BECOMING HUMAN: BIPEDALITY

Walking upright seems so obvious and natural to us humans. In fact, the ability to walk on two legs is the trait that separated the first hominids from other four-legged apes, making bipedality one of the most fundamental human characteristics. Since the 1960s the fossil record has significantly increased, however scientists still cannot agree on why and how this radical change came about.

So let me start with what we can be the most certain about: the timeline of bipedality development. Here are some examples of the ancient roots of upright walking:

• The shape of the thigh bones of Orrorin tugenensis who lived 6 Ma suggest Orrorin was bipedal.
• The reconstruciton of the skeleton of Ardipithecus ramidus from 4.4 Ma shows extensive evidence for bipedality too.
• Lucy, the 3.2-million-year-old Australopithecus afarensis fossil, leaves no doubt for the species’ upright walking.

However, while showing signs of bipedality, all of the above species retained some features (such as long arms, short legs and long fingers and toes) indicating that they spent a considerable amount of time on trees. Homo erectus, ‘the upright man’ who appeared 1.89 Ma, was the first fully terrestrial hominid with an anatomy very similar to ours (Wayman: 2012).

What can we conclude from the current evidence then? Firstly, it suggests that the transition between quadrupedalism and bipedalism was gradual. Although first signs of walking upright appeared 6-7 Ma, it wasn’t until around 3 Ma when the early hominins became nearly as efficient at bipedal locomotion as us. They developed foot and pelvic bones which provided more support and stability for bipedal movement. The figure below represents the comparison of foot and pelvic bones between chimpanzees, Australopithecus africanus and Homo sapiens (Arsuaga and Martinez: 2006).  

 

Source: http://anthro.palomar.edu/hominid/australo_2.htm

The second conclusion we can draw is that, again, the past might have been much more complicated than we thought. We know that the hominin family tree used to be very diverse and it’s likely there was a high degree of locomotor diversity within early hominins too (Harcourt-Smith and Aiello: 2004). Different patterns of locomotion represented adaptations to a variety of habitats – which fits nicely with what I said about climate change and habitat fragmentation in my previous posts. In the next post I will explore different theories on the development of bipedality in hominins, as well as the possible relationships between walking upright and the changing climate.