In his Essay 'Darwin's last laugh' (Nature 460, 175; 2009), Frans de Waal suggests that ape vocalizations are homologous to human laughter, which they could be — but that does not necessarily imply that apes have a sense of humour. Adaptive divergence could be at play, making humans the ones who get the jokes.
Darwin's singular contribution to the issues raised by de Waal is that, in spite of universal appreciation for the behavioural similarities between humans and other animals, these continuities can be divided into two classes. The first is a symbolic, or literary, relationship, familiar to us through Aesop: the ant is industrious, the fox is clever. The second is the result of a shared biological history, or common ancestry. Our scientific interest in the apes is presumably based on the expectation that the similarities we can identify between them and us are principally of the second type. Although critical to de Waal's argument, this type of overlap is, unfortunately, much easier to assume than to demonstrate.
The circumstances in which similarities between chimps and humans manifest often indicate the opposite — that the behaviours are not directly homologous and are being overenthusiastically interpreted. For example, infanticide by chimps is carried out by non-relatives (for reasons only they know) whereas in humans it may be carried out by the mother, or by someone acting on her behalf, mainly for cultural, social or economic reasons. In one species, the act is aggressive, violent and often cannibalistic, with no indication of the remorse that can accompany the act in the other species.
And although our feet are homologous to ape feet, the chimp's foot is principally adapted for grasping and the human foot to weightbearing. They are similar, and are descended from a common ancestral structure, but they are by no means the same.
A genuine Darwinian approach to primate behaviour may have to acknowledge that the brains of apes (and their capabilities) may simply be different from our own, like their feet. Evolution, after all, is the production of difference. If one scholar acknowledges the adaptive divergence that has occurred between a human and a chimp over 7 million years or so of separation, and another insists that they are the same, then who is really in denial of evolution?
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