It seemed like a plausible idea: a massive asteroid crashed into the Yucatan Peninsula 65 millions years ago, wiping out dinosaurs and clearing the way for mammals to replace them as Earth's highest profile life form. The theory was accepted widely enough that even cartoon faves The Simpsons pondered it.

But new evidence uncovered during a decadelong study challenges the long-held belief. The finding was part of an effort to explore the evolutionary relationships among all extant (living) mammal species over the past 160 million years.

As part of this project, a collection of researchers from five countries (the U.S., U.K., Germany, Australia and Canada) compared molecular evidence (based on the evolution of 66 genes found in all mammals) with the fossil records to create a so-called "supertree"—a chart of the diversification of a particular lineage, in this case 4,500 extant mammals. Their findings, published in the current issue of Nature, suggest that although the ancestors of modern-day mammals, were already present during the mass extinction, they did not rapidly evolve until roughly 10 million to 15 million years after the dinosaur die-off.

The new study stems from initial efforts by teams at Imperial College London and the University of Georgia. The project ballooned from there, incorporating more contributors, until an entire construction of mammals was cobbled together from the different sources.

"We fully expected that we would find the peak [of mammalian diversification] right after the K-T boundary" [(the transition from the Cretaceous to the Tertiary period)], "when the mass extinction event occurred," says lead study author Olaf Bininda-Emonds, an evolutionary biologist currently completing a scholarship at the University of Jena in Germany. "When we didn't find it, we immediately went back and checked our results."

Bininda-Emonds says that although the supertree shows a spike at the K-T boundary, the animals involved in the rapid speciation are now largely extinct—examples include the rodentlike multituberculates, a wolflike cow called Andrewsarchus and the plesiadapiformes, animals similar to primates but not part of the primate lineage.

Bininda-Emonds says that the current ancestral species of modern mammals "did exist," prior to the K-T boundary and, according to the supertree, are at least 75 million years old "but were in such small numbers that they weren't likely to fossilize." He adds that there was a point about 50 million to 55 million years ago during the Eocene epoch (34 million to 56 million years ago) when mammals began to rapidly evolve.

To pin down these dates, the researchers compared the sequences of 66 genes found in all living mammals. For instance, the human version of cytochrome b, (according to Bininda-Emonds the most frequently sequenced mammalian gene, which is associated with electron transport in mitochondria) could be compared with its chimp counterpart. The number of changes in the two varieties of that particular gene (plus the other 65) between the two species could allow researchers to approximate a divergence time. Estimates of these divergence times were adjusted according to the fossil record—for instance, whereas the molecular evidence indicated two species diverged 80 million years ago, if study of a known fossil stated the split took place 60 million years ago, the latter number was used.

"The only truly controversial part of this study is whether orders of extant mammals began to appear in the early part of the Late Cretaceous [about 90 million years ago]," says J. David Archibald, an evolutionary biologist at San Diego State University, whose own research supports the conclusion that they may have existed during that time period. As to the findings on mammalian evolution at the K-T boundary, he believes the new analysis clearly suggests a need to reformulate previous notions about the effects of the dinosaur mass extinction.

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