Descent of Man
In this lecture, beginners can familiarize themselves with basic information and terms used to describe the evolution of humanity beginning with the origin of primates through the comings and goings of Genus Homo.
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Evolution of human bipedalism
Posted: Tuesday, December 12, 1995
The causes of the evolutionary change from quadrapedalism to bipedalism in early humans have been unknown for many years. Early studies showed that bipedalistic movement was much less energetically efficient than moving on all fours at identical speeds. This idea had many anthropologists question why man moved to an upright posture. More recent studies, however, have proven that walking on all four limbs is less efficient than using only two(Leonard & Robertson). The adaptation of upright movement in early humans had great advantages for energy efficiency and for feeding .
Geological evidence shows that climate changes during the Miocene period probably thinned out forests in East Africa. The rich tropical forests became open savannah with trees scattered in clumps throughout. Some of the primates in this changing climate stayed in the trees, and the members of this group became the ancestors of the gibbons and siamangs. Many hominoid species, however, left their arboreal homes for the ground in order to forage(Jennings and Hoebel). These species, the early humans, evolved to be able to forage with both hands while moving, and this evolutionary change led to upright movement(Jolly and White). Even though this theory explains why hominids began walking on two legs, it does not explain how. Australopithecus, the first primate with upright bipedal locomotion, lacked many of the morphological adaptations necessary for efficient bipedal movement. The pelvis, sacrum, and femur of the australopithecine resemble those of modern humans, but it also shows many arboreal characteristics, such as long, slender appendices and a funnel shaped rib cage. As time passed, Australopithecus adapted to life on the ground, and began to move more and more upright, until the rise of the genus Homo.
The efficiency of bipedalism has been questioned, and the evolutionary causes of bipedalism have been unknown for many years. Until recently, increased efficiency in locomotion had been accepted as one theory as to why man moved to bipedalism. Few studies have examined the energetic costs of human upright movement in an evolutionary context. Studies conducted by Taylor, Schmidt-Nelsen, and Raab in 1970 showed that human running was much less energetically efficient than moving quadrupedally at the same speed. Certain primate species, such as chimpanzees and cebus monkeys, show no difference in energy consumption moving on four legs or two(Jennings and Hoebel). Taylor, Schmidt-Nelsen, and Raab concluded from these results that the evolution of human bipedality was not due to increased locomotor efficiency. These data were wrong, however, and more recent studies have shown that human walking is more efficient than quadrupedal movement in chimpanzees(Leonard & Robertson).
As the vegetation in Africa became more widely spread, the daily distance traveled by early humans increased. The amount of energy spent traveling, therefore, increased greatly, from 500 kilojoules a day to nearly 3000 kJ. A fifteen percent increase in locomotor efficiency would save the average hominid almost 500 kJ a day(Leonard & Robertson), so increasing efficiency in locomotion would have been a great advantage to early humans. The actual change in energy used due to moving bipedally as opposed to moving quadrupedally is great. The average male mammalian biped moving between 2.4 and 6.0 kilometers an hour saves between 100 and 500 kJ by moving bipedally, the average female saves between 100 and 600 kJ. Compared to the energy used in primates for locomotion, bipedal walking is almost half as expensive(Leonard & Robertson). Bipedal movement in humans is more energetically efficient than quadrupedal movement in mammals and primates, and it can be implied that Australopithecus found a great advantage to walking.
As early humans began to travel greater distances in order to find food, energy consumption and conservation became more important issues. The ability to move bipedally allowed the australopithecine to have their hands free. The hands could therefore be used to forage on the move, giving Australopithecus a greater reach. Free hands also allowed for the ability to use tools. The energy saved by moving upright also allowed for increased traveling time, and enabled the early humans to increase the daily distance traveled. In general, bipedal movement had great benefits for the early humans, and because of this, man evolved to a fully upright animal.
Leonard, William R., and Marcia L. Robertson. "Rethinking the Energetics of
Bipedality." Current Anthropology 38.2 (1997): 304-309.
Jennings, and Hoebel. Readings in Anthropology. New York: McGraw-Hill, 1972.
Jolly, Clifford, and Randall White. Physical Anthropology and Archeology. New York:
McGraw Hill, 1995.
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