When did our ancestors stop sleeping in trees?

ResearchBlogging.orgAustralopithecus afarensis, of which AL-288-1* was a member, lived between 3.9-2.9 million years ago and was fairly adept at walking upright. Whilst not as good at being bipedal as later species, such as Homo sapiens, they could still walk well.

However, whilst their anatomy was adapted for upright walking, they still retained some characteristics that would also allow them to be good climbers. Their fingers, for example, are curved like a modern ape’s.

Thus the question arises – did they mostly climb or walk? Did they merely traverse between groups of trees, where they spent most of their time climbing, or did they only retreat into the canopy for safety from predators; or is there something else going on entirely?

These footprints were left behind by Au afarensis 3.5-3.5 mya. Just think about that.

New Scientist suggests a soon to be published paper may provide some answers to this mystery, although I’m not so sure.

The research in question focused on Guinea chimpanzees which were discovered to nest on the ground more frequently than other troupes had been recorded doing (14% of their nests were on the ground versus 5%).

This would suggest that chimps will nest on the ground despite the fact they lack fire (for now) or other tools to fend of predators, which in turn opens up a range of options for Au. afarensis.

Might they too have nested on the ground? Could their tree climbing features merely be vestigial then? There’s a range of interesting possibilities here.

All lies! (kinda, perhaps)

However, this train of thought – the one New Scientist seems to be taking – suffers from a fatal flaw which threatens to derail the whole thing: The jungle in which this group of chimps live has very few predators in it.

Now, this makes sense; after all, why would the chimps be putting themselves in a more vulnerable position if there are things to exploit them? But it also means that applying this logic to Australopithecus is only valid if they too live in an environment with few predators.

Which doesn’t seem to be the case. Hominins, after all, were living on the increasingly open Savannah type environment. It would not have been a safe place for them, so camping out on the ground seems to be an unlikely option.

The frequency with which different genders camp out on the ground.

Some chimp groups have been seen to nest on the ground despite their being some predators in the area, but the frequency with which this happened is lower than the troupe studied (10% versus 14%).

Further, the citation for this figure is both in Dutch and under an embargo until November this year so I have no way of finding out how frequent predation is.

Even if it is more often than for the group under investigation, is it similar to what hominins were experiencing? If not then these findings still wouldn’t be applicable to them.

At best this research suggests hominins would’ve slept on the ground prior to predator defeating technology if there were no predators. I hate to be crude but no shit Sherlock.

But who said they were safe in trees?

So the implications for human evolution aren’t especially valid. So doesn’t this render the research useless?

Hardly, it’s an interesting paper that’s trying to answer a puzzling question in its own right: why do the chimps sleep on the ground? There are perfectly good trees for them to nest in nearby, it doesn’t make any sense.

One hypothesis was that it was a form of mate guarding – it is mostly males who sleep on the ground, perhaps they are doing it to protect their mates in the nearby trees from rivals.

However, this research uses a rather clever genetic trick (or at least I think it’s clever, but I know little about genetics so it might not be) to demonstrate that the chimps sleeping on the ground are closely related to those in the nearby trees.

As such, they wouldn’t be mating to the tree-dwelling chimps so the “mate guarding” hypothesis can be dismissed. And so the question of why they sleep up there remains.

It's just one big happy family

That is, in my opinion, more interesting (and accurate) than the relevance of this study to human evolution. However, this angle is hidden by New Scientist’s focus on how this research pertains to hominins and the best part of the research lost.

Of course, it isn’t entirely their fault; the paper itself also tries to tie this work into human evolution. Quite why I do not know, the research is fascinating without trying to tart it up by linking in humans.

Take that away and you’re left with an interesting, creative paper. Unfortunately, we’re also left with two rather puzzling mysteries. How arboreal was Australopithecus and why aren’t these chimps sleeping in the trees?

Koops K, McGrew WC, Matsuzawa T, & Knapp LA (2012). Terrestrial nest-building by wild chimpanzees (Pan troglodytes): Implications for the tree-to-ground sleep transition in early hominins. American journal of physical anthropology PMID: 22460549

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*I’m going to start calling Lucy Al-288-1 (unless I feel lazy). Hopefully after a little bit we’ll all remember the number and can feel that tiny bit superior to people who just call her Lucy.

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