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.
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