Four great waves of animals have spread out from the tropics

Trilobite fossils

Trilobites dominated the first group of complex animals

imageBROKER / Alamy Stock Photo

The tropics are the most biodiverse regions on Earth. Now there is evidence that they are also the main source of evolutionary innovation and diversity.

Complex animals have dominated Earth for 541 million years, a time span called the Phanerozoic eon. Huge numbers of species have evolved and gone extinct during this time, in a complex story that includes fish, giant reptiles and whales.

However, in the 1980s palaeontologist John Sepkoski analysed the overall pattern of evolution in the sea, where the fossil record is best. He concluded that …