million years ago. Finally, about 4 to 7 million years ago, humans parted from chimpanzees, which are our closest modern relative.
HUMAN STRUCTURAL CHANGES There have been several changes that have occurred throughout evolution to lead to modern man. Bipedalism was the key evolutionary adaptation that led to major skeletal changes. The first bipedal species was Sahelanthropus or Orrorin, which both arose about 6 million years ago. These were believed to have primitive forms of bipedalism. The genus Ardipithecus was fully bipedal around 5.6 million years ago. What was the value of bipedalism? Some believe that it better enabled long distance running, enhanced our field of vision, kept the hands free for tools and food carrying, and avoided hyperthermia through decreased sun exposure on the back. It uses less energy than quadrupedal walking. The arms shortened and the legs lengthened over time and the big toe began to be lost as a grasping object. The main changes to the skeletal system was to the legs and pelvis. Other changes involved those to the vertebral column, which gained a new shape, change in alignment of the big toe so it would help in moving forward, and shortening of the arms for easier running. The pelvis changed the most, with the disadvantage of narrowing the birth passage. This has made the birth process more difficult and more complex, and limited the brain size. It shortened the gestation time and made human babies ambulatory at a later age. Menopause evolved and the menstrual cycle changed so that it led to the advantage of women to help raise their grandchildren rather than continue to raise their own children. Girls arrive at menarche at a later age. The human brain is greater than that of other primates. Humans have about a 1330-milliliter brain size, which is three times that of gorillas and chimpanzees. Ardipithecus and Australopithecus had stable and relatively small brain sizes, while greater brain size was first seen in Homo habilis. Neanderthals had the largest brain size; it has fallen since that time. Along with this has been an increase in brain growth after birth so that there could be social learning and the acquisition of language in young humans. The brain structure has changed, even as the size has decreased. The temporal lobes, responsible for language, have increased as has the prefrontal cortex, which handles decisionmaking and social behavior. There have been changes in the overall morphology of the skull,
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