To explain bipedalism, he pointed to the feeding posture of the gelada baboon which has been (somewhat unconvincingly) described as bipedal. Geladas live on the savannah and feed mainly on grass. Grass eaters have to spend many hours a day feeding because grass is not very nourishing, so in moving from one patch of grass to the next geladas do not waste time in walking on all fours. While both hands are still occupied plucking grass and carrying it to their mouths, they shuffle along in a squatting position. Jolly reasoned that picking up seeds would make similar demands on a busy pair of hands, so that a seed-eating ape might adopt the same strategy.
The vegetarian approach aroused a great deal of interest, but the gelada model was unconvincing because when a gelada moves in this way it is only erect from the pelvis upwards. Many primates move with their torsos erect, from the brachiating gibbon to the leap-and-cling lemurs of Madagascar. It has never induced any of them to develop habitual bipedalism. Besides, bipedalism strictly implies only two points of contact with the ground, and a closer inspection showed that geladas for much of their feeding time have three points of contact with it. They are shuffling along on their bottoms.
The search for a model was getting nowhere. Almost any primate can be trained with patience and bribery to walk bipedally over short distances, and their respective gaits were being analysed with the aid of high technology, using filmed images and electromyographs to measure activity in the various muscles. Some people were arguing that chimpanzees and gorillas in the wild often rise up when they are excited and charge along on two legs, and therefore the only difference is that they practise bipedalism incidentally whereas we practise it habitually. It looked like a possible solution. Once you have reduced a difference to ‘only a matter of degree’ it is a short step to assuming that the difference is unreal and there is no need to explain it.
The locomotor analysts would have none of this. Jack Prost of the University of Illinois at Chicago, comparing chimp bipedalism with ours, reported, ‘The motions of their limbs and the forces around their joints are so different that there is a question whether the two activities ought to be given the same name … The two are quite distinct and only the similarity of names can be offered to argue that one is a primitive form of the other.’ M. H. Day, anatomist at Guy’s Hospital Medical School, London, concluded that hopes of solving the problem must await further fossil discoveries, ‘with known extant locomotor models looking less and less likely to fill the bill’.
The next new approach to the problem was made by C. Owen Lovejoy in 1981, in a paper entitled ‘The Origin of Man’. He did not allow himself to get bogged down in the debates over whether the hominid’s food was animal or vegetable. That was irrelevant to his theory. The key question lay in what they did with the food when they found it. In the case of most primates, what they do with food when they find it is to eat it. But there are exceptions.
Sometimes a monkey or a chimpanzee will pick up the food and carry it to a place where it can be eaten undisturbed – especially if she is female and fears it is going to be taken away from her. Sometimes a Japanese crab-eating macaque, if the food is left on the seashore, will carry it down to the water and wash off the sand before eating it. And if the food is rather bulky or awkwardly shaped, it may require two hands to carry it and the animal walks on two legs.
These incidents are far too brief and rare to affect a species’ behaviour in general or its evolution. The only reason an animal in the wild transports food regularly and over fairly long distances is in order to give it to another animal. Ospreys carry fish to their nestlings; wolves carry meat back to a mate which has produced a litter. Lovejoy felt that the origins of bipedalism might lie less in the diet and more in the pattern of reproduction.
He was the man to whom Don Johanson turned for an expert’s opinion on Lucy and the other Hadar fossils, and how they fitted into the story of human origins. As Johanson summed up the situation, ‘We had added our bit to the when and where of erect walking. But the why of it was something else.’ Lovejoy’s approach to the question was unexpected. As reported by Johanson, when asked why the hominid and not the apes became bipedal, he responded:
‘Would you like to talk about sex?’
In Lovejoy’s thesis as finally presented there was one new idea and a modified version of an old one.
The new idea was that bipedalism did not evolve on the savannah, but in the forest. The savannah idea seemed to him absurd on the grounds that ‘… no hominid could ever have ventured out on the savannah as a stumbling imperfect walker and learned to do it better there. If it had been unfitted for erect strolling on the savannah, it would not have gone. If it had gone, it would not have survived the trip.’ (And if not the savannah, he automatically assumed, it had to be the forest. After all, where else was there?)
The old idea was the sex angle – the concept that the hominid may have been pair-bonded. The most popular aspect of the hunting hypothesis had been the picture of a roving male bringing back sustenance to a home-based female. It made the commuter-and-housewife partnership seem part of Nature’s plan. Since bipedalism came too early to be caused by hunting, Lovejoy rearranged the order of events, so that pair-bonding came first. The ancestral ape became pair-bonded in the forest. Through carrying handfuls of vegetable food to his family he gradually perfected bipedalism. That made him better qualified to walk out at a later date onto the savannah where he eventually learned to add meat to the menu.
That is a somewhat brutally compressed summary of a thesis set out by Lovejoy in a complex scientific exposition full of loops and feedbacks. Even if we accept for the moment that the ancestral apes may conceivably have become pair-bonded, there are some basic weaknesses in the scenario.
One is that male primates do not make a practice of offering food to females – not even when they are pair-bonded. The only pair-bonded ape is the gibbon, and he cements the bond not by offering sustenance but by chasing off any potential rivals, including his own growing sons, that may threaten his monopoly. Lovejoy urges that primate relationships are very varied. He cites the marmoset as an example of a species where the input of paternal child-care makes it possible to rear more offspring (marmosets normally have twins).
But what he does not make clear is that the care consists not of feeding the young, but of minding the babies while the female goes off to forage for herself and then returns to suckle them