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Scientists have long thought that the mass extinction of the dinosaurs around 65 millions years ago opened the door for modern mammal species to proliferate. But an international team of scientists has created a mammoth record of evolutionary timing, showing that the origins and diversification of existing mammal species – including human ancestors – don’t synch with the demise of the dinosaurs.

In the March 29th issue of Nature, the research team presents the first comprehensive history of the evolution of virtually all mammal species currently in existence, a huge group of animals that includes placentals, marsupials and monotremes.

“This is the first time such a large history of mammal evolution has been constructed,” said team member Dr Marcel Cardillo, a Visiting Fellow at the School of Botany and Zoology at The Australian National University.

By comparing data from a range of sources including the fossil and molecular records, the researchers show that most existing mammal orders first appeared between 100 and 85 million years ago, well before the Cretaceous-Tertiary extinction event that wiped out the dinosaurs. The team also found that there was a second spike of mammalian species diversification beginning around 55 million years ago.

“These two divergent spikes suggest that the rise of present-day mammals was delayed for a very long time,” Dr Cardillo said. “This suggests that the ‘fuse’ period – the time between a mammal order appearing and then diversifying into a range of different species – is much longer than previously thought. Our study shows that modern mammal ancestors appeared millions of years before the dinosaurs disappeared, and then chugged along at low rates of species diversity for many more millions of years before exploding into the multitude of species we see today.”

Dr Cardillo said it would take many more years of study to find out the reasons why modern mammal ancestors were around for so long before they diversified, but suggested that it could be the result of complex relations between mammals and other animal species, and variations in the Earth’s climate, geology and atmosphere.

The research team included Olaf Bininda-Emonds from the Technical University of Munich, Kate Jones from the Zoological Society of London, Ross MacPhee from the American Museum of Natural History, Robin Beck from the University of New South Wales, Richard Grenyer from the Royal Botanic Gardens (UK), Samantha Price from the National Evolutionary Synthesis Centre (US), Rutger Vos from the University of British Columbia, John Gittleman from the University of Georgia, and Andy Purvis from Imperial College (UK).