In my last rant I tried to convey the idea that, when it comes to explaining the difference between humans and chimpanzees - to me the ultimate question - we can rank various ideas on a ladder (a kind of 'scala natura') in terms of how satisfying they are with, most satisfyingly of all, the "God did it" explanation right at the bottom.
I suggested, rather cheekily I thought, that random forces - although certainly involved to a huge extent - would appear only one up from God on the next rung of the ladder, closely followed by "Evo-Devo" (pro neoteny) arguments and then ones to do with sexual selection. Bravely holding the adaptationist banner up high, I claimed that only adaptation can offer truly satisfying explanations in Biology and therefore adaptationist explanations for the differences between human and chimpanzees are really the most interesting and important ones.
Here though, we get into potentially sticky ground because there is a danger of following in the footsteps of Rudyard Kipling and building "just so" stories. All sorts of nice-sounding 'explanations' have been put forward, for example, to explain the evolution of hominin bipedalism. (Bipedalism that evolved on the evolutionary lineage leading to man.) I've been studying this subject for several years now and I think there are probably at least 33 distinct ideas that have been published to explain this difference. Anything from "it was to reduce our body's profile in the mid-day equatorial sun" to "it was to enhance penile display." I won't list them all out but anyone who knows anything about anthropology will be familiar with at least six or seven of them. I say there are about 30 but, in truth, they can be categorised into several broad groups. Among the more sensible ones are those that suggesting carrying things was the key driver, those that food procurement was the most important reason for upright posture and others that it was simply more energy efficient to move on two legs than four.
Now if you look at these ideas in detail, and I have, what tends to strike you is that they all seem to make sense at some level but, equally clearly, they all seem to have a few problems too. None of them are wholly satisfying but all of them are somewhat satisfying. Frustrating, isn't it? Of course, faced with such a dilemma it is easy to think maybe they were all involved to some degree. It is certainly a common sense position that must be closer to the truth than the unlikely possibility that, actually, one of the proponents of these models got it completely right and everyone else got it completely wrong.
So what is the mainstream view on this today? What do university level students get taught about it? Surely, after 150 years since Darwin we should have something solid and sensible to say on the matter, shouldn't we? Well this is basically what I returned to academia to find out and I have to say I have been rather disappointed with what I discovered, to say the least. At University College London (UCL) and then at the University of Western Australia (UWA) and, I suspect, also at the vast majority of universities in the world where evolution is taught as fact, rather than fiction, what seems to be taught is basically the savannah theory.
The what? ... The savannah theory.
Now before people start objecting that there never was a savannah theory, that it was actually a straw man invented purely for the purpose of knocking it down (yes, this view even reached the respectable pages of the Journal of Human Evolution in 1997), let's be clear - practically every text book written on human evolution in the past 75 years aludes to, or explicitly cites, the idea that climate change in Africa - and by that we mean a change to increased aridity - was the thing that did it. It was the resulting change in habitat from closed forest to open woodland and grassland, sometimes called "parkland" but more often called "savannah", that was responsible for the process of 'homininisation'.
Most of the 30-odd explanations of bipedal origins assume this quite openly although there are some clear exceptions. In fact just this year (2007) a paper was published in the journal Science with front page prominence, by Robin Crompton's very well respected team of researches in Liverpool, espousing one such view. Their model, if it can be caled that, is that orang-utan-like, tree-wobbling, upright posture whilst reaching for food in the thinner branches of trees was a likely precursive form of locomotion not only for hominins but actually for all the great apes.
Now it seems to me that this is an explanation that can be equally accurately labelled as "obvious" and "silly", at the same time. It's obvious in the sense that, of course, the ancestor of all the great apes was almost certainly arboreal and would have, to a large extent, had to feed from thin branches. Whilst doing so a precarious form of upright posture is likely to have been held for a few minutes and this upright orientation of the body, encouraged too by vertical climbing in relatively large mammals, would have orientated the body in the direction of greater bipedalism. I do not think it is silly, by the way, that the authors suggest that this form of 'arboreal proto-bipedalism' was also likely to have been the ancestral condition of the other great apes too. Since the discovery of Orrorin in 2000, I've been persuaded that the fossil evidence suggests that bipedalism was actually so old it may well pre-date the last common ancestor (LCA) of the human and the chimp, and maybe also the gorilla and the orang-utan too. It strikes me as a classic case of anthropocentrism that we are tempted to imagine the LCA as very similar to a chimpanzee. It's us that have done all the evolving. The stupid chimps have basically stayed the same. I doubt it. It seems much more likely that the precursor of bipedalism and knuckle-walking (let's face it, a rather off form of quadrupedalsim) was some kind of semi-arboreal bipedalism.
