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Canadian researcher helps put humans on the tree of life

International effort traces placental mammals back to a scampering common ancestor that appeared after extinction of dinosaurs

University of Toronto Scarborough

A University of Toronto Scarborough researcher was part of a team that reconstructed the family tree of placental mammals - a diverse group that includes cats, dogs, horses and humans. The research traces placental mammals back to a small, scampering, insect-eating creature that got its start 200,000 years or more after the extinction of the dinosaurs.

The work is featured in this week's Science magazine.

Mary Silcox, assistant professor of anthropology at UTSC, is a co-author on the paper and the only Canadian member of the team. She was responsible for organizing the dental traits used in the analysis and contributed to the collection of the data that were used to classify primates, including humans.

"We were responsible for putting humans in the tree of life," she says, referring to her work with Drs. Jonathan Bloch of the Florida Museum of Natural History and Eric Sargis of Yale University.

The research team used the world's largest dataset combining genetic and physical traits to reconstruct the placental mammal tree of life. A major finding is that placental mammals diversified much later than previous theories had suggested, with all of the major groups alive today originating after the extinction of the dinosaurs. Genetic evidence alone had suggested that placental mammals were already a diverse group in the Late Cretaceous period, before the event that drove the dinosaurs and 70 percent of other then-existing species extinct.

But by adding evidence from fossils, the team concluded that placental mammals arose a few hundred thousand years after the extinction event.

To carry out the study, the researchers built a database that recorded physical - or phenomic - traits for 86 placental mammal species, including 40 species that are extinct and known only from fossils. More than 4,500 traits, including the presence or absence of wings, teeth, and certain bone types, were recorded in the database and used to construct the tree of life, in combination with genetic data.

The phenomic dataset is 10 times larger than those that had previously been used to study mammal relationships, is publicly available online, and illustrated with over 12,000 images.

Silcox says that the work will serve as a model for future projects that will give us a better idea of how species evolved and are related to one another.


The research was funded by the National Science Foundation, and led by researchers at the American Museum of Natural History and other institutions from the US, Canada, and elsewhere.

Mary Silcox
Assistant Professor, Department of Anthropology
University of Toronto Scarborough
(416) 208-5132
Kurt Kleiner
Department of Communications and Public Affairs
University of Toronto Scarborough
(416) 287-7008

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