Even after dino dieoff, our mammal forebears laid low
Mar 30,2007 00:00
When the cat's away, the mice will play. And for somewhat similar reasons, biologists have long believed that the extinction of dinosaurs caused the great flourishing of mammals on Earth - a process that produced species including ours.
That's not quite the way things happened, a study has found.
A complete new family tree tracing the history of all Earth's 4,500 mammals shows they didn't start to diversify right after the dinosaurs' demise, as conventional wisdom holds, researchers say. Rather, the process took at least 10 million years to start in earnest.
|A Cape Hyrax (Procavia capensis), a small African mammal that looks like a rodent but is actually related to elephants. Their common ancestor lived 83 million years ago, long before the dinosaurs died out. - Photo: Richard Grenyer |
The scientists, with Imperial College London and the Zoological Society of London, described the findings in the March 29 issue of the research journal Nature.
They found that many of the genetic ancestors of the mammals living today existed 85 million years ago, and largely survived a meteor crash thought to have killed the dinosaurs 65 million years ago. Throughout the Cretaceous era, when dinosaurs reigned, these mammal species had been relatively few, presumably blocked from diversifying and evolving in dinosaur-dominated habitats.
The family tree indicates that after the mass extinction, some mammals did undergo a quick diversification and evolution, the scientists said. But most of these groups have since either died out, such as Andrewsarchus—an aggressive wolf-like cow—or declined in diversity, such as the group containing sloths and armadillos.
The researchers contend that our actual "ancestors," and those of living mammals, began to diversify around the time of a sudden increase in the temperature of the planet—10 million years after the dinosaur disaster.
Andy Purvis of Imperial College said: "For the first 10 or 15 million years after the dinosaurs were wiped out, present-day mammals kept a very low profile, while these other types of mammals were running the show. It looks like a later bout of 'global warming' may have kick-started today's diversity—not the death of the dinosaurs.
"This discovery rewrites our understanding of how we came to evolve on this planet, and the study as a whole gives a much clearer picture than ever before as to our place in nature."