Bidepedalism, or upright walking, is a key characteristic of human beings. There is general agreement that the evolution of bipedalism is the benchmark adaptation which set hominins on the path away from ape and toward human (Lovejoy et al, 2009; Schrenk, 2011; Lieberman, 2012).
The common ancestor of all hominins (organisms along the pathway to humans, after the split from the chimpanzee line) was believed to be an arboreal quadruped (Reynolds & Gallagher, 2013). Somewhere around 6 million years ago, the climate in Africa began to change. As the climate became dryer, due to a tectonic upthrust of the eastern part of the continent, what was formerly all tropical rainforest began to open up. Eastern Africa changed over to savannahs, grasslands, and other more open ecoregions (Simpson, 2012) .
By examining the skeletal structure of fossil hominids, paleontologists have been able to determine that the earlier, more ape-like species were almost exclusively arboreal. Ardipithecus ramidus represents a transitional form; its anatomy indicates that it could walk quite well, but could still climb trees easily. It probably didn’t run especially well (Lieberman, 2012). By the time Australopithecus fossils appear, all the evidence indicates they were fully bipedal (White, 2013).
Not only did upright walking allow our ancestors to better spot predators in an open environemnt. Once our they began walking, their hands were now free -- free to carry things, free to create and manipulate, to develop more and more sophisticated tools -- to eventually become what we recognize as human.
Lieberman, D. (2012). Human evolution: Those feet in ancient times. Nature, 483 (7391).550-1.
Lovejoy, C.O., Suwa, G., Spurlock, L., Asfaw, B., Whit, T.D. (2009). The pelvis and femur of Ardipithecus ramidus: The emergence of upright walking. Science, 326 (5949). 71-71e6.
Reynolds, S.C. & Gallagher, A. (Eds.) (2012). African Genesis: Perspectives on Hominin Evolution. NY,
Cambridge University Press.
Schrenk, F. (2011). Earliest Homo, in A Companion to Paleoanthropology (ed D. R. Begun),
Blackwell Publishing Ltd, Oxford. ch25.
Simpson, (2011) Before Australopithecus: The earliest hominins, in A Companion to
Paleoanthropology (ed D. R. Begun), Blackwell Publishing Ltd, Oxford. ch22.
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