The unusually large brains of mammals apparently didn't evolve so that we could ponder philosophy _ but so we could sniff our way to success. A new analysis of some of the earliest mammals and mammal-like creatures shows their complex brains evolved in stages, starting with the regions that handle the sense of smell.
The tiny creatures that evolved into today's mammals "exploited a world of information dominated to an unprecedented degree by odors and scents," report researchers led by Timothy B. Rowe, a paleontologist at the University of Texas.
"If I had to tell a freshman class what it means to become a mammal, it means to become a superb smeller," Rowe said in a telephone interview.
Enlargement of the brain's smell-sensing region was followed by upgrades in the brain areas that deal with touch sensitivity from body hair, and then parts providing improved movement, Rowe and colleagues report in Friday's edition of the journal Science.
Among mammals, today's humans have traded away some of that ability to smell for improved vision and hearing, Rowe observed, but we still have close companions called dogs who heavily exploit the sense of smell.
As the mammal brain evolved, the area involved in sensory response "underwent particularly spectacular development," said R. Glenn Northcutt of the Scripps Institution of Oceanography, who was not part of Rowe's team. The studies "provide the first solid evidence of the stages of mammalian brain evolution," Northcutt said.
The report is "very significant because it outlines, for the first time, the evolutionary history of major brain regions in the closest relatives of mammals, and early mammals," said Hans-Dieter Sues, curator of vertebrate paleontology at the Smithsonian's National Museum of Natural History.
Scientists think these tiny creatures were active at dusk or during the night, said Sues, who was not part of the research team. "Thus, smell and tactile senses, but also improved hearing, would have been really important to these animals."
Mammals are warm-blooded animals with backbones, and with females that have milk-secreting organs to feed their young. They include humans, apes, many four-legged animals, whales, dolphins and bats.
Rowe studied the brains of several examples of Morganucodon and Hadrocodium using CT scans to produce images of the inside of the skulls of the animals. The creatures lived about 190 million years ago when mammals were just beginning to evolve. Morganucodon, which weighed in at less than an ounce, and the even smaller Hadrocodium, existed in a transition period between pre-mammals and the earliest mammals.
Studies of their teeth and jaws indicated these animals were eating insects, worms and grubs, Rowe said. Improved sense of smell could help them find food. Improved sensitivity to things touching their body hair helped the creatures sense their environment, when they were scurrying under leaves for example. It also would have allowed them to be aware of parasites on their bodies.
Eventually mammals developed complex brains several times larger, relative to body size, than their ancestors, and the researchers were interested in how that process began and proceeded.
What they are learning could one day lead to construction of machines or robots with the ability to smell, which could be valuable in security situations, for product inspections and other uses, Rowe said. Every person, he pointed out, has an individual odor.
Rowe's co-author, Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh, participated in the discovery of the fossils for this study and first described the paper clip-sized Hadrocodium 10 years ago.
"I have spent years studying these fossils, but until they were scanned it was impossible to see the internal details," Luo said in a statement. "I was absolutely thrilled to see what the brains of our 190 million-year-old relatives were like."