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Evolution of Mammalian Brains

992 BioScience • November 2001 / Vol. 51 No. 11 We know what distinguishes us from the rest of the animal kingdom: our brains. The increase in brain capacity throughout hominid evolution brought with it language, complex social structure, creativity, curiosity, and the capacity to reason, among other things. So it’s only human nature to wonder how we got where we are. As scientists continue to probe the dramatic transformations mammalian brains have undergone, the types of analyses they perform also evolve. A recent study by Sam Wang, Damon Clark, and Partha Mitra (Nature, 11 May 2001) is a thought-provoking reanalysis of 20year-old data that have been reanalyzed before. The original data are volumetric measurements of the five main parts of the brain (telencephalon, diencephalon, mesencephalon, medulla, cerebellum) for extant representatives of several groups—insectivores (28 species), tree shrews (three species), and primates (44 species). What emerges from the recent reanalysis is a comparison of relative changes in brain architecture, independent of a species’ absolute size. To compare the structure of brains from animals of disparate size, Wang and his colleagues used the total brain volume for each species as a reference, arguing that it is more appropriate than body size, which can be variably influenced by other factors such as diet and environment. Each brain component for a species was expressed as a volume fraction of that species’ total brain volume. Wang and company dubbed the set of volume fractions for each species its cerebrotype. The dominant structure in human brains is the telencephalon, which comprises the two cerebral hemispheres and is the locus of higher-order, informationprocessing functions. As might be expected, the volume fraction of telencephalon increases from 60% and 61% in insectivores and tree shrews, respectively, to 74% in primates, primarily at the expense of diencephalon, mesencephalon, and medulla. By contrast, the volume fraction of cerebellum, which was previously thought to vary with the telencephalon, remains relatively constant: 13.2%, 12.7%, and 12.4% in insectivores, tree shrews, and primates, respectively. Functionally, the cerebellum coordinates motor activity with sensory information. The authors expanded the study to members of 19 mammalian taxa (15 orders and four primate suborders) to test how consistent the cerebellar fraction is generally, and it held up. Interestingly, the two orders for which the cerebellar fraction is significantly higher than for other mammals are Microchiroptera (microbats) and Cetacea (whales and dolphins), whose members use echolocation to gather information about their surroundings. Those bats that do not echolocate (Megachiroptera) have cerebellar fractions resembling other mammals, indicating the enlarged cerebellar fraction of microbats is probably tied to their sophisticated sensory adaptation. The same whole-brain approach was used to study evolutionary changes within the telencephalon. Wang and coauthors normalized volume fractions of the seven subcomponents (neocortex, hippocampus, schizocortex, septum, piriform cortex, olfactory bulb, striatum) to the total volume fraction of telencephalon for each species. The volume fraction of neocortex climbs from 28% in insectivores to 55% in tree shrews to 81% in primates (95% in Homo sapiens). These increases were shown to come at the expense of the other subcomponents, except the striatum, which was relatively constant: 8%, 8%, and 6% in insectivores, tree shrews, and primates, respectively. The striatum functions in motor activity and forms extensive connections between parts of the brain, including much of the telencephalon, which may be why its relative size is maintained throughout mammalian evolution. Wang and coauthors map the relationships between cerebrotypes several ways, and from these analyses they conclude that mammals can be grouped taxonomically according to brain structure. Cerebrotypes are clumped within taxa, and they are clearly distinct between taxa. For example, cerebrotype comparisons show tree shrews to be distinct from primates and insectivores; at a finer scale, the taxonomic subgroups within the larger groupings of insectivores and primates have distinct cerebrotypes as well. The few overlaps that are found, between certain New World monkeys (four species) and Old World monkeys, for example, can be attributed to other similarities, such as social organization, between the groups. These findings suggest mammalian brains are scalable structures, meaning the relative proportions of brain components are constrained within taxonomic groups, even when absolute brain sizes vary considerably. They also indicate that major shifts in brain architecture accompany the appearance of new taxa. Of course, the next thing we’ll want to know is what underlying factors determine the cerebrotype, so we can understand more precisely how we came to be.