 | Recently discovered skeletons of plesiadapiform species, including Ignacius clarkforkensis, depicted here, show that the ancient creatures were primitive primates, according to a new study. |
A Yale researcher was part of a team of scientists that has shed new light on the origins and earliest stages of primate evolution, showing that the roots of the primate family tree are 10 million years older than previously estimated.
The study, published on the cover of the Jan. 23 edition of the Proceedings of the National Academy of Sciences, offers evidence that a group of archaic mammals called plesiadapiforms are more closely related to modern primates than to flying lemurs, as had previously been proposed.
The research was led by University of Florida paleontologist Jonathan Bloch. The team included Eric Sargis, associate professor of anthropology at Yale, as well as scientists from the University of Winnipeg and Stony Brook University.
The team reconstructed the base of the primate family tree by comparing skeletal and fossil specimens representing more than 85 modern and extinct species. The team also discovered two 56-million-year-old fossils, the most primitive primate skeleton ever described.
The scientists analyzed 173 characteristics of modern primates, tree shrews and flying lemurs with plesiadapiform skeletons to determine their evolutionary relationships. High-resolution CT scanning made fine resolution of inaccessible structures inside the skulls possible.
"This is the first study to bring it all together," says Sargis, who is also assistant curator of vertebrate zoology at Yale's Peabody Museum of Natural History. "The extensive dataset, the number and type of characteristics we were able to compare, and the availability of full skeletons let us test far more than any previous study."
At least five major features characterize modern primates: relatively large brains, enhanced vision and eyes that face forward, a specialized ability to leap, nails instead of claws on at least the first toes, and specialized grasping hands and feet. Plesiadapiforms have some but not all of these traits. The article argues that these early primates may have acquired the traits over 10 million years in incremental changes to exploit their environment.
While the study did not include a molecular evaluation of the samples, according to Sargis, these results are consistent with molecular studies on related living groups. Compatibility with the independent molecular data increases the researchers' confidence in their own results, he says.
Bloch discovered the new plesiadapiform species, Ignacius clarkforkensis and Dryomomys szalayi, just outside Yellowstone National Park in the Bighorn Basin with co-author Doug Boyer, a graduate student in anatomical sciences at Stony Brook. Previously, based on the skulls and isolated bones that had been discovered, some scientists proposed that Ignacius was a gliding mammal related to flying lemurs, while others hypothesized that it was the ancestor of modern humans -- an idea that prompted much debate within the scientific community.
However, analysis of a more complete and well-preserved skeleton by Bloch and his team offers strong phylogenetic evidence supporting the connection between plesiadapiforms and primates.
"These fossil finds from Wyoming show that our earliest primate ancestors were the size of a mouse, ate fruit and lived in the trees," says Bloch, a vertebrate paleontology curator at the Florida Museum of Natural History. "It is remarkable to think we are still discovering new fossil species in an area studied by paleontologists for over 100 years."
This study places the origins of plesiadapiforms in the Paleocene, about 65 million to 55 million years ago, in the period between the extinction of the dinosaurs and the first appearance of a number of undisputed members of the modern orders of mammals.
"Plesiadapiforms have long been one of the most controversial groups in mammalian phylogeny," says Michael J. Novacek, curator of paleontology at the American Museum of Natural History. "First, they are somewhere near primates and us. Second, historically they have offered tantalizing, but very often incomplete, fossil evidence. But the specimens in their study are beautifully and spectacularly preserved."
Co-author Mary Silcox, professor of anthropology at the University of Winnipeg, notes, "The results of this study suggest that plesiadapiforms are the critical taxa to study in understanding the earliest phases of human evolution. As such, they should be of very broad interest to biologists, paleontologists and anthropologists."
The research was supported by grants from the National Science Foundation, Field Museum of Natural History, Yale University, Sigma Xi Scientific Research Society, Natural Sciences and Engineering Research Council (Canada), University of Winnipeg, the Paleobiological Fund and The Wenner-Gren Foundation for Anthropological Research.
-- By Janet Rettig Emanuel
T H I S
Team casts new light on
roots of primate family tree
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