No, what I think is silly about the Science paper is that it is very thin on the ground when it comes to giving good reasons as to why only one lineage from that ancestral quasi-bipedal great ape became obligate bipeds, whilst all the others reverted to quadrupedalissm. Well no reason, that is, apart from the good old savannah theory. What? Didn't I say that this was an idea that didn't posit bipedal origins in a savannah context? Yes I did. And, to be fair, they didn't. They posited bipedal origins to have begun in a forested environment. It's just that once it had begun, it was the ones that left the trees, they argue, that somehow were left standing on two legs, whereas the ones that stayed in the woodland became quadrupedal. Now that's what I think is silly.
I could ramble on all night about other ideas of bipedal origins and how I think they're silly too but I won't. Actually, I find some of them rather compelling. I actually quite like the energy efficiency model of Rodman & McHenry fame, for example. It does make sense that moving on just two legs, especially when they're long and able to lock into a straight, inverted-pendulum gait, has to be more efficient than moving on four, relativly short, bendy legs. When doing so, you're using half as many limbs but mainly you're using the kinetic energy of falling forward to power most of your propulsion. Muscle power is really only used to fine tune the positioning of the limbs as they swing into position for the next step.
It makes sense although there are problems: It's clear that we move more efficiently than chimpanzees do today, but only because our anatomy is manifestly adapted to efficient walking and only when that locomotion is conducted on certain special substrates.
So here's problem one: All the evidence which shows humans being more efficient at locomotion than chimpanzees have been conducted on that most special substrate - remarkably rare in the natural world - the treadmill. It strikes me that this is a bit of a loaded dice. Humans naturally find walking on very flat, firm, relatively vegetation free substrates not only easy but quite appealing. I have often wondered just how appealing walking on a treadmill is to a chimpanzee. No matter how well the animal is desensitised to doing so, it must be quite unnatural at best and, at worst, quite terrifying. Experiments have shown that humans are much less efficient when moving through less perfect substrates. I have myself done experiments with douglas bags which show that even walking through long grass adds about 20% to the cost of walking as compared to that on concrete. It makes me think that when it comees to moving through bushland or especially dense forest, the supposed energetic advantage of bipedalism quickly disappears.
The second problem is that there was probably an energetic 'rubicon' to cross for an early hominin biped not quite adapted to bipedalissm. How or why was bipedalism practised even before the anatomical adaptatations which make it efficient evolve? A recent paper gave good evidence that perhaps, by chance, some chimps started exhibiting a more upright gait and because they got some slight energetic benefit from that it became selectd for. A chain of events started at the end of which some populations just happenned to move more bipedally than others. Now this is possible of course, perhaps it is even likely, but, as you will have gathered already, I just don't find that sort of explanation very satisfying. Why didn't all chimpmanzillas (my label for the LCA of chimp, man and gorilla) encounter similar scenarios? Maybe it is the explanation we will end up adopting but first, perhaps we should first try to find a better one.
I think there is a more satisfying one and, you will not be surprised to read, it is based on the crazy, outrageous idea that our ancestors actually spent a significant amount of time (but note not all that much time) wading through shallow water. The thing about shallow water and apes is this: they are far, far more likely to move bipedally there than on dry land. And I mean "move". In trees or on the edges of bushes, whilst doing the behaviour "postural feeding", apes do not do much moving at all, they do a lot of standing and reaching and grabbing and eating. When they do move they almost always move with support, by holding onto a handy branch. Only in water are apes compelled to move bipedally and almost always it is without support. Clearly the actual depth of water is an important factor to consider. In very shallow water, say a few centimetres only, there is obviously no such compulsion. In fact, as has been reported in bonobos, if they were foraging food in the water, perhaps on the underside of floating leaves, they are actually compelled to move quadrupedally. But, equally clearly, the deeper the water gets the more likely they are to switch to bipedalism. In waist deep water a chimp simply has no choice and all the evidence shows that they do, indeed, move bipedally there.
Now, that to me is as satisfying an explanation as you can get: An ancestral scenario where if our ancestor moved quadrupedally it would die but if it moved bipedally it would live. Can we think of any other scenario where bipedalism would confer such a clear cut advantage? No. Can we think of any other taxa on the planet that exhibit this peculiar shift in locomotor pattern in this scenario? No. (Do dogs, horses, pigs, cats etc switch from quadrupedalism to bipedalism in shallow water? No.)
It is almost ceratinly not the only reason our lineage adapted bipedalism but I think it must be a big part of the reason. In the next rant, I'll go on about why, if it's such a good idea, it's not already in the mainstream and what must happen before it will get there.
All the best