What's the difference between a chimp and a human?

We are the Chimpanzees' (and also bonobos, equally) closest relative in the animal kingdom, being even closer to them than they are to gorillas are according to molecular data. Only an estimated 1.4% of our DNA is different from theirs. Even gorillas are further from chimpanzees than we are. We are closer to chimpanzees than zebras are to horses, and yet just look at us...

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Clearly the ancestors of these two species became adapted to radically different niches since the the Pan/Homo split.
  • Chimps are hairy like all but one of the other 250+ types of primate and most mammals. Body hair makes sense not only to keep the body warm but also to protect the skin from abrasion when moving through thick vegetation.
  • Like most mammals, chimpanzees tend to move on the ground on all fours. The scenario they are most likely to move (as opposed to stand) with unsupported bipedalism, however, is in waist deep water. Couldn't this be a clue to why there is a difference here? Interestingly their sister species, the bonobo, which is slightly longer legged and has a slightly greater tendancy for bipedalism also has less fear of water as they live in habitats which are typically much wtter.
  • Chimps have brains less than 400 cc in cappacity and have relatively large teeth. Their diet largely comprises of fruits, shoots and other vegetarian matter although they do eat meat ocassionally.
  • Chimps are terrified of water and are very poor swimmers. Only one (as far as I know) anecdotal instance of a chimpanzee swimming has been reported, in Attenborough's book "Life of Mammals" to accompany the BBC documentary series. And even that was a second hand account. No such incidents have ever been reported in bonobos, orang-utans or any of the 14 species of gibbon.
  • Chimp babies are born relatively skinny with fat levels around 2-3% of body weight to carry around with them.
  • Chimp faces are flat and have outward pointing nostrils.
  • Humans are naked. This is very unusual for a primate and quite rare amongst mammals. The only other mammalian lineages that have evolved this trait are burrowing mammals and aquaatic ones. The part of the body most likely to be covered with hair in a (sub-adult) human is, oddly, the very part of the body most likely to be above the surface of the water whilst swimming the breast stroke.
  • Humans, uniquely amongst primates move bipedally pretty much 100% of the time.
  • Humans have much bigger brains than our ape cousins which consumes a far larger percentage of our daily energy budget. The high energy requirement of the brain must have required a switch in the type of food eaten from a predominently vegetarian diet to one much higher in energy. Oddly, our brain growth coincided with dental reduction.
  • With a little help babies learn to swim excellently and can grow up into superb skin-divers. Babies can learn to swim months before they can learn to walk.
  • Human babies are born comparatively fat (approximately 15% of body weight) making them much more buoyant in water than chimps. Interestingly mature women are fatter than their male counterparts, whereas there is little sexual dimorphism in adipocity in apes.
  • Human faces appear to be more strealined with, especially, much larger, downward pointing nostrils.

Zoologists know that nakedness in mammals has evolved rarely. Desmond Morris, in his book "The Naked Ape" (1967) suggested that “... the zoologist is forced to the conclusion that either he is dealing with a burrowing or an aquatic mammal, ot there is something very odd, indeed unique, about the evolutionary history of the naked ape.” Morris (1967:11) It occurs to me that we can be quite confident in discarding the burrowing explanation but that the other two are hardly contradictory. Couldn't our nakedness be explained by very low levels of selection for ocassional swimming despite the fact that this is actually rather unusual and actually quite unique?

The simplest explanation for our superior swimming & diving ability (compared to chimpanzees) is that since the Pan/Homo split, the ancestors of human beings lived in an environment where natural selection favoured that ability. /font>

Other explanations for this are comparatively complicated: The ability to swim, it is argued, is just another attribute of human intelligence. Infant fat is due to increased energy demands for our large growing brains. Nakedness has nothing to do with swimming abilities since many semi-aquatic mammals (e.g. otters, beavers, many types of seal) have retained their fur.

TThese arguments may, of course, be right. But it is curious why the most basic one: our ancestors evolved by water more than their's did - is not even considered as a possibility in the university-level text books on human evolution.

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Photo taken from the beautiful book "We are all Water Babies" Johnson & Odent (1995).

References:

Morris, Desmond (1967). The Naked Ape. Vintage