"Long before it's in the papers"
January 28, 2015


Even after dino dieoff, our mammal forebears laid low: study

March 28, 2007
Courtesy Imperial College London
and World Science staff

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.

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. (Cred­it: Rich­ard Gren­yer)

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.

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 other liv­ing mam­mals, be­gan to di­ver­si­fy around the time of a sud­den in­crease in Earth’s tempe­rature. That would have been 10 mil­lion years af­ter the di­no­saur dis­as­ter.

Sa­id An­dy Purvis of Im­pe­ri­al Col­lege: “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.”

* * *

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When the cat’s away, the mice play. And for somewhat similar reasons, biologists have long believed that the extinction of dinosaurs caused the great flourishing of mammals on Earth—a phenomenon that produced species including humans. 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 get going in earnest. 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 were relatively few, presumably prevented from diversifying and evolving in dinosaur-dominated habitats. The family tree indicates that after the mass extinction, some mammals did experience 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 all other mammals around now, began to diversify around the time of a sudden increase in the temperature of the planet—10 million years after the dinosaur disaster. Said Andy Purvis of Imperial College: “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.”