It’s fascinating to think about humankind’s evolutionary history and to speculate over what caused us and apes to split from our last common ancestor about five or six million years ago and develop in the way we did. I’ve really been gripped by the aquatic ape hypothesis since chancing upon a copy of Elaine Morgan’s ‘The Aquatic Ape Hypothesis’ in a second hand bookshop shortly before moving to Dublin. Morgan is an amateur paleontologist who picked up on Alister Hardy’s speculation that humans underwent a period of semiaquatic existence in the evolutionary past that would account for the differences between humans and our nearest genetic relatives, the chimpanzees.
Principally, the claim is that such traits as hairlessness, bipedalism, the fact that humans are comparatively fat animals and that we store our fat subcutaneously (immediately under the skin) rather than only at certain points around internal organs, can all be accounted for by the suggestion that these traits are all in one way or another an adaptation for spending a lot of time in water. There are a number of other traits as well. Breath control, useful for speaking, is supposedly an adaptation for diving underwater. Actually, I suppose the claim is that a semiaquatic past BEST explains those traits. The burden of proof has to be stronger than simply accounting for those traits because other theories might account for those traits too.
Before I get too carried away, I should say that a sceptical eye is cast on the hypothesis over at this site by another amateur paleontologist and philosopher of science, Jim Moore. The site is a little polemical, but behind the rhetoric lie a great many hard questions for proponents of the theory. The only claim I think he’s fatally undermined is the hairlessness claim. Although I haven’t made it through the full suite of pages on the site, so he might tear down some o the others too.
On the face of it, not having a hair is a pretty maladaptive for life under the hot african sun. Hair protects the skin from UV radiation and is a very good insulator because it traps air against the skin. It keeps you warm at night and cool during the day. The aquatic ape theory suggests that going bald on our bodies made humans better swimmers and the insulating properties were less efficient in the water, where blubber provides more insulation (which explains our being so fat). The problem is, as Moore points out, that aquatic mammals are mostly furry. They don’t call them fur seals for nothing. Polar bears have fur. Otters have fur. The list of aquatic mammals without fur is quite short (and mainly consists of whales). So you don’t explain how we became hairless by saying we took to water. I think the explanation for hairlessness lies in the second theory I’m interested in at the moment, the long distance runner hypothesis.
The long distance runner hypothesis suggests that humans are adapted to be superlative long distance runners, much better than virtually anything else on the African plains. It claims we hunted in the time before bows and arrows by literally running other animals to death; chasing them to the point when they died of heat stroke as a result of not being able to cool down. They’re wearing fur coats after all. Human hairlessness is a really extreme adaptation because we were sacrificing that insulaton and protection from the sun so we could cool down better than anything else. We sweat copiously all over our body, which is unusual in the animal kingdom and massively wasteful of water. Together, hairlessness and sweating mean we stay cooler when we run than other animals.
I think the long distance runner hypothesis and the aquatic ape hypothesis are actually quite compatible. The aquatic ape hypoethsis is best seen as an explanation of how we separated from the last common ancestor (I think about 6 million years ago), but the long distance runner hypothesis can relate to a much later period of human evolution, like the origin of the genus homo (about 2 million years ago, I think). So perhaps we were hairy when we were in the water and only shed the hair when we took to the African savanna?
But what does the aquatic ape hypothesis have going for it? To my mind, it is still a good explanation of bipedalism. The problem with bipedalism is that you have to explain why something that naturally walks on four legs when it’s on the ground (like an ape), would prefer to walk on two legs, when, given that it had yet to adapt to walking on two legs, it would have been hopelessly bad at it. Animals command a good view of their surroundings by standing on two legs, but why not drop down to four legs after having a look around like merecats do? Why stay on two legs? The suggestion is that if walking on two legs allowed the animal to continue to breathe, it would confer an immediate benefit no matter how slow the animal was. If our early ancestors spent all their time in swamps, lakes or river margins, they would be on two legs often enough for mutations that made it easier to walk on two legs to be adaptive.
This is a point where I think criticism of the aquatic ape hypothesis actually supports it. There’s a paper in an anthology on the Aquatic Ape hypothesis that discusses the bearing of the fact that humans are comparatively poor swimmers and drown frequently. From body design we’re not quite buoyant enough to keep our nose out of the water and our mode of locomotion on land doesn’t naturally transfer to water (for dogs, paddling is pretty much what they do anyway on land, but for humans swimming involves quite a different set of movements from walking). So, we are actually quite poorly adapted, or even maladapted to water. But the author also notes that apes are among the few animals as prone to drowning as humans are. Their body design is subject to the same problems. They’re not buoyant enough to keep their nose out of the water and their method of locomotion doesn’t transfer over to swimming. But you see how this actually supports the aquatic ape hypothesis? We started out as apes. That meant we were natural born drowners. Imagine the environment changed so that we were suddenly forced to spend a lot of our time in water. Would we swim or would we walk upright? What do apes do if they have to cross a river? They walk upright.
