Extinction events

Species die out all the time. Occasionally, though, the Earth experiences mass extinctions, where catastrophic environmental change wipes out huge numbers of livings things.

In 1909, according to legend, Charles Walcott of the Smithsonian Institute was exploring in the Canadian Rocky Mountains when his horse stopped by a rock. He cracked open the rock and discovered beautifully preserved fossils. Walcott named the site the Burgess Shale. More than 50 years later, palaeontologists dug deeper – discovering an extraordinary diversity of life forms, many with no recognisable living relatives.

The fossils were a snapshot of marine life after the Cambrian explosion of about 500 million years ago. This was one of life on Earth's most significant events. Single-celled life forms gave rise to more complex, multicellular ones. With a whole planet to colonise, an extraordinary diversity of living things evolved. Then came a mass extinction.

Five main extinction events have been recognised:

1. The late Ordovician event 438m years ago, when 100 families went extinct.

2. Late Devonian 360m years ago, when 30 per cent of families went extinct.

3. End Permian 245m years ago, the biggest extinction of all time, when over 50 per cent of all families were lost.

4. Late Triassic 200m years ago, when 35 per cent of families died out.

5. The Cretaceous Tertiary (K-T) 65m years ago, which ended the reign of the dinosaurs.

The Burgess Shale contains evidence of a remarkable diversity of life. Life may have been more diverse then than it is now, 500 million years later.

Mass extinctions have a profound effect on evolution. With so many species vanishing, there is new competition to replace them in their 'environmental niches' – their positions in the world's ecosystems. The dinosaurs' loss was the mammals' gain.