Evolutionary Contradictions

Mass extinction effect on mammals disputed

A new phylogenic tree suggests ancestors of present-day mammals diversified millions of years after dinosaurs were wiped out, contradicting a popular theory

By Graciela Flores, The Scientist, March 28, 2007

New findings are contradicting a popular theory that the late Cretaceous mass extinction, which eliminated non-avian dinosaurs and most existing fauna 65 million years ago, helped trigger the rise of present-day mammals. Instead, in a report in Nature this week, researchers suggest that ancestors of modern mammals coexisted with dinosaurs, survived this mass extinction event, and only experienced major diversification millions of years after dinosaurs disappeared.

“We think that the death of the dinosaurs didn’t really have a strong effect, either positive or negative, on the extant groups,” including today’s mammals, lead author Olaf Bininda-Emonds of the University of Jena in Germany told The Scientist.

Bininda-Emonds and a multidisciplinary team of researchers assembled and analyzed a mammalian “supertree,” a comprehensive phylogenetic tree that combines some 2,500 smaller trees, and brings together existing fossil record data and new molecular analyses. “The supertree shows two major diversification peaks: A quick burst of evolution between 100 million years and 85 million years ago and a second peak at about 50 million years ago,” Bininda-Emonds said.

According to Bininda-Emonds, by 75 million years ago, 10 million years before the dinosaurs went extinct, all the 18 placental orders were already there. “Then, 50 million years ago, after sitting there for 25 to 40 million years, they suddenly exploded, giving all the forms that we know nowadays,” he explained. The authors did find a small peak of mammalian diversification immediately after the dinosaurs disappeared, but most of those mammals went extinct 50 to 55 million years ago.

David Penny, of Massey University in New Zealand and an author of a related News & Views article, considers the supertree a major achievement in itself. “Now there is an evolutionary tree for almost 99% of all the 4,500 species of extant mammals, complete with divergence times estimates, available for further research,” Penny told The Scientist.

To date the supertree, the authors used 51,089 base pairs from 66 genes present in the mammalian groups they analyzed. Working under the hypothesis of a molecular clock (a dating method that infers elapsed time from the number of differences in DNA sequences between groups), they chose 30 well-dated fossils to “calibrate” the clock. To Penny, the addition of timing in the tree makes it even more remarkable. But not everybody agrees.

“Most researchers using molecular data in mammal evolution pretty much stop at coming up with the divergence time estimates, but the authors have used them to infer the diversification rate of modern lineages,” John Hunter of Ohio State University at Newark told The Scientist. “It’s a very creative approach, but I think that applying these early divergence time estimates is probably premature and a little problematic,” Hunter cautioned. “The paper’s conclusions depend on the accuracy of those estimates, and some are considerably older than a lot of the more modern molecular divergence time estimates that I’ve encountered lately.”

Bininda-Emonds partially agreed, noting that the date estimates contain errors, but even factoring in approximate values for those errors eliminates the possibility that the mass extinction triggered a major mammalian diversification. “Each new molecular study seems to be pushing the divergence dates for mammals a little further back in time, and this is a general pattern in the literature.” Bininda-Emonds said.

Hunter also suggested the early divergence time estimates reported in the Nature paper could actually be an artifact, a consequence of higher rates of gene evolution during the big adaptive radiation that affected all mammals as a group after the dinosaur extinction. “To suppose that modern mammals were somehow immune to that is kind of puzzling.”

“What we’ve done is to present a hypothesis for testing, and it’s up to others now to verify our claims or to disprove them,” Binnda-Emonds concluded.

Graciela Flores

Editor’s note (posted March 29): This story has been updated from its original version, which contained a misleading title and first paragraph.

Links within this article

O. R. P. Bininda-Emonds et al., “The delayed rise of present-day mammals,” Nature, March 29, 2007.

Olaf R.P. Bininda-Emonds

L. Pray, “Modern phylogeneticists branch out,” The Scientist, June 2, 2003.

David Penny

D. Penny and M.J. Phillips, “Mass survivals,” Nature 446: 501-502, March 29, 2007.

S. Bunk, “The molecular clock,” The Scientist,, November 13, 2000.

John P. Hunter

M. L. Phillips, “Genetic evidence for punctuated equilibrium,” The Scientist, October 6, 2006.

Source: http://www.the-scientist.com/news/home/53035/

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