Why do mammals — including the human kind — have bigger brains than other animals, relative to the size of their bodies? The answer may lie in the least appreciated of the five senses.

A team of paleontologists from San Antonio, Austin and Pittsburgh think that the earliest mammals grew larger, more complex brains in order to accommodate a heightened sense of smell.

It's not as strange as it sounds. Mammals evolved from tiny, shrewlike animals that navigated burrows and dense underbrush where eyes and ears were of limited use. Smell was vital for finding food, detecting threats — and for mating.

“The single most important thing that was driving the origin of mammals would be their sense of smell,” said Tim Rowe, professor of geosciences at the University of Texas at Austin. “It could smell more different kinds of things, and it probably had greater sensitivity so it could smell them in lower concentrations.”

The findings were published today in the journal Science.

“We wanted to know, what was the driving force towards the increase in brain size in mammals?” said Thomas Macrini, now an assistant professor of biological sciences at St. Mary's University, who did much of the research for his doctoral dissertation at UT-Austin. “This is a question that paleontologists have been interested in for probably 100 years.”

Using a high-resolution CT scanner, Macrini scanned the skulls of a number of species, both living and extinct. The most important of those were rare, intact skulls of tiny pre-mammalian animals, Morganucodon and Hadrocodium, found in China and dating back almost 200 million years to the early Jurassic Period.

He was able to make fairly accurate digital models of the brain's shape and features from the cavity of the fossilized skull. The researchers then could compare those with the skulls and brains of others.

After studying those scans, they speculate that those early mammalian brains evolved in three major stages: first by improved sense of smell, then by touch — particularly through the movement of fur — and finally improved muscle control and coordination. The importance of smell was determined by an enlarged nasal cavity and the parts of the brain that process smell.

As for the importance of touch, it was found in changes to the neocortex of the brain.

“If you can feel a bug crawling across your skin, it's mostly because it's deflecting your hair,” Rowe said. “Animals that have a lot of fur use that in navigation, so they know when they are brushing up against something. It helps you to then react, to shake the bug off or be more precise in your movements so you don't bump into something and create a noise. So this sensory motor system began to improve. And it's pretty darn good in mammals.”

The Hadrocodium, which weighs two grams and was roughly the size of a paper clip, was discovered by co-author Zhe-Xi Luo, now with the Carnegie Museum of Natural History in Pittsburgh.

“I have spent years studying these fossils, but until they were CT scanned it was impossible to see the internal details unless you were willing to destroy the skulls to look inside,” Luo said. “I was absolutely thrilled to see the shape of the brain of our 190-million-year-old relatives.”