Bipedal locomotion refers to walking on two legs in an upright position: our ancestor primates lived in trees and rarely set foot on ground; our ancestor hominids moved out of those trees and lived primarily in the savannas. The ability to walk upright is thought to be an evolutionary step forward, if you will, and one of the hallmarks of being human.
Scholars have often argued that walking erect would have been evolutionarily advantageous. Walking erect improves communication, allows visual access to a wider horizon, and changes throwing behaviors. By walking upright, a hominid's hands are freed to do all sorts of things, including holding and throwing weapons.
Evidence for Bipedal Locomotion
There are four main ways scholars have used to figure out whether a particular ancient hominid is primarily living in the trees or walking upright: skeletal foot construction of ancient hominids themselves, other bone configurations above the foot, footprints of those hominids, and dietary evidence from stable isotopes.
The best of these of course, is foot construction: the only issue is that ancient ancestral bones are difficult to find under any circumstances, and foot bones are very rare indeed. Foot structures associated with bipedal locomotion include a plantar rigidity from when the foot is flat through toe-off; and flat earth walkers generally have shorter toes than hominids who live in trees. Much of this was learned via the recent discovery of a nearly complete Ardipithecus ramidus, an ancestor of hours who apparently walked upright sometimes, some 4.4 million years ago.
Skeletal constructions above the feet are slightly more common, and scholars have looked at the configurations of the vertebral column; the tilt and structure of the pelvis; and the way the femur fits into the pelvis to make assumptions about the ability to walk.
Footprints are also rare, but when they are found, they have been examined for evidence that reflects the gait, length of stride and weight transfer. Sites include Laetoli in Tanzania (3.5-3.7 million years ago, probably Australopithecus afarensis; Ileret (1.5 million years ago) and GaJi10 in Kenya, both likely Homo erectus; the Devil's Footprints in Italy, H. heidelbergensis about 345,000 years ago; and Langebaan Lagoon in South Africa, early modern humans, 117,000 years ago.
Finally, a case has been made that diet infers environment: if a particular hominid ate a lot of grasses rather than fruit from trees, it is likely the hominid lived among grassed savannas. That can be determined through stable isotope analysis, and is discussed in detail at that link.
So far, the earliest known bipedal locomoter was Ardipithecus ramidus, who sometimes--but not always--walked on two legs. Fulltime bipedalism is currently thought to have been achieved by Australopithecus, the type fossil of which is the famous Lucy, approximately 2.2 million years ago.
Climbing Trees and Bipedal Locomotion
Biologists have argued that foot and ankle bones changed when our primate ancestors "came down from the trees", and that after that evolutionary step, we no longer had the facility to regularly climb trees without the aid of tools or support systems. However, a 2012 study points out that there are some modern humans who do regularly and quite successfully climb tall trees, in pursuit of honey, fruit and game.
Venkataraman and colleagues investigated behaviors and anatomical leg structures of two modern day groups in Uganda: the Twa hunter-gatherers and Bakiga agriculturalists, who have coexisted in Uganda for several centuries. The scholars filmed the Twa climbing trees and used movie stills to capture and measure the maximum flexion capabilities of tree climbing. They found that there is a difference in the flexibility and length of soft tissue fiber between the people who could climb trees with ease and those who cannot. That difference does not, however, get reflected on the bones: in other words, the flexibility that allows people to climb trees can't be seen in skeletal remains. Venkataraman caution that the foot and ankle construction of Australopithecus, for example, does not rule out tree climbing, even though it does allow upright bipedal locomotion.
This glossary entry is a part of the About.com Dictionary of Archaeology.
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