Origin of Life on Earth: History of Earth in 24 Hours

One day for a whole planet

Picture a clock face. Midnight is the moment Earth formed, 4.5 billion years ago, and the next midnight is today. One hour on this dial equals about 187.5 million years, and a hand begins to sweep. The trick turns a number too big to imagine into something you can feel. For hours nothing seems to happen: no plants, no animals, no sound but wind and waves. The ground is bare rock, the sea is warm and the sky has almost no oxygen. Scientists rebuild this age from ancient rocks and minerals, because no animal fossils exist from such an early time.

The first living cells

At around 04:16 on our dial, life appears. The oldest evidence is at least 3.7 billion years old, though some of it is still debated: layered rocks called stromatolites, built by mats of microbes, and chemical traces in ancient stone. These were single cells without a nucleus, far smaller than a grain of dust. How life began is still an open question. In the 1920s Alexander Oparin and J. B. S. Haldane proposed a warm “primordial soup”. In 1952 Stanley Miller sparked gases in a flask and made amino acids. Another idea points to hot vents on the sea floor. A third, panspermia, says life arrived from space; it is unproven.

The air changes

Now it is 11:12. Tiny microbes called cyanobacteria had learned to use sunlight to make food, and they released oxygen as waste. At first the oxygen was soaked up by iron dissolved in the oceans, which rusted and sank, leaving red stripes in the rock. Then, around 2.4 billion years ago, oxygen began to fill the air. This is the Great Oxidation. It was a disaster for many microbes that could not live with oxygen, and a gift for others. Oxygen later made big, active bodies possible. Those rusty stripes, which we mine today for iron, are the proof that it happened.

Animals with shells and eyes

At 21:08 the dial is almost full, and life is still mostly small and soft. Then, about 538 million years ago, the fossil record suddenly fills with animals: trilobites in armour, hunters with claws, creatures with eyes. This is the Cambrian explosion. “Suddenly” here means tens of millions of years, which is only a few minutes on our dial. Why then? Scientists debate it. Candidates are more oxygen, predators and prey racing each other, and new genes for building bodies. Places such as the Burgess Shale in Canada and Chengjiang in China preserve even soft bodies, so we can see these first animals in detail.

The age of dinosaurs

At 22:46 the first dinosaurs appear, about 230 million years ago. They rule the land for more than 160 million years: huge plant-eaters, fast hunters and the ancestors of birds. On the dial this is almost an hour. Their bones, tracks and even eggs are found on every continent, and that is how we know them. Dinosaurs were not a failure waiting to end. They were the most successful large animals of their time, and the clock hand is about to reach the moment when everything changes.

The asteroid

At 23:39 the story is cut short. Sixty-six million years ago an asteroid about ten kilometres wide hit what is now the Yucatán coast of Mexico and made the Chicxulub crater. Dust and soot darkened the sky, and about three quarters of all species died, including every dinosaur that was not a bird. How do we know? A thin clay layer all over the world is rich in iridium, a metal common in asteroids, and the buried crater itself was found. Some small feathered dinosaurs survived. Today we call them birds.

Us, in the last seconds

Now the hand rushes to the end. Mammals, which survived the asteroid, spread and diversified. Homo sapiens, our species, appears about 300,000 years ago: at 23:59:54, only six seconds before midnight. All recorded history, from the first writing to today, fits into the last tenth of a second. Everything else in this book happens there. That is a reason for modesty, and also for wonder. For most of its life Earth was a world of microbes, and the story of people is very new. Our date comes from fossils such as those found at Jebel Irhoud in Morocco.

Earth’s Day in 24 Hours