Compared with other mammals, primates—from lemurs to humans—have huge brains. But scientists still don't know exactly why—or even when—our brains ballooned. A new study, published yesterday in Proceedings of the National Academy of Sciences, postulates that even without big brains, early primates were able to do a lot of primatelike things—a finding that calls into question many of the prevailing evolutionary theories.

"At the beginning, we didn't have an exceptionally large brain," says Jonathan Bloch, an associate curator of vertebrate paleontology at the University of Florida's Florida Museum of Natural History and co-author of the paper, talking about primates in general. To explain the expansion in primates' brains, researchers have put forth various possible mechanisms. Tree dwelling, for instance, may have required more brainpower to coordinate muscles and joints for life off the ground. Changes in diet, too, such as consuming fruits instead of leaves, could have led to more calories being available for brain development.

Bloch and his colleagues, however, propose that the owner of the rare, 54-million-year-old skull, an Ignacius graybullianus from an extinct side branch of the primate family tree, was already doing these things with a decidedly nonprimate brain. Not only was the I. graybullianus's brain about two thirds the relative size of those of the smallest modern primates, "it doesn't look anything like a primate's brain," says Richard Kay, a professor of evolutionary anthropology at Duke University who wasn't involved in the study but was first to describe this particular skull (in 1989). If forced to come up with a modern-day comparison, he says, "I'd be looking at a hedgehog."

Bloch and his team used high-resolution CT scans to image the brain cavity. From there, they were able to create a full model of the brain using a three-dimensional printer. "To see a whole 3-D construction is kind of unbelievable," Bloch says.

One of the big differences, Bloch and the other authors noted, was the large olfactory area of the brain along with the smaller parts of the temporal lobe, which helps with visual processing. "They were smell-first animals, rather than sight-first," unlike today's primates, Bloch says.

Although the I. graybullianus was on its way to developing a differentiated brain, Bloch notes, a truly recognizable primate brain probably didn't emerge until the Eocene, 55 million to 34 million years ago.

The new findings establish "a serious baseline," says Kay, who notes that until now there wasn't a reliable starting point for primate brain evolution, leaving researchers to draw correlations between modern-day physiognomy and behavior. But, Kay says, "you can't construct the pattern of evolution without knowing what was going on in the past."

Mary Silcox, an associate professor of Anthropology at the University of Winnipeg, was the lead author of the study.