The tropics are the most biodiverse regions on Earth. Now there is evidence that they聽are also the main source of聽evolutionary innovation and聽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聽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聽evolution in the sea, where聽the fossil record is best. He concluded that marine evolutionary history could be聽broken down into , which he called 鈥済reat evolutionary faunas鈥�.
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The first group was dominated by trilobites, which聽resembled woodlice, and聽bristle worms; the second by shellfish-like creatures called brachiopods; and the third by molluscs, which have persisted to the present day. Other animals like land mammals probably followed similar patterns, but their fossil record isn鈥檛 complete enough for us to know.
Now, by analysing nearly 18,300 marine genera from the聽Phanerozoic fossil record, Alexis Rojas-Briceno of Ume氓 University in Sweden and his聽colleagues have found that聽the evolution of complex聽marine life is best described using four great groups of fauna, not three (bioRxiv, ).
The first supergroup existed between 541 and 494 million years ago, spanning the Cambrian explosion in which many animal groups first emerged. As in the original analysis, trilobites dominated.
The second supergroup, dubbed the Palaeozoic, lasted from 494 to 252 million years ago. Creatures with hard outer shells were now widespread, including brachiopods. This phase ended when the end-Permian extinction wiped out almost all complex life on Earth.
This matches Sepkoski鈥檚 analysis, but the new study splits his third great fauna into two. In this version, the third supergroup is called the Mesozoic. It began in the wake of the Permian extinction and ended 129 million years ago, in the middle of the dinosaur era. This time cephalopods, the group that includes squid and聽octopuses, were the rulers.
The fourth and final supergroup, the Cenozoic, is still dominant today. During this time, clams and snails have made up much of the diversity.
The team found that all four supergroups originated in the tropics, then expanded through the oceans. This is in line with the idea that the tropics are hotbeds of evolutionary innovation, which has been promoted by David Jablonski at聽the University of Chicago.
It may be that the warmer temperatures and ample sunlight ensure there is plenty of food to nourish a multitude of species. However, John Alroy at Macquarie University in Australia isn鈥檛 convinced. 鈥淚 have always been sceptical about the out-of-the-tropics hypothesis, because they have never seriously dealt with sampling biases, as far as I know,鈥� he says.
For Alroy, the problem is that聽more fossils are known from North America, Europe and Asia, whereas we have few聽from聽South America and聽Africa.聽鈥淭hat鈥檚 the fundamental problem with this聽kind of analysis, the concentration of聽data in a part of the world that聽has moved significantly,鈥� he聽says.
