The rise of modern mammals to become the world’s dominant large animals was not caused by the extinction of the dinosaurs as is commonly thought, a genetic study has revealed.

It has long been assumed that mammals started to flourish and diversify into their present forms only as a result of the cataclysmic event, probably an asteroid impact, that wiped out the dinosaurs 65 million years ago.

The prevailing theory holds that this sudden mass extinction opened up ecological niches that were previously monopolised by dinosaurs, allowing small mammals that survived the catastrophe to evolve rapidly into forms as different as apes and elephants.

An international team of scientists has now overturned this view, with research which shows that the rise of the mammals had little to do with the dinosaurs’ demise.

A reconstruction of the mammalian family tree, using genetic data collected from 99 per cent of the 4,500 species that exist today, suggests that the diversification of mammals began between 80 million and 100 million years ago, long before the dinosaurs died out.

The next rapid phase of mammalian evolution, which gave rise to groups we know today such as primates and rodents, did not take place until between 50 million and 55 million years ago, well after the dinosaurs had disappeared. The findings are published in the journal Nature.

Although there was a small “pulse” of mammal expansion 65 million years ago, most of the new species that evolved then have died out. Animals such as the andrewsarchus, a wolf-like predator, are extinct, and others that expanded at this point, such as armadillos, have declined in diversity.

The evidence suggests that the asteroid strike that brought the Cretaceous period to an end and killed off the dinosaurs was only a relatively minor event in the evolutionary history of mammals.

Professor Andy Purvis, of Imperial College London, one of the study leaders, said: “It looks like a later bout of global warming may have kick-started today’s diversity — not the death of the dinosaurs. This discovery rewrites our understanding of how we came to evolve on this planet, and the study gives a much clearer picture than ever as to our place in nature.”

Modern mammals owe their existence to two phases of evolution that took place tens of millions of years on either side of the dinosaurs’ demise.

The conclusions reconcile evidence from DNA, which pointed towards an important phase of mammalian evolution between 80 million and 100 million years ago, and evidence from fossils, where modern forms do not emerge until about 55 million years ago.

Kate Jones, of the Zoological Society of London, said that the work also had wider significance for the study of extinctions. “It provides us with a wealth of information. Vitally, scientists will be able to use the research to look into the future and identify species that will be at risk of extinction. The benefit to global conservation will be incalculable.”

A cataclysmic mistake?

— Until now it had been assumed that mammals flourished after a cataclysmic event, probably an asteroid strike, wiped out the dinosaurs 65 million years ago

— Scientists now believe that mammals were well established long before dinosaurs disappeared

— The next rapid phase of mammalian evolution happened about 50 million years ago, well after the dinosaurs had become extinct

Source: Nature

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