New family tree suggests the predecessors of rodents, horses, and humans did not emerge until after the dino extinction

By Erin Wayman

Web edition: February 7, 2013
Print edition: March 9, 2013; Vol.183 #5 (p. 5)

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PUZZLE PIECE

Ukhaatherium nessovi (fossil shown) was one of 40 extinct mammal species that scientists used to construct a new mammal family tree. The tree indicates that modern placental mammals emerged after the dinosaurs went extinct 65 million years ago.

Credit: Courtesy of S. Goldberg and M. Novacek/AMNH

Tyrannosaurus rex may never have had the chance to terrorize the grandfathers of rodents, rabbits or primates. A new family tree using both anatomical and genetic data indicates that the lineages of modern placental mammals — those that give birth to well-developed young — arose after the dinosaurs went extinct 65 million years ago.

The new study, published in the Feb. 8 Science, adds to a debate over the emergence of a diverse group that includes whales, cats, bats, horses and humans. Since the 1990s, some scientists have concluded from family trees based largely on molecular evidence that at least some lineages of modern placental mammals originated as early as 100 million years ago, during the Cretaceous period. But paleontologists have been skeptical because they have found no fossils resembling these mammals that are older than 65 million years.

“What the [new] analyses do is vindicate the fossil record,” says Ken Rose, a paleontologist at the Johns Hopkins University School of Medicine.

But some molecular biologists aren’t as convinced. “This paper really doesn’t add anything new to the debate,” says Olaf Bininda-Emonds of the University of Oldenburg in Germany.

To build the family tree, paleontologist Maureen O’Leary of Stony Brook University in New York and colleagues began with 46 species of living mammals, including placental, marsupial and egg-laying mammals. The team also included 40 extinct species known from fossils. To determine how the species relate to one another, the researchers looked at differences in 27 genes in the living species. They also analyzed 4,541 physical traits related to bones, teeth and soft tissue in both the living and extinct mammals.

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VIEW FROM THE TREE

A hypothetical ancestor of modern placental mammals (shown in an artist’s rendering) may have been a small, insect-eating creature, based on researchers’ construction of a new mammal family tree.

Credit: Courtesy of Carl Buell

Some previous family trees had combined molecular and anatomical evidence. But in such studies the large amounts of available DNA always overshadowed the sparser anatomical data, O’Leary says. Using new measurements and published findings about the mammals, she and colleagues created the largest dataset of mammalian physical traits ever amassed, containing 10 times as much anatomical data as previous family trees had used. (The resulting database, MorphoBank, is online and freely accessible.)This gave the genetic and anatomical data equal weight in constructing the tree, she says.

Once the tree took shape, the researchers used the ages of the fossil species and where they sit in the tree to date the origin of the tree branches. The oldest fossil that grouped with the living placental mammals dates to 64.85 million years ago.   

The timing suggests that the disappearance of the dinosaurs and changes in the post-extinction ecosystem might have opened up opportunities for placental mammals, says Jaelyn Eberle, a vertebrate paleontologist at the University of Colorado Boulder.

But other researchers say the new work doesn’t resolve fundamental issues of how to date the origins of placental mammals. Evolutionary biologist Mark Springer of the University of California, Riverside says using fossils to date a family tree is tricky because of convergent evolution: An extinct species, for example, might be inadvertently grouped wrongly with a living species not because they share the same common ancestor but because they independently evolved similar physical traits. “I don’t think convergence has been effectively dealt with,” he says.

Convergent evolution is less of a problem in trees based on genetic evidence, he says. Such trees use steady mutation rates to calculate when lineages diverged from each other. Yet this method also has its drawbacks, says J. David Archibald, an evolutionary biologist at San Diego State University. For example, he says, it assumes mutation rates are constant, but researchers have found that these rates may change with time and vary by gene.

Although the new study has provoked controversy, some critics are open to changing their opinions. “I think Cretaceous placentals are distinctly possible and out there to find,” Bininda-Emonds says. “I’m also willing to say that if the paleontologists find nothing of the sort in the next 20 to 30 years and haven’t left some big obvious clumps of rock unturned, then I’ll switch camps and say we need to re-examine our [molecular] models.”

Back Story | HUMBLE ORIGINS

Researchers have reconstructed a hypothetical last common ancestor of all living placental mammals by working backward on the new family tree they constructed using both genetic and anatomical data. Maureen O’Leary of Stony Brook University in New York and colleagues determined which set of physical features would have been the simplest starting point for the evolution of the roughly 5,100 placental species alive today. The team concluded that the hypothetical creature probably weighed between 6 and 245 grams and had dark fur on its back with lighter hair on its belly. It also probably ate insects, climbed trees and gave birth to one offspring at a time. Its offspring was likely born hairless and with its eyes closed.