Dino Die-Off Didn't Cause Rise of Mammals, Study Suggests

Anne Minard
for National Geographic News

March 28, 2007

The death of the dinosaurs 65 million years ago didn't spark the rise of modern mammals, as has long been believed, suggests a massive new study of genetic and fossil data.

Instead, mammals were already beginning to assume their modern forms tens of millions of years before the mass extinction—and finished doing so well afterwards—write researchers in this week's issue of the journal Nature.

The findings contradict the long-held "short fuse" theory, which says that the die-off jumpstarted mammal evolution and led to a rapid diversification into the varieties we see today.

"As it turns out, the death of the dinosaurs had absolutely no impact on the modern groups whatsoever, good or bad," said study leader Olaf R.P. Bininda-Emonds, of Friedrich Schiller University in Jena, Germany.

"It was simply business as usual for them."

Long Fuse

Bininda-Emonds and colleagues from Canada, Australia, and the U.S. arrived at their conclusion after creating the most comprehensive family tree yet of modern and ancient mammals.

The so-called supertree is based on more than 2,500 estimates of mammal relatedness. It shows the genetic relationships of 99 percent of the 4,500 known mammal species and the approximate length of time since each genetic lineage split off.

The team found that the explosion in mammal diversity wasn't really an explosion at all but rather developed over a period of time as long as 50 million years—a "long fuse."

The modern diversity of mammals emerged primarily in two waves, one millions of years before the mass extinction and the other millions of years after, the researchers said.

About 93 million years ago, the first burst in diversity occurred when the placental mammals appeared. Placental mammals nourish their offspring—usually born live—through a temporary organ called a placenta.

The second burst, about 50 million years ago, gave rise to the ancestors of most modern mammals.

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.

Free Email News Updates
Best Online Newsletter, 2006 Codie Awards
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).