Humans, Anatomical adaptations

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Summary

Modern humans (Homo sapiens or Homo sapiens sapiens) are the only extant members of the hominin clade, a branch of great apes characterized by erect posture and bipedal locomotion; manual dexterity and increased tool use; and a general trend toward larger, more complex brains and societies. Early hominids, such as the australopithecines who had more apelike brains and skulls, are less often thought of or referred to as "human" than hominids of the genus Homo some of whom used fire, occupied much of Eurasia, and gave rise to [[anatomically modern humans|anatomic

Details

Human evolution is characterized by a number of morphological, developmental, physiological, and behavioral changes that have taken place since the split between the last common ancestor of humans and chimpanzees. The most significant of these adaptations are 1. bipedalism, 2. increased brain size, 3. lengthened ontogeny (gestation and infancy), 4. decreased sexual dimorphism. The relationship between all these changes is the subject of ongoing debate. Other significant morphological changes included the evolution of a power and precision grip, a change first occurring in H. erectus.

Bipedalism is the basic adaption of the hominin line, and it is considered the main cause behind a suite of skeletal changes shared by all bipedal hominins. The earliest bipedal hominin is considered to be either Sahelanthropus or Orrorin, with Ardipithecus, a full bipedal, coming somewhat later. The knuckle walkers, the gorilla and chimpanzee, diverged around the same time, and either Sahelanthropus or Orrorin may be humans' last shared ancestor with those animals. The early bipedals eventually evolved into the australopithecines and later the genus Homo. There are several theories of the adaptational value of bipedalism. It is possible that bipedalism was favored because it freed up the hands for reaching and carrying food, because it saved energy during locomotion, because it enabled long distance running and hunting, or as a strategy for avoiding hyperthermia by reducing the surface exposed to direct sun.

The human species developed a much larger brain than that of other primates – typically 1,330 cc in modern humans, over twice the size of that of a chimpanzee or gorilla. The pattern of encephalization started with Homo habilis which at approximately 600 cc had a brain slightly larger than chimpanzees, and continued with Homo erectus (800–1100 cc), and reached a maximum in Neanderthals with an average size of 1200-1900cc, larger even than Homo sapiens (but less encephalized). The pattern of human postnatal brain growth differs from that of other apes (heterochrony), and allows for extended periods of social learning and language acquisition in juvenile humans. However, the differences between the structure of human brains and those of other apes may be even more significant than differences in size. The increase in volume over time has affected different areas within the brain unequally – the temporal lobes, which contain centers for language processing have increased disproportionately, as has the prefrontal cortex which has been related to complex decision making and moderating social behavior. Encephalization has been tied to an increasing emphasis on meat in the diet, or with the development of cooking, and it has been proposed that intelligence increased as a response to an increased necessity for solving social problems as human society became more complex.

The reduced degree of sexual dimorphism is primarily visible in the reduction of the male canine tooth relative to other ape species (except gibbons). Another important physiological change related to sexuality in humans was the evolution of hidden estrus. Humans are the only ape in which the female is fertile year round, and in which no special signals of fertility are produced by the body (such as genital swelling during estrus). Nonetheless humans retain a degree of sexual dimorphism in the distribution of body hair and subcutaneous fat, and in the overall size, males being around 25% larger than females. These changes taken together have been interpreted as a result of an increased emphasis on pair bonding as a possible solution to the requirement for increased parental investment due to the prolonged infancy of offspring.

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External Links

  • WikipediaArchaeology InfoHomo sapiens

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