Brains show evolutionary designs.Mammalian brains come in a variety of shapes, sizes, and organizations. Just how these neural disparities evolved remains mysterious. Damon A. Clark of Princeton University Princeton University, at Princeton, N.J.; coeducational; chartered 1746, opened 1747, rechartered 1748, called the College of New Jersey until 1896. Schools and Research Facilities 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 Years, The the seven decades of Eleanor Pargiter’s life. [Br. Lit.: Benét, 1109] See : Time researchers propose, the brain's composition undergoes sufficient remodeling remodeling /re·mod·el·ing/ (re-mod´el-ing) reorganization or renovation of an old structure. bone 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, mole, hedgehog, tenrec, and solenodon. , 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. The cortex of the frontal lobe of the cerebral hemisphere. Also called frontal area, prefrontal area. Frontal cortex turns up in prosimians (lemurs, lorises, and tarsiers). 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, 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 the·o·rize v. the·o·rized, the·o·riz·ing, the·o·riz·es v.intr. To formulate theories or a theory; speculate. v.tr. To propose a theory about. . Although brain composition differs considerably across mammalian species, the proportional size of the cerebellum cerebellum (sĕr'əbĕl`əm), portion of the brain that coordinates movements of voluntary (skeletal) muscles. It contains about half of the brain's neurons, but these particular nerve cells are so small that the cerebellum accounts for 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 echolocation Physiological process for locating distant or invisible objects (such as prey) by emitting sound waves that are reflected back to the emitter by the objects. Echolocation is used by an animal to orient itself, avoid obstacles, find food, and interact socially. . |
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