Bipedalism is a defining characteristic of human lineage. It has long been believed that our ancestors began to walk on two legs in the more open environments of the African savannah. A recent study centered on chimpanzees, our closest cousins, nevertheless suggests that bipedalism may have first appeared in trees.
The Savannah Hypothesis
Mandatory terrestrial bipedalism is a defining characteristic of modern humans. This morphological adaptation is also essential for distinguishing fossils belonging to the human clade (hominines) from those of other apes (hominoids) over the last seven million years. But how did we go from quadrupeds to bipeds?
The prevailing theory (the savannah hypothesis) suggests that our ancestors gradually rebounded when rainforests began to recede due to natural climate change in the late Miocene-Pliocene. The first hominids, who then evolved in a more fragmented environment dotted with groves and meadows, would indeed have had to take a little height both to orient themselves, but also, and above all, to observe and follow prey, while watching for predators.
However, not everyone fully agrees with this idea. A study published recently in the journal Science Advances and conducted by researchers from University College London, the University of Kent and Duke University also refutes it as well.
An arboreal origin?
Chimpanzees living in early hominid-like habitats offer a unique opportunity to study the drivers of bipedalism. Until now, studies of locomotion have focused only on forest chimpanzees, neglecting critical comparative data on how these behaviors vary across habitats. The researchers therefore observed a group of thirteen wild adult chimpanzees living in the Issa Valley, western Tanzania, for fifteen months.
During these observations, the researchers found that chimpanzees spent as much time in trees as their counterparts in dense forest environments. In other words, they very rarely ventured into the grasslands, despite the fact that they were accessible.
Moreover, even when they ventured out into the open, chimpanzees still tended to walk on all fours. In fact, more than 85% of observed occurrences of bipedalism have taken place in trees.
“Our study suggests that late Miocene-Pliocene forest retreat about five million years ago and more open savannah habitats were in fact not a catalyst for the evolution of bipedalism. “Summarizes Dr Alex Piel of University College London.
Naturally, this study only focused on one group of chimpanzees over a short period of time. It therefore seems premature to draw definitive conclusions. However, these results suggest that trees may have remained an essential component of the hominid adaptive niche. A bipedalism evolving in an arboreal context would then probably have been motivated by a foraging strategy.