[Photo] A Malayan colugo (Galeopterus variegatus)and an attached baby gliding between trees with a baby attached. Credit: Norman Lim, National University of Singapore

A group of creatures resembling large flying squirrels is the closest living relative of primates, the group that includes apes and humans, according to a new genetic study.

The finding, detailed in the Nov. 2 issue of the journal Science, contradicts a study published earlier this year by another team, which concluded that the squirrel-like colugos are more closely related to Scandentia, a group that includes tree shrews, than to primates.

Found in Southeast Asia, colugos are colloquially called "flying lemurs," although they are not lemurs and they don't truly fly. The animals are larger than flying squirrels but have a similar skin fold, called a patagium, which they use for gliding. Coasting from tree to tree at dusk, they look like furry kites.

Unclear relationships

Colugos belong to a classification of mammals known as Dermopterans. Together with primates and Scandentia, they make up the single taxonomic unit, or "clade," known as Euarchonta (meaning "true ancestors").

The exact evolutionary relationships among the three groups are a topic of debate among scientists. There are three possibilities:

-Colugos and primates shared a common ancestor that split from tree shrews

-Tree shrews and colugos are more closely related to each other than to primates

-Primates and tree shrews are sister groups, and colugos are the odd ones out

A study published in a January issue of the journal for the Proceedings of the National Academy of Sciences (PNAS) used morphological comparisons of the three groups to determine that tree shrews and colugos are more closely related to each other than either is to primates.

A different picture

The new study, based on genetic comparisons, paints a different picture. Jan Janecka of Texas A&M University and colleagues compared rare genetic changes, called indels, in the genomes of members of the three groups. Indels are regions of insertion or deletion in areas of the DNA that code for proteins.

The team found that colugos and primates have seven indels in common. Only one indel matched up between primates and tree shrews, and no indels were shared between tree shrews and colugos.

"In short, these molecular data strongly suggest that colugos are the sister group to primates," said study team member Webb Miller of Penn State University.

In a second experiment, the team fed genetic data from five mammalian groups, including Primates, Dermopterans, and Scandentia, into a computer model to calculate when they diverged. The results suggested Primates, Demopterans and Scandentia shared a common ancestor as far back as 87.9 million years ago, when dinosaurs still walked the Earth.

According to the model, the three groups separated relatively quickly soon after. At 86.2 million years ago, the ancestors of tree shrews split from that of primates and colugos, and primates and colugos went their separate ways about 79.6 million years ago.

Based on the new findings, the team urges an effort to create a draft of the colugos genome. "Colugos are going to be a much more important species to study now that we know their relationship to primates," Miller said.

For selfish reasons

Mary Silcox, an anthropologist at the University of Winnipeg in Canada who was a co-author on the PNAS study had an open mind about the new findings and the final word on the evolutionary relationship between the three Euarchonta groups.

"Even though it's in conflict with our morphological findings that we've published, I'm not totally closed to the idea that we have the branching pattern among the three Eurochonta groups wrong," Silcox told LiveScience.

She added that a genome of colugos would be "spectacularly wonderful," but that the genome of the most ancient living tree shrew, Ptilocercus lowii, is also needed.

Comparing the full genomes of members from all three groups would allow scientists to chart the evolutionary relationships among them with much more confidence, Silcox said.

Why is knowledge of these relationships important? For one, it will help to answer the question of the origins of our own species, Silcox said. "To some extent, to understand where we came from, we need to put that in a larger context of mammalian evolution," she said.

But Silcox also chalks it up partly to vanity. "I think it's because humans are remarkably self-centered animals," she said. "It's because we're more interested in our evolution than we are the evolution of slime molds that we tend to focus on this stuff."