Introduction to Bipedal Locomotion

Bipedal locomotion refers to walking on two legs in an upright position, and the only animal to do that all the time is the modern human. Our ancestor primates lived in trees and rarely set foot on the ground; our ancestor hominins moved out of those trees and lived primarily in the savannas. Walking upright all the time is thought to have been an evolutionary step forward if you will, and one of the hallmarks of being human.

Scholars have often argued that walking erect is an enormous advantage. Walking erect improves communication, allows visual access to farther distances, and changes throwing behaviors. By walking upright, a hominin's hands are freed to do all sorts of things, from holding babies to making stone tools to throwing weapons. American neuroscientist Robert Provine has argued that sustained voiced laughter, a trait which greatly facilitates social interactions, is only possible in bipeds because the respiration system is freed to do that in an upright position.

Evidence for Bipedal Locomotion

There are four main ways scholars have used to figure out whether a particular ancient hominin is primarily living in the trees or walking upright: ancient skeletal foot construction, other bone configurations above the foot, footprints of those hominins, and dietary evidence from stable isotopes.

The best of these, of course, is foot construction: unfortunately, 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—flat foot—which means the sole stays flat from step to step. Secondly, hominins that walk on the earth generally have shorter toes than hominins who live in trees. Much of this was learned from the discovery of a nearly complete Ardipithecus ramidus, an ancestor of ours 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 spine, the tilt, and structure of the pelvis, and the way the femur fits into the pelvis to make assumptions about a hominin's ability to walk upright.

Footprints and Diet

Footprints are also rare, but when they are found in a sequence, they hold evidence that reflects the gait, length of stride, and weight transfer during walking. Footprint sites include Laetoli in Tanzania (3.5-3.8 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 hominin ate a lot of grasses rather than fruit from trees, it is likely the hominin lived primarily in grassed savannas. That can be determined through stable isotope analysis.

Read More

Laetoli - 3.5 Million Year Old Hominin Footprints in Tanzania

By K. Kris Hirst

Earliest Bipedalism

So far, the earliest known bipedal locomotor was Ardipithecus ramidus, who sometimes—but not always—walked on two legs 4.4 million years ago. Fulltime bipedalism is currently thought to have been achieved by Australopithecus, the type fossil of which is the famous Lucy, approximately 3.5 million years ago.

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 lost the facility to regularly climb trees without the aid of tools or support systems. However, a 2012 study by human evolutionary biologist Vivek Venkataraman and colleagues points out that there are some modern humans who do regularly and quite successfully climb tall trees, in pursuit of honey, fruit, and game.

Climbing Trees and Bipedal Locomotion

Venkataraman and his 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 how much their feet flexed while tree-climbing. They found that although the bony structure of the feet is identical in both groups, there is a difference in the flexibility and length of soft tissue fibers in the feet of people who could climb trees with ease compared with those who cannot.

The flexibility that allows people to climb trees only involves soft tissue, not the bones themselves. Venkataraman and colleagues 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. 

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