Humans like to think of themselves as uniquely well endowed when it comes to gray matter. Our brains are, after all, quite large relative to our body size. But a new study, published today in the journal Nature, indicates that it may be the relative sizes of different brain partsnot the entire brainthat set us apart from our mammalian kin. In fact, each mammal group appears to have its own unique brain structure.

To investigate brain evolution, Sam Wang of Princeton University and his colleagues turned to a database compiled 20 years ago by German researchers who had examined the brains of some 300 animals. Comparing the sizes of 11 different brain regions to total brain size, the team identified a number of different "cerebrotypes" that characterize the different mammal groups. Whales and dolphins, for example, have larger cerebellums that do most mammals, perhaps because these creatures rely heavily on sonar. Similarly, whereas the brains of most insect eaters devote only 16 percent to the neocortex, which is known to control social interaction and other complex cognitive tasks, the human neocortex takes up about 80 percent of the brain.

Perhaps not surprisingly, the team found that animals with the most similar cerebrotypes are in fact the most closely related evolutionarily. Shifts in cerebrotype, they note, emerged with the appearance of new groups. Furthermore, the findings also suggest that, considering the limited ways in which the sizes of the various brain areas shifted over time, only relatively few genes must direct the development of the brain's structure. "It provides a little insight," Wang says of the study, "into who we are and how we got here."