WHAT FORCED US TO WALK UPRIGHT?

There are numerous theories trying to explain hominin bipedalism; however, none of them is entirely satisfactory, as each of them leaves many unanswered questions. In today’s post I will discuss the most important hypotheses of the hominin bipedalism evolution.

The earliest theory of bipedalism, suggested by Darwin in 1871 and widely supported until the 1960s, linked walking upright to the use of tools; free hands were supposed to facilitate making weapons for defence and hunting. However, fossil and archaeological records show that there is at least a 1.5 million year gap between the development of bipedalism and the earliest stone artefacts which date back to about 2.6 Ma (Harcourt-Smith: 2007). Even if we assume that the first tools were made of wood, and so weren’t preserved, the timing difference makes this theory rather unlikely.

More recent theories tend to take into account the relationship between the emergence of bipedalism and the climatic shift towards cooler and drier conditions during the Plio-Pleistocene. Initially, according to the Savannah theory, scientists thought that walking upright was an adaptation to shrinking forests and spreading grasslands. Upright posture would help to see over tall grass, reduce skin’s exposure to the sun, allow more efficient body cooling and facilitate carrying resources across open spaces.

However, as new paleontological evidence appeared, bipedalism started to be seen as one of the adaptations to the extreme climatic variability (not the encroaching savannah) in East Africa during the Plio-Pleistocene. This might be why the fossils of our ancestors show signs of both walking upright and climbing trees – such flexibility could have been crucial to succeeding in diverse habitats. This adaptation made it easier to gather food from the trees and the ground; it also facilitated carrying what was gathered over long distances.

Decreasing the dependence on trees could have favoured the development of bipedalism: some researchers believe that such a locomotion mechanism is very energy efficient on the ground.  A study published by Sockol et al. in 2007 showed that human walking needs 75% less energy than both quadrupedal and bipedal walking in chimpanzees. Then again, some other studies (like this one conducted by Halsey and White in 2012) suggest that, compared to other types of mammalian locomotion, bipedalism is similar in terms of its energy efficiency.

There is one more interesting hypothesis I want to talk about, put forward by Lovejoy in the 1980s. He argued that upright walking in humans was linked to monogamy. According to Lovejoy and his supporters, increased bipedalism facilitated carrying food to desired locations. As our ancestors became monogamous, females would choose a partner with the ability to provide plenty of food to her and their offspring. Consequently, upright-walking males would have been more likely to reproduce, passing bipedalism onto the following generations. However, this theory is quite controversial as it is unclear whether the early bipedal hominins were indeed monogamous.

It is evident that when it comes to human bipedalism, there is still a lot to be understood. Although the last 50 years were exceptionally abundant in fossil discoveries, the evidence is far from complete. There is still no clear answer or agreement on how hominins became bipedal; however, there is no doubt that the development of bipedalism was crucial to human evolution. It was fundamental to establishing our flexibility and resilience – the features that helped us become the dominant species.