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Ancestor of All Placental Mammals Revealed

7 February 2013 2:20 pm
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Image courtesy of Carl Buell; (inset) Image courtesy of AMNH/S. Goldberg/M. Novacek

Family resemblance. The ancestor to all placental mammals was a tiny insect-eating creature that evolved soon after the mass extinctions that wiped out the dinosaurs. Although generally similar in appearance, the shrew-sized Ukhaatherium nessovi (inset), which lived alongside dinosaurs, wasn't a placental mammal.

The ancestor of all placental mammals—the diverse lineage that includes almost all species of mammals living today, including humans—was a tiny, furry-tailed creature that evolved shortly after the dinosaurs disappeared, a new study suggests.

"These new findings call for a reevaluation of the evolutionary story of placental mammals," says Anne Yoder, an evolutionary biologist at Duke University in Durham, North Carolina, who wasn't involved in the work.

Placental mammals are those that have live birth and nourish their embryos throughout gestation via a specialized organ—the placenta—attached to the wall of the mother's uterus. More than 5000 species exist today, from the 1.5-gram bumblebee bat to the 190-tonne blue whale. (Egg-laying mammals such as the platypus aren't included in this major group. Neither are marsupials, whose use of a placenta during gestation is short-lived.)

The fossil record has long suggested that even though mammals existed long before dinosaurs died off, most likely at the hands of an asteroid impact, the furry critters didn't really diversify or reach a large size until their reptilian competitors were out of the picture and ecosystems had recovered, says Maureen O'Leary, a paleontologist at Stony Brook University in New York. And even though the earliest placental mammals don't appear in the fossil record until after the dino die-offs, previous genetic analyses of living species have hinted that placental mammals may have evolved as much as 100 million years ago, tens of millions of years before that mass extinction.

To help settle the debate, O'Leary and her colleagues reconstructed the family tree of placental mammals using evidence from a large number of living and extinct species. The team's database included more than 4500 characteristics for each of 86 species. They chose creatures that represent all major groups of placental mammals, which vary in traits such as size, fur color, and various other aspects of anatomy and physiology, including the number and arrangement of bones and teeth. They also compared 27 different genes common to all placental mammals. (For the 40 extinct species included in the analysis, the team could include only information related to dental and skeletal traits.)

"This is an extraordinary set of data," Yoder says. Such databases, although large, are becoming more and more useful, heralding a new era when paleontological reconstructions of family trees can easily tap into the wealth of data available about a creature's anatomy and physiology, she notes.

Results suggest that the ancestor of all placental mammals evolved less than 400,000 years after the mass extinctions that wiped out the dinosaurs, the researchers report online today in Science. The hypothetical creature, not found in the fossil record but inferred from it, probably was a tree-climbing, insect-eating mammal that weighed between 6 and 245 grams—somewhere between a small shrew and a mid-sized rat. It was furry, had a long tail, gave birth to a single young, and had a complex brain with a large lobe for interpreting smells and a corpus callosum, the bundle of nerve fibers that connects the left and right hemispheres of the brain.

The period following the dinosaur die-offs could be considered a "big bang" of mammalian diversification, with species representing as many as 10 major groups of placentals appearing within a 200,000-year interval, O'Leary says.

Among the surprises is the study's conclusion that the ancestor of a group of placental mammals called Afrotherians—a diverse group that includes aardvarks, sea cows, elephants, and elephant shrews and was presumed to have evolved in Africa—actually lived somewhere in the Americas. That's unexpected because by the time the ancestor appeared, Africa had split from South America and was thousands of kilometers across the Atlantic, O'Leary says. It's not clear whether the ancestral Afrotherian reached Africa via a land route or by island-hopping across northern latitudes, or by riding rafts of material floating across the southern Atlantic, she notes.

The new study used the genetic information to arrange the branches on the family tree of placental mammals but didn't use a molecular clock inferred from rates of mutation to determine when the various branches first appeared, Yoder says. Statistical methods that help researchers determine the length of those branches as well as their arrangement will certainly shed more light on mammalian evolution, she says.