid arriving a few minutes later would have hit deep ocean instead of a sulphur-rich shelf. Rerun the tape and you very likely do not get us.

The hard steps were early and invisible. Getting from chemistry to a cell, and from a simple cell to a complex one, took billions of years. Getting from a complex cell to an elephant took a few hundred million. If that ordering holds generally, it says something about how common intelligent life is likely to be elsewhere, though with one sample nobody should be confident.

And everything human happens in the last flicker. Every name in Parts 4 through 12 lived in the final thirty seconds of the year-long calendar.

What the next page covers

Around seven million years ago an ape in Africa began walking upright, and the fossil record from that point to the present is unusually good. Chapter 3.2 covers becoming human — what actually distinguishes us from other apes, why walking upright came before the big brain, what fire and cooking had to do with paying for that brain, how many other kinds of human there were and what happened to them, and the honest state of the evidence on when our species left Africa and where it went.