Feb 23 2007

Chimps with Spears

There is something endlessly fascinating about chimpanzees – all primates, really, but chimps especially. The most often cited reason is the fact that they are so eerily similar to humans, and I think this obvious speculation is probably correct. Humans have a well-documented obsession with humans, and chimps are almost human.

Historically we have tried to erect barriers between our lofty selves and mere animals, but they have been systematically smashed by scientific investigation – mostly of our closest cousins, the chimps. Chimpanzees can recognize themselves in mirrors, they can anticipate and plan for the future, they can mourn the death of loved-ones, and they can use tools. It has previously been observed that they can fashion sticks for later use as tools, for fishing termites out of mounds, for example.

The evidence is overwhelming that chimps are closely evolutionarily (phylogenetically) related to homo sapiens. We share more than 99% of our DNA, and our common ancestors were likely interbreeding unti about 5.4 million years ago. Clearly humans are much more intelligent than chimps, reflected in our larger brain size, especially the frontal lobes, or neocortex – responsible for complex thought and planning. But the human brain is just a modification of the chimp brain. One time a researcher asked me to review an MRI scan of the brain of a rhesus monkey in their care (not even as similar as a chimp). It looked pretty much like a human brain – all the same parts and structures were there. I showed it to a few colleagues of mine, without telling them what it was, and at first they mistook it for a human brain. They did immediately sense that something was wrong – the proportions were not correct and the overall shape was off. But it took them a moment to realize they were not looking at a human.

Now two recent studies have added to the chimpanzee repertoire, and brought them even closer to humans in ability. It had previously been described that chimpanzees from the Tai Forest used stone tools to crush nuts and process other foods. However, some speculated that the chimps may have just been mimicking the behavior observed in humans. Stone tool use would still be an impressive behavior for chimps, but coming up with it on their own is more impressive, and has greater implications, than if they were “merely” copying humans.

Now follow up study published online last week in the Proceedings of the National Academy of Sciences shows evidence that chimps have been engaged in stone tool use for the past 4300 years, predating human farmers in the area by 2000 years. The evidence comes from stones that show the characteristic “incidental flaking” that results from their use, and also crusting with food residue.

If it is true that chimps, on their own, figured out how to use stones as tools and that they have been doing this for at least thousands of years – prior to contact with humans, then this hold implications for both human and chimp paleoanthropology. First, it is possible that some evidence of stone tool use that has previously been attributed to human ancestors may have in fact been left by chimpanzees. Second, it is possible that the common ancestor of modern chimps and humans may have used tools (wood and or stone) and therefore tool use is a homologous feature common to chimps and humans – derived from a common ancestor rather than developed independently.

Although chimps have been seen crafting sticks as tools, so far there is no evidence that they fashion or modify the stones for use as tools. They do, however, select stones that are appropriate to the task. (This has been a fine point of some confusion due to some erroneous secondary reporting.)

Now, just a week after the recent publication on stone tool use, another study published in Current Biology documents that the chimpanzees of Senegal are using spears to hunt bushbabies. Unfortunately, there are no pictures or film of this behavior, so we only have the researchers’ descriptions. If confirmed, this would be yet another once exclusively human behavior now shared with our chimp cousins.

In this case the spear use might be a recent adaptation. Local chimps typically hunt colobus monkey, but they are lacking in this environment. The chimps in this study hunt the much smaller bushbabies and may have developed spear use to adapt to this smaller prey. They use the spears to probe small hiding spaces, but also with enough force to injure their prey.

Also, spear use is more common among females and younger chimps. This is evidence of recent adoption, since newly acquired skills tend to be passed down more quickly by mothers and adopted more quickly by the younger generation. Adult males are the slowest to adopt new behaviors.

That chimps are so skilled and versatile is interesting in itself, but it is also useful for what it tells us about humans. Our nearest relatives are an indirect window into human ancestors – and therefore into human nature. I highly recommend the book Shadows of Forgotten Ancestors by Carl Sagan and Anne Druyan for a thorough discussion of this topic.

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