The History of Life in Geological Time

A story almost too big to imagine

The history of life on Earth is so long that our minds struggle to grasp it. Our planet is about 4.6 billion years old. If you squeezed that entire history into a single 24-hour day, humans would appear only in the last couple of seconds before midnight. Scientists call this vast stretch deep time, and learning to read it changes how you see your own place in the world. The tool for organising it all is the geological time scale.

How rocks keep time

How can anyone know what happened hundreds of millions of years ago? The answer is written in the rocks. Layers of sedimentary rock build up one on top of another over time. The simple but powerful rule called the law of superposition says that, in undisturbed rock, lower layers are older than the layers above them. So a fossil deep down is older than one near the top.

By comparing the fossils in rock layers around the world — a process that depends on understanding how fossils form — geologists can match up rocks of the same age across continents. Modern radiometric dating then pins exact ages on the layers by measuring the steady decay of radioactive elements, which acts like a built-in clock.

Dividing up deep time

To handle billions of years, scientists split the time scale into nested chunks, from largest to smallest: eons, eras, periods and epochs. The broad story runs like this:

• Precambrian (4.6 billion to 540 million years ago): the longest stretch by far. Earth forms, oceans appear, and life begins as simple single cells. Tiny organisms slowly fill the air with oxygen.
• Paleozoic Era (540–252 million years ago): the Cambrian explosion unleashes a burst of complex animals. Fish, then plants, insects and amphibians colonise the land. It ends with the largest extinction of all.
• Mesozoic Era (252–66 million years ago): the age of the dinosaurs, but also the first mammals, birds and flowering plants — connecting to evolution and natural selection.
• Cenozoic Era (66 million years ago to today): after the dinosaurs vanish, mammals diversify and dominate. Eventually, very recently, humans appear.

Mass extinctions: resetting the story

The history of life is not a smooth climb. At least five mass extinctions have struck, each wiping out a huge share of species in a geologically short time. Causes have included massive volcanic eruptions, climate swings and asteroid impacts.

The most famous was the end of the Mesozoic, 66 million years ago, when a city-sized asteroid smashed into what is now Mexico. The impact threw so much debris into the air that sunlight was blocked, food chains collapsed, and the non-bird dinosaurs died out. Yet each extinction also opened the door for survivors to flourish — the death of the dinosaurs is exactly what gave mammals, and eventually us, our chance.

Our place in the picture

Anatomically modern humans appeared only about 300,000 years ago, and all of recorded history — every pyramid, empire and invention — fits into the last few thousand years. On the scale of geological time, that is barely an eyeblink. Realising this is humbling: we are a very recent chapter in an enormous book, and the chapters before us shaped the planet, the climate and the life we inherited.

Try it yourself: build a deep-time timeline

1. Find a roll of paper or a long hallway and mark a line 4.6 metres long. Let each metre stand for 1 billion years.
2. Mark these events at the right spots (measuring from the start, 0 m = Earth forms):
3. First simple life: about 0.8 m from the start.
4. Cambrian explosion (complex animals): about 4.06 m.
5. Dinosaurs die out: about 4.53 m.
6. First modern humans: about 4.5997 m — almost at the very end.
7. Stand back and look at how tightly everything familiar is crammed into the final few centimetres.

Seeing the whole of human existence shrink to the width of a fingertip is the best way to feel what "deep time" really means — and why the slow processes of plate tectonics and moving continents have had more than enough time to rearrange the entire face of the Earth.