54-million-year-old skull reveals surprises about early evolution of primate brains

by Bill Kanapaux

Researchers at the Florida Museum of Natural History and the University of Winnipeg have developed the first detailed images of a primitive primate brain, unexpectedly revealing that cousins of our earliest ancestors relied on smell more than sight.

The analysis of a well-preserved skull from 54 million years ago contradicts some common assumptions about brain structure and evolution in the first primates. The study, also narrows the possibilities for what caused primates to evolve larger brain sizes.

The skull belongs to a group of primitive primates known as Plesiadapiforms, which evolved in the 10 million years between the extinction of the dinosaurs and the first traceable ancestors of modern primates.

This image shows a translucent rendering of the brain as it would fit inside the skull of the 54-million-year-old primitive primate, Ignacius graybullianus.

The 1.5-inch-long skull was found fully intact, allowing researchers to make the first virtual mold of a primitive primate brain.

"Most explanations on the evolution of primate brains are based on data from living primates," said lead author Mary Silcox, an anthropologist at the University of Winnipeg and research associate at the Florida Museum. "There have been all these inferences about what the brains of the earliest primates would look like, and it turns out that most of those inferences are wrong."

Researchers used CT scans to take more than 1,200 cross-sectional X-ray images of the skull, which were combined into a 3-D model of the brain.

"A large and complex brain has long been regarded as one of the major steps that sets primates apart from the rest of mammals," said Florida Museum vertebrate paleontologist and study co-author Jonathan Bloch. "At our very humble beginnings, we weren't so special. That happened over tens of millions of years."

The animal, Ignacius graybullianus, represents a side branch on the primate tree of life, Bloch said. "You can think of it as a cousin of the main line lineage that would have given rise ultimately to us."

Nose first

In previous research, Bloch and Silcox established that Plesiadapiforms were transitional species. Ignacius was similar to modern primates in terms of its diet and tree-dwelling but did not leap from tree to tree like modern fast-moving primates.

In many ways, the early primate behaved like living primates but with a brain that was one-half to two-thirds the size of the smallest modern primates. This means factors such as tree-dwelling and fruit-eating can be eliminated as potential causes for primates evolving larger brain sizes, Silcox said, because "the smaller-brained Ignacius was already doing those things."

The mould suggests a "startling combination" of features in the early primate that requires a re-thinking of primate brain evolution, said Florida State University anthropologist Dean Falk, who was not involved in the study.

"Hypotheses about early primate brain evolution often link keen smell with nocturnal insect-eating, and a more recently evolved increase in visual processing with fruit-eating in arboreal habitats," Falk said.

The move to larger brain size occurred during an evolutionary burst that happened 10 million years after the extinction of the dinosaurs. At that point, visual features in the brain became more prominent while the olfactory bulbs became proportionately smaller.

More than likely, Bloch said, this change in brain structure and size was related to primates living in closed canopy forests that brought trees closer together and allowed for more leaping. But answering that will require the discovery and analysis of new fossils.

Changes in brain size and structure in the early stages of primate evolution have generated enormous debates for decades. But until now, fossil evidence has been lacking. Many models of the ancestral primate brain are based on tree shrews, which come from Southeast Asia and are distantly related to humans. But with some 70 million years of evolution between them and humans, "it turns out tree shrew brains are not a good model," Silcox said.

The early primate's brain had relatively large olfactory bulbs, indicating the animal relied on smell rather than sight. "Being visually directed is one of the things that is a primate characteristic, but we can tell from the brain that's not something that came in right at the base of primates but evolved later," Silcox said.

Detailed picture

The fossil record provides the best test of inferences about brain evolution, but until recently, fossil evidence for primates has been mostly limited to teeth and fragmented jaws. But in past two decades, limestone deposits in Wyoming have yielded well-preserved skeletons and skulls.

The CT scan technique has become an accepted method for fossil studies but most often involves medical scanners that are re-purposed for fossils.

By contrast, the primate brain study used a more powerful industrial-strength high-resolution scanner at Penn State University.

Claire Dalmyn, an undergraduate student at the University of Winnipeg and paper co-author, traced more than 800 X-ray images of the braincase. The effort took nearly a year and produced one of the best endocasts they had ever seen for an extinct mammal.

"I couldn't believe what we were seeing," Bloch said.

Primate brains tend to be dominated by cerebrum, the most highly evolved part of the brain, making it difficult to look at functional areas and forcing researchers to focus solely on brain size.

"Brain size is interesting, but it's quite difficult to interpret because in one brain, 50 percent could be made up of olfactory bulbs, and in another brain, 50 percent could be made up of visual processing areas," Silcox said.

In the primitive primate, the cerebrum had not yet evolved to the point where it covered all of the other functional regions of the brain.

As a result, the relative sizes of different parts of the brain provide a better picture of brain function and the early stages of primate evolution.

The specimen came from north central Wyoming, near the entrance to Yellowstone National Park. The intact skull is a rare find that allowed the primate to be studied this way for the first time, Silcox said.

"This is very exciting in terms of the history of paleo-neurology, the history of the study of brains in the fossil record," Silcox said.

Courtesy: Natural History