Next of Kin

Pat Shipman
February 18, 2008
This colugo shows its remarkable patagium, or skin wing, which allows it to glide from tree to tree in the forests of southeast Asia.
Norman Lim, National University of Singapore

This colugo shows its remarkable patagium, or skin wing, which allows it to glide from tree to tree in the forests of southeast Asia.

Evolutionary biologists have filled in most of the branches on the primate family tree. But which group of mammals are primates' closest relatives? With the emergence of new fossil and molecular evidence, the question has been hotly debated over the last 10 years.

Some scientists have suggested that the group Scandentia, which includes the small tree shrews that scamper up and down trees in Asia, deserves the honor. Others favor dermopterans, a relatively little-known group that includes two living species of colugos, sometimes called flying lemurs, although technically they are neither lemurs, nor do they fly.

Since 1999, both scandentians and dermopterans have been lumped with primates into a single taxonomic unit or clade known as Euarchonta, or "true ancestors." But the exact evolutionary relationships among the three have proven elusive.

"The triad of the Euarchonta was too important to let this question remain unresolved," says Webb Miller, professor of biology and computer science and engineering at Penn State. That's why Miller joined a team, led by William J. Murphy of Texas A&M, that set out to untangle the puzzle.

Three theories had been generated by earlier researchers. The Primatomorpha hypothesis states that colugos and primates together comprise a single group, the Primatomorpha, which branched off from a common ancestor with the tree shrew group. The Sundatheria hypothesis suggests that tree shrews and colugos—inhabiting the geographic region known as Sunda and having many features in common—are more closely related to each other than to primates. In this view, primates split off from the sundatherians, which later subdivided into scandentians and dermopterans. The third premise—dubbed the tree shrew hypothesis—asserts that primates are most closely related to tree shrews alone, with colugos being more distant from both.

"We decided to design an exacting study that would test the three hypotheses using two independent molecular approaches," says Miller. But first they wanted to make sure the Euarchonta was a valid evolutionary grouping. To do so, the team screened a set of 197,322 known exons—areas of the human genome that code for proteins—looking for insertions or deletions—collectively called "indels"—which are rare genetic changes. From 300 candidate indels, and adding genomic information from a number of living primates for comparison, they found strong support for the idea that Euarchonta was a valid clade.

The researchers then looked to see which indels were shared by which groups. Seven showed a close relationship between primates and colugos. Only one suggested a closer relationship between tree shrews and primates than between colugos and primates. No indels supported the idea that tree shrews and colugos together made a single group. "In short, these molecular data strongly suggest that colugos are the sister group to primates," says Miller. "The Primatomorpha hypothesis is well-supported by the indel data."

To confirm their conclusion, the team assembled genomic data from two genera of living colugos, three living tree shrews, and six living primates, and used it in a molecular-clock analysis to estimate divergence times of the different branches within the Euarchonta group.

"Based on these data," Miller says, "the most probable phylogeny is that the Euarchonta arose as a distinct taxonomic group at 87.9 million years ago—during the heyday of the dinosaurs." This fundamental divergence was followed relatively rapidly by the separation, about 86.2 million years ago, of the Scandentia group—which included tree shrews and their relatives—from the Primatomorpha group—which includes primates and some gliding mammals (dermopterans). Within the Primatomorpha, the split between primates and dermopterans came at about 79.6 million years ago. These rapid-fire evolutionary divergences left relatively short periods of time for genetic differences to accumulate, which is why the team needed two different approaches to resolve the question, Miller notes.

A surprising by-product of the study—with implications for species-conservation efforts—was genetic information emphasizing the uniqueness of the pen-tailed tree shrew, which diverged from the other tree shrews approximately 63 million years ago and today is represented by a single species, Ptilocercus lowii. "Because this is such an early divergence, and because the only living species in this group is found in lowland forests in a restricted part of Southeast Asia, we suggest that a global priority be placed on the conservation of this species and its habitat," Murphy says.

Further, because of the newfound importance of understanding colugos as the nearest relative to primates, the team has urged sequencing of the colugo genome as soon as possible. According to Miller, "colugos are going to be a much more important species to study now that we know their relationship to primates."

Webb Miller, Ph.D. is professor of biology and computer science and engineering in the Eberly College of Science, William J. Murphy, Ph.D. is associate professor in the Department of Veterinary Integrative Biosciences at Texas A&M University.

Other researchers on the project include Jan E. Janecka, Texas A&M; Thomas H. Pringle, Sperling Foundation, Eugene, Ore.; Frank Wiens, University of Bayreuth in Germany; Mark S. Springer, University of California, Riverside; Annette Zitzmann, Zoological Institute at Johann Wolfgang Goethe-University in Frankfurt, Germany; and Kristofer M. Helgen, National Museum of Natural History at the Smithsonian Institution.

The study was supported by National Science Foundation grants to William J. Murphy and Mark S. Springer, as well as by a National Institutes of Health grant to Webb Miller. A paper announcing the results was published in the journal Science on Nov. 2, 2007.

Last Updated February 18, 2008