An international team of scientists has come up with a new timetable for the evolution of mammals, saying they began to emerge millions of years after the extinction of dinosaurs.  VOA's Jessica Berman reports the researchers reached their conclusion by piecing together the ancestral family trees of thousands of animal species.

Every school child learns that mammals began to evolve and diversify almost immediately after dinosaurs died out, leaving a biological niche for the furry creatures to thrive in.

But the director of the University of Georgia's Institute of Evolution, John Gittleman, says that is not the case.

"What we found is that when for the first time you piece together a complete evolutionary tree of the mammals, and this includes all of the species, well over 4,000 species, you find a very different picture," he explained.

Gittleman is co-author of one of a number of papers published in the journal Nature describing the creation of supertrees, which trace mammals back millions of years using all available information, including fossil records, and molecular and genetic data.

Scientists concluded that, although some ancient mammals began to emerge around the demise of the dinosaur 85 million years ago, they were relatively few in number and had not taken full advantage of the ecosystems left after the dinosaurs.

Researchers say that changed 10 million to 15 million years later, when the data show there was a burst of mammalian diversification, or early evolution, among the prehistoric forerunners of today's animals.

Gittleman says scientists do not know why this diversification suddenly took off millions of years after the demise of the dinosaur.  They speculate it might have had to do with the emergence of flowering plants. 

Gittleman says another possibility is that early mammalian activity was triggered by global warming.  He says clues from the past may offer further information about our current climate.

"In contemporary terms, we know that global climate change is something that brings about how species live, where they move, what they start to eat," he added.  "And so it may be that by gaining a clear perspective on the past, it may help us explain what is going on today and possibly in the future."

John Gittleman of the University of Georgia's Institute of Evolution.