Wednesday, January 2, 2008

Waterside hypotheses of human evolution - the most satisfying solution

In previous rants, I've tried to argue the case, and I think I succeeded, that the most satisfactory explanations for biological observations, including the major differences between apes and humans, are those that are adaptationist. I dismissed the 'God made us that way' explanations with a deft flick of my little finger and suggested that they were the very least satisfactory of all possible explanations. They just beg a much bigger question: So, how do we explain God? Next up, on my scala naturae of satisfaction, were those explanations that rely only on purely random forces such as drift, mutation and extinction. I admitted, grudgingly, that they are certainly a big part of the story and certainly happening all the time but my point is that they just don't satisfy my curiosity about the way we are. Ok, random effects are happening all the time but other forces of evolution are happening too, sitting on top of that fluid foundation of randomness. It's these forces that have shaped things most effectively to make them work in the real world. Next up were the effects of ontogeny, or "evo devo." They too, again, are clearly very important but are they explanations in themselves? I don't think so. They are proximate (or how?) explanations, not ultimate (or how come? as my tutor at UCL, Volker Sommer, would put it) ones. Sean Carrol's great book on Evo Devo offers brilliant insights into how relatively tiny mutations to toolbox and switch genes can make profound differences to phenotypes. But none of these changes would last five seconds if they weren't actually useful for the organism's survival or, at least, attractive to the opposite sex. This brings me to the next level up on my ladder of satisfaction: Sexual selection. Again, clearly, it happens. It must explain a lot in nature. It explains peacock feathers, huge, "fuck off!" silver back gorillas and, maybe beards and deep voices in men. But again it ultimately begs other questions like "why would one sex find that attractive in the first place?"
So, I have come to the conclusion that pure adaptive explanations are by far the most satisfactory.
In the study of human evolution most of the ape-human differences have, according to paleoanthropological tradition, been explained as adaptations to more open habitats. It's logical enough. After all, we are 100% terrestrial creatures today (to the nearest integer) and we certainly did come down from the trees (even though some, like Carsten Niemitz, have tried to argue otherwise.) According to this orthodoxy, we became bipedal so that we could move more efficiently. Or, maybe we became bipedal so we could free our hands to carry things like food, infants and weapons - also useful on grassy plains. Or maybe we became bipedal so that our upper bodies were lifted above waist high grass into the cool breeze and so that our backs were not perpendicular to the hot equatorial sun - also useful on the grassy plains. Having to scrape out a living on the harsh savannah, competing with all those fierce predators, made us need to be smart and as we cleverly procured more meat this high energy food fuelled yet more brain growth in an elegant positive feedback loop. As we hunted more for meat we needed to keep cool, so we evolved glabrous skin, well adapted to sweat cooling, to compensate for our large brain's vulnerability to over heating. To keep warm at night we evolved increased fat. As we got smarter and our brains grew bigger we learned to use stone tools and weapons and thus relied less on the more natural armoury of teeth.
All of this is orthodoxy and has been for generations. It might also be right. If there were no other competing hypothesis we'd surely conclude that it was right and happily tick off that box as another achievement of modern science. Indeed many anthropologists appear to have ticked it off already for that very reason - they just don't know any better.
Trouble is, there is another hypothesis. Some of us think the savannah hypothesis outlined above, although it kind of works to a degree if you want it to and if you turn off you critical faculties a bit, it just isn't as satisfactory as it could be.
So why did our ancestors adopt bipedalism? If it were for the myriad of reasons given, a few of them listed above, how come no other primate did so too? How come we don't see great apes moving bipedally for those reasons? Isn't there a better scenario where quadrupedalism might actually kill you? a place where an early hominid would still be expected to move bipedally long before any anatomical adaptations evolved to make doing so efficient? Can we imagine a place which provides a smooth continuum between this scenario and the dry, flat earth that we are clearly adapted to walking on today? Well I can. I think it's actually very obvious. Shallow water provides the perfect scenario for this to have happened and it does so in a way that in complimentary, not contradictory, to the other ideas listed.
It's not just wading that offers a better explanation for our bipedalism though. The peculiar phenomenon of encephalisation and simultaneous dental reduction just doesn't make sense in the context of increased carnivory, sorry, even if you can somehow squeeze in stone tools into the mix. It surely makes far more sense on the coasts where high-energy, high brain-nutrient foods can be procured without having to wait for Tarzan to come back with the meat. In some coastal habitats there is a potentially vast supply of sessile, very easy to procure (even for a child) shellfish. All you need is a pebble, nimble hands and a curious brain. Humans, probably for the first time, could have been a species to exploit such habitats. Now doesn't that explain encephalisation, dental reduction and - as a bonus - the adoption of stone tools far, far more satisfactorily? I think so. And, while we're at it, it also explains better another key trend in human evolution - increased altriciality. I've always wondered how it is imagined mothers and babies are supposed to fit into life on the savannah. Where did extended play time come in? Life on the beach is much safer and more cosy for would-be humans. Nakedness is another. Sweat cooling is not a bad explanation but it too is really a waterside one. The quickest and most efficient way of cooling down, as everyone knows, is to go for a dip. Seen in this context, sweat cooling is nothing more than an adjunct to going for a dip. Basically you're borrowing from a 'cooling bank'. There's more. If you think of the typical hairline of the scalp of a sub-adult human and wonder where that shape makes most sense I suppose you might conclude it's on the savannah. The hair on the top of the head is to protect our sensitive brains from the sun's rays and the discrepancy that the forehead is bare might be explained by being a social species where face recognition is a key aspect of life. Perhaps. But isn't it better explained in the water? It seems much more accurate simply to say that the part of the body (of a sub-adult, before other manifestations, such as signalling sexual maturity, come into play) most likely to be covered with hair is simply that part most likely to be above the surface of the water whilst swimming. What about fat? It can' make sense to be fat on the open savannah, whether chasing antelope or fending of big cats. It makes even less sense in trees. But it does make some sense in water. Infants are far less likely to die of hypothermia and for more likely to float and hence survive near drowning scenarios if they're fat. If mum's fat too, she's more likely to be able to rescue her infant from such scenarios. If these traits became established one would expect that it would follow soon that males would find sexually attractive mature women with visible displays of fat in areas which would also, amazingly coincidentally, help infants to survive too. Of course all of this is speculation but I think they're all scientifically testable. All that is needed is a willingness for the science of anthropology to open it's eyes to the fact that waterside hypotheses of human evolution are plausible and in need of some serious study.
In my next rant I'll bang on about why they haven't tested them yet and how this must change if we are to make progress.
Algis Kuliukas

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