I think bipedalism provides the strongest support for the hypothesis, because in virtually every other respect for getting about on land four legs is better than two (faster, more stable, more forgiving if one leg or foot is injured and so on). And although our bipedal gait has some advantages over four legged movement now, it wouldn’t have at the beginning of the evolutionary change. So it’s very hard to see what else than a semi aquatic environment could have induced us to give quadrupedalism up.
Then there’s the fat. Even humans in the scientifically defined normal range (which is not especially common these days) are fat in comparison with animals (up to a quarter of our body mass is pure lard). In my own case, despite jogging several times a week, it’s a few percentage points higher than that. That’s ridiculous. What do I need all that fat for? Elaine Morgan contends that we need it to keep warm in the water, which seems commonsensical given how fat whales and seals and polar bears are. I think it’s got to be right, but Caroline Pond, also in the anthology linked to above and an expert on adipose tissue in animals denies there’s good evidence to say insulation is the primary function of fat. It’s an entirely sufficient explanation to say fat is a store of energy. She claims fat is not even a very good insulator. In tests in 1962, it was found that even after prolonged immersion the skin of an alaskan fur seal was only five degrees cooler than its core temperature (meaning most of the insulation was being done by its fur).
But suppose the insulation angle has indeed been overplayed. Nevertheless being in water makes carrying a lot of fat less of an encumbrance. Suppose fat is primarily a store of energy (it’s not an especially accessible store of energy or it would be a lot easier to lose weight, but forget about that). Apes need energy stores just as much as humans do but they are much less prone to putting on weight. In water the disadvantages of maintaining a large energy store are less, and there may indeed be additional advantages such as insulation and added buoyancy that might make a tendency to corpulence indeed adaptive. If apes are our starting point, apes + water = humans seems to give a strong explanation of why we carry so much fat. On land, apes with large energy stores couldn’t outrun the lions (or more to the point, couldn’t outrun the apes with no energy stores when the lions came calling), but the energy store didn’t pose a similar disadvantage in the aquatic environment.
Now back to the running man theory. The advocates point to a couple of specific adaptations that make humans good long distance athletes. The achilles tendon, connecting the calf muscle to the back of the heel, and the nuchal ligament, connecting the spine at the base of the neck to the base of the head. Both are supposed to be adaptations for running, with the achilles serving as a sort of elastic store of energy (a different kind of ‘energy store’ obviously!) and the nuchal ligament assisting to keep your head stable when you run. Interestingly, neither of these things is present in chimpanzees or in Australopithecus. Chris McDougall (‘Born to Run’ 220) says these adaptations began to appear about two million years ago (with homo erectus). I think the dates are very interesting, if important adaptations for running only appeared two million years ago, the advantages of bipedalism for distance running cannot be cited as a rival to the aquatic ape hypothesis. Because without the achilles tendon and the nuchal ligament to stop our heads going boingy boingy we would have been quite poor runners for about three to four million years after we first started walking upright.
The running man theory contends that in ages past humans practiced ‘persistence hunting’, or running after animals until they die of hyperthermia. This is still practiced by the Kalahari bushmen today, so it can be done. Humans can do this because we jog faster than four-legged plains ungulates walk, and although they are much faster than us when they run, they cannot run and cool down at the same time. Most animals don’t sweat, which is wasteful of water. Instead, they pant. But they can’t pant and run at the same time. Humans tracked and harried their prey over long distances, always forcing them to run rather than walk to the point where the animals were utterly exhausted.
I think the theory has some plausibility. The one thing that I might question is how comparatively intolerant we humans are of changes to our core body temperature. There are some interesting figures given (p.185/6) in the Aquatic Ape anthology mentioned earlier relating to the average body temperature of humans and other animals. Humans maintain a body temperature of 37*C and can get in serious trouble if their temperature rises as high as 41*C. The body temperature of the savanna dwelling oryx on the other hand varies from 38*C at night to 45*C during the day. And a gazelle’s body temperature can increase by 5 or 6*C on a single run. These gazelle can apparently cope with a temperature as high as 46.5*C for several hours. So some other animals are in some respects clearly better placed to withstand the heat load generated by long distance running. This doesn’t completely overturn the theory though. It suggests there are different mechanisms for coping with heat with the human one being a rather extreme solution.
On the aquatic ape theory
Books by Elaine Morgan ‘The Aquatic Ape Hypothesis‘, ‘The Descent of Woman‘, ‘The scars of evolution‘
Another aquatic ape website (I only just chanced upon this site while writing this post, so I haven’t had a chance to read all of it yet.)
And the sceptic site mentioned earlier (Aquatic Ape theory Sink or Swim?)
On the running man theory
Chris McDougall “Born to Run”
Daniel Liebermann and Dennis Bramble”Endurance Running and the Evolution of the Genus Homo” Nature vol 432, 2004: 345-352. (available from Daniel Liebermann’s website).
Update: It just occurred to me that I was once told the ancient Greeks believed that purpose of the brain was as an organ for cooling the blood (because the heart was the seat of the soul). Is it possible that for millennia it was just that?