Why we became bipedal

ResearchBlogging.orgOh bipedalism, where would we be without you? A lot closer to the ground, that’s for sure; and probably far from the civilisation we find ourselves in. After all, it was walking purely on our hind limbs that freed up our hands and enabled them to develop the fine motor control that makes tool use possible. Even today, after years of training, chimps find it difficult to create the simple stone tools our ancestors were using 2.6 million years ago.

And it's pretty easy to do. Even I've managed to get some nice sharp flakes off a core (then cut myself on them)

So, like just about every other aspect of human evolution, the development of bipedalism is an interesting topic that demands investigation. However, unlike most other aspects of human evolution, this is quite easy. Unlike language or modern behaviour, bipedalism involves many features that have preserved.

We can look at the skeletons of our ancestors and watch as they become more adapted to upright locomotion; as our feet become robust to support the body’s weight, as our hips change shape, as our limb proportions change…all of it can be seen in the fossil record.

EvoAnth: 1; Decay: 0

Further, such detailed information on the development of bipedalism allows many of the selection pressures driving it forwards to be identified. We can see there was a push to become more efficient and to be able to traverse longer distance, amongst other things. All in all several factors have been identified which seem to have driven us to become the walkers we are today.

However, issues arise when trying to identify which one of these factors was the “first cause”, so to speak. What was the original reason we became bipedal?

These are just a few of the possible "first causes" as to why we became bipedal.

But now some of the mystery surrounding why we started to walk on two legs may be laid to rest as a new study provides evidence for the “bipedalism started so our hands could carry more food” hypothesis.

The researchers found a clearing in a forest inhabited by a troupe of chimps and used this opportunity to do some experiments on why they might be bipedal, providing them with a range of nuts, some of which are rare in their natural environment, and seeing what happened.

Experiments on chimps don't count as animal cruelty because chimps are evil.

They gave the chimps stones to crack nuts open and either lots of a rare nut, only a few rare nuts or lots of a common nut. They found that when they only gave them a few rare nuts the chimps transported them around the area more frequently, ~4 times more than when the rare nut was provided in abundance or the common nut was given.

They also noted that when the rare nut was present – either in large or small numbers – the chimps carried them bipedally ~3 times as often than they carried the uncommon nut bipedally. Further, by transporting the nuts bipedally they were able to carry more of the resource.

The researchers also supplemented this information by studying the troupe when they stole food from nearby farms (yes, chimps do that). In accordance with their previous observations they found that the chimps walked bipedally more often when stealing these rare resources and were able to carry more when they did so.

Their ascent to humanity begins here!

So it would seem that chimps walk bipedally to enable the hoarding of more valuable resources. Early hominins, being similar to chimps, are extremely likely to have engaged in this behaviour as well. Therefore it provides evidence that this was one of the first factors to motivate us into walking upright, after which other pressures might start to push our bipedal evolution.

Perhaps as our environment changed and resources became thinned out such hoarding would be a more valuable behaviour, providing increased selection for bipedalism. Alternatively it might be as the forest became patchy moving tools to the food (another reason these chimps walked upright) required traversing longer distances thus selecting for more bipedal.

After a while tool use, efficiency and all those other evolutionary factors would also start to exert their influence and drive bipedal evolution further.

Overall this is a good study documenting over 700 instances of food transport making the conclusions regarding what motivates chimp bipedalism rather strong. However, they are only studying one troupe. Whilst this doesn’t render these results invalid, they would be significantly strengthened if they could be demonstrated in other chimp groups.

However, I don’t think these researchers will be doing that since they seem to think that, as this troupe’s forest is decreasing the bipedalism in this group is a unique behaviour that has emerged as a response to this environmental change – similar to early humans, who also started being bipedal as the environment changed.

They even cite the “variability selection” – the same process responsible for humans – as the factor driving this change. To me this is a fascinating claim, suggesting that these chimps are in a similar position to that which hominins found themselves in ~7 million years ago. However, it is also currently unsupported. Further research should see if this behaviour can be identified in chimp groups that are not believed to be under variability selection pressures.

If it cannot, then chimp (r)evolution similar to humanity may well have begun.

Carvalho S, Biro D, Cunha E, Hockings K, McGrew WC, Richmond BG, & Matsuzawa T (2012). Chimpanzee carrying behaviour and the origins of human bipedality. Current biology : CB, 22 (6) PMID: 22440797

A quick reminder, Wednesdays are “wondering wednesdays.” Submit your questions on human evolution via the “feedback” page and I’ll do my best to answer them. Even non-experts evolved, so you can ask questions too even if you think they’re stupid.

6 thoughts on “Why we became bipedal

  1. Pingback: Why you shouldn’t trust 96% of Evolutionary Psychology | EvoAnth

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