During the many millions of years between those events—a time period that include the dino die-off—mammals kept a low profile, evolutionarily speaking.
The extinction event, which many scientists believe was caused by a massive asteroid impact and subsequent global cooldown, did pave the way for some new mammal species. (Related: "'Dinosaur Killer' Asteroid Only One Part of New Quadruple-Whammy Theory [October 30, 2006].)
But most of those went extinct quickly, the scientists added.
Controversial Theory
Already the new view of mammalian diversification is drawing some criticism.
The 93-million-year-old pulse idea mirrors a theory that's been touted by molecular biologists for years, said Ken Rose of the Johns Hopkins University School of Medicine.
But the new theory doesn't get much support from the fossil record, he said.
"If the modern orders did diverge from each other 93 million years ago, why it is that we've found no fossil evidence of them in the Cretaceous?" he said, referring to the geologic period that lasted from 144 to 65 million years ago.
"There's a complete lack of fossil evidence for it."
Future Implications
Meanwhile, some of the study authors argue that the new findings and the comprehensive family tree may have implications for understanding modern extinctions.
Andy Purvis of Imperial College London, a study co-author, suggested in a Nature press release that an episode of global warming much like the one the planet is experiencing today could have been responsible for mammalian divergence.
Purvis created a comprehensive family tree for primates in 1995 that spurred the supertree project.
If correct, said co-author Kate Jones of the Zoological Society of London, the work may help predict how groups of animals might fare during future climate change.
Bininda-Emonds is more conservative in his interpretations.
"We don't discuss it, because we really have no evidence one way or another, other than the fact that there was a global warming event around 50 million years ago when the modern groups finally took off," he said.
But if such a warming event had taken place, he said, it would have drastically changed Earth's landscape, expanding certain habitats and shrinking others.
"It very well could be that the present-day mammals were ... better able to adapt to the changing landscape than the archaic mammals," Bininda-Emonds said.
He added that the paper illustrates the unpredictability of the effects of climate change on species survival. (Related: "Global Warming Is Spurring Evolution, Study Says" [June 8, 2006].)
The entire research team was surprised, for example, that the nuclear winter-like period believed to have wiped out the dinosaurs showed little impact on mammals.
The evolutionary history contained in the supertree could be combined with other measures of vulnerability, such as the World Conservation Union's Red List, to determine which species should get priority in conservation efforts, Bininda-Emonds said.
That's already the goal of the Evolutionarily Distinct and Globally Endangered (EDGE) project at the Zoological Society of London. (Related: "'Weirdest' Animals to Get Conservation Attention" [January 16, 2007].)
The supertree can also help reveal factors common among closely related mammal groups that may increase the risk of extinction, he added.
And the findings may not be good news for us.
"Larger species and those with a slower reproductive mode"—such as humans—"tend to be at greater risk," Bininda-Emonds said.
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