Fossil

This is a fossil: the preserved remains in rock of something that was once alive. The original living material has been replaced (in one of several possible natural ways) by rock that preserves the shape of the animal or plant, often in great detail. This is the fossil of a trilobite, which was a marine-dwelling arthropod. There were many different types of trilobite, and they lived from about 525 million years ago until about 250 million years ago.

This trilobite is much older than the dinosaurs. They came along around 230 million years ago – after the trilobites had died out. These time scales are huge. It is over these immense periods of time that all the plants and animals on the earth have slowly evolved from earlier living things. This is the story of the origins of life that scientists have discovered by studying not just fossils but the geology of the earth. They have gradually worked out this amazing story over the last few hundred years, and continue to evolve their theories as new evidence emerges.

Humanists find this story of our origins marvellous, inspiring but humbling. Try to understand the timescale. Suppose we compress the whole life of the earth – 4 1⁄2 billion years – into a single year. On that scale, the universe came into existence with the Big Bang three years ago. The earliest single-cell life began nine months ago. This trilobite lived one month ago. Dinosaurs appeared just under 20 days ago and died out five days ago. Our ancestors diverged from those of the other apes seven hours ago, modern humans began twelve minutes ago, and the earliest records of human history – about 4000 BCE – are found 42 seconds ago. When you look at evolution in this perspective, it becomes much easier to understand how the huge variety of different forms of life have evolved from, originally, just one simple cell able to replicate itself. Everything that modern human beings are capable of evolved from that starting point.

Fossils and geological time | British Geological Survey

_{With thanks to David Pollock}