Brains show evolutionary designs.Mammalian brains A computer's "brains" are its central processing unit. See CPU. come in a variety of shapes, sizes, and organizations. Just how these neural disparities evolved remains mysterious. Damon A. Clark of Princeton University and his colleagues are trying a new approach to this puzzle by comparing the proportions of nearly a dozen brain structures among many mammalian species. Like other related groups of species, mammals all share a basic arrangement of these structures. But in each species, these structures, as well as the overall brain, evolved to have specific sizes, the scientists report in the May 10 NATURE. About every 10 million years, the researchers propose, the brain's composition undergoes sufficient remodeling to account for the arrival of dramatically new types of mammals. Clark's team analyzed data, published 20 years ago by other researchers, on the volumes of various brain structures in insectivore insectivore (ĭnsĕk`təvōr'), term broadly given to any insect-eating animal or plant. More specifically, the term refers to mammals of the order Insectivora (see Chordata), including the shrew shrew, common name for the small, insectivorous mammals of the family Soricidae, related to the moles. Shrews include the smallest mammals; the smallest shrews are under 2 in. (5.1 cm) long, excluding the tail, and the largest are about 6 in. (15 cm) long. Light-boned and fragile, shrews have mouselike bodies and long, pointed snouts with tiny, sharp teeth., mole, hedgehog, tenrec, and solenodon. Insectivores are small animals, ranging from 2 to 16 in., bat, and primate species. The team compared the sizes of 11 brain regions with total brain size for each species. The researchers found, for example, that the fraction of the brain occupied by the telencephalon--which includes the cortex--reaches 28 percent in insectivores, 55 percent in tree shrews, 81 percent in primates, and 95 percent in people. Major primate groups exhibit differences in brain design that reflect their presumed evolutionary histories, Clark and his coworkers contend. For instance, the relatively smallest frontal cortex frontal cortex n. turns up in prosimians prosimian: see primate. (lemurs lemur (lē`mər), name for prosimians, or lower primates, of two related families, found only on Madagascar and adjacent islands. Lemurs have monkeylike bodies and limbs, and most have bushy tails about as long as the body. They have pointed muzzles and large eyes., lorises, and tarsiers tarsier (tär`sēər), small, nocturnal, forest-dwelling prosimian primate, genus Tarsius. There are at least three species found in the Philippines, in Sumatra and Borneo, and in Sulawesi. Tarsiers are about 6 in. (15 cm) long with a 10 in. (25 cm) hairless tail, and weigh about 4.5 oz (130 g).). Its relative size is progresively larger in New World monkeys (such as squirrel monkeys), Old World monkeys (such as baboons), and finally great apes (chimpanzees, gorillas, orangutans orangutan (ōrăng` The cortex of the frontal lobe of the cerebral hemisphere. Also called frontal area, prefrontal area.  tăn), an ape, Pongo pygmaeus, found in swampy coastal forests of Borneo and Sumatra. Highly specialized for arboreal life, it usually travels by grasping branches with hands and feet and moving from tree to tree., and humans). New World and Old World monkeys display some overlap in brain design. What's more, several New World species lead social lives as complex as those of many Old World species, suggesting that the brains of the two groups evolved in similar ways, the researchers theorize. Although brain composition differs considerably across mammalian species, the proportional size of the cerebellum cer·e·bel·lums or cer·e·bel·la (-b  l remains largely the same. The cerebellum, which is located at the brain's base, participates in too many brain functions to be trimmed back in evolutionary changes, Clark's team argues. Much research implicates this brain region in the control of muscles and balance. An unusually large cerebellum occurs only in species such as dolphins and bats, that move about using echolocation. |
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