The evolution of the species: a story of thriving mammals and dying dinosaurs

They are the only warm-blooded, furry creatures that can feed their young on milk and now it seems the dominance of mammals has more to do with flower power than dinosaur disaster.

A comprehensive family tree of the 4,500 mammalian species alive today has punctured the enduring tale that links the sudden death of the dinosaurs 65 million years ago with the rise of mammals.

Instead, the study found that the period when mammals underwent their greatest evolutionary explosion coincided with the arrival of flowering plants during the Eocene epoch 56 to 34 million years ago.

Alternatively, the rise might have been the result of a change in the climate - a period of global warming long after the dinosaurs became extinct - which allowed mammals to flourish.

The scientists said it was not their intention to debunk the dinosaur hypothesis, but their study indicated that the disappearance of the largest land animals the world has known had no immediate impact on the subsequent rise of mammals. "The previous evidence showed that we did see a die-off of the dinosaurs and an increase in the rise of the mammals roughly 65 million years ago," said John Gittleman, director of the University of Georgia Institute of Ecology. "But the fossil record, by its very nature, is patchy. We have found that when you fuse all of the molecular trees with the fossil evidence, the timing does not work. The preponderance of mammals really didn't take off until 10 to 15 million years after the demise of the dinosaurs."

The ancestors of many mammal species emerged more than 85 million years ago, according to the study published in the journal Nature. By combining 2,500 partial family trees, and analysing the physical and genetic relatedness of present-day mammals, the scientists found two periods of evolutionary divergence - one long before and another after the time when dinosaurs disappeared.

Although there was a small spurt of evolutionary divergence immediately after the dinosaur extinction, this occurred among a group of mammals that did not leave many descendants today. True mammalian diversity began long after this time, perhaps because the emergence of flowers and pollinating insects provided a new source of food, said Dr Gittleman.

Andy Purvis, of Imperial College London, said that a period of higher temperatures may have also played a role. "It looks like a later bout of global warming may have kick-started today's diversity, not the death of the dinosaurs," he said."This discovery rewrites our understanding of how we came to evolve on this planet and the study gives a much clearer picture than ever before as to our place in nature."

Molecular biologists studying DNA believed the common roots of mammals went back 100 million years, said Ross MacPhee, of the American Museum of Natural History. "But many palaeontologists have been dubious of this claim given the lack of ancestral-looking fossils until 50 to 55 million years ago. This new work helps to reconcile those differences. Now we know the ancestors of living mammal groups were there, but in very low numbers. The big question now is what took the ancestors of modern mammals so long to diversify."