Read more: “10 biggest puzzles of human evolution“
CHARLES DARWIN suggested that our ancestors first stood upright to free their hands for toolmaking. We now know that cannot be right since the oldest tools yet discovered are a mere 2.6 million years old, whereas the anatomy of hominin fossils reveals that bipedalism emerged at least 4.2 million – and possibly even 6 million – years ago.
The trouble with bipedalism, says Chris Stringer at the Natural History Museum in London, is that proficient walking has many advantages, but acquiring the skill requires anatomical changes, and in the meantime you will be slow, clumsy and unstable. “It could have begun in the trees,” he suggests, pointing out that orang-utans and other primates walk upright along branches when feeding. This fits with what we know about the lifestyle of the first bipeds but does not explain why they evolved specialist anatomy. By 4 million years ago, for instance, the tibia in the lower leg was held upright to the foot, whereas it is angled to the outside in apes living now, even those that spend the most time on two legs.
Advertisement
In a more compelling evolutionary explanation bipedalism would substantially boost survival, which is why some people believe it evolved to allow males to access more food so that they could help feed their partners and offspring (Odyssey, vol 2, p 12). But this idea presupposes a very early origin of monogamy, which the evidence doesn’t support, says Donald Johanson of Arizona State University in Tempe, who in 1974 discovered Lucy, a 3.2-million-year-old, upright Australopithecine. He points out that among early hominins, males were much larger than females, which in primates is a sign that there is competition rather than cooperation between the sexes.
“The real question is what were the benefits,” says Johanson. One possibility is that individuals who could wander further than others had access to a wider variety of food sources, allowing them to live longer and produce more surviving offspring. In addition, bipedalism would have left their hands free to carry things and, being taller, they may have been better at spotting predators. “There might have been a whole package of advantages,” he says, adding that bipedalism may have emerged more than once.
All of which would have set the stage for a second phase of evolution around 1.7 million years ago, when our ancestors left the forests for the savannah. This is when the greatest anatomical changes took place, with shoulders pulled back, legs lengthened and a pelvis adapted to life on two legs.
There are many possible reasons why bipedalism took off at this point. Walking upright might have helped individuals deal with the scorching heat of the open grassland, allowing air to circulate around the body while minimising direct exposure to the sun (). It would also have increased mobility. “I think the argument comes down to travel efficiency and travel distance,” says Robin Dunbar at the University of Oxford. Bipedalism allowed our ancestors to walk long distances, enabling them to track down prey on the savannah. One study even suggests that we become adapted for endurance running (), although modern couch potatoes may consider this idea a step too far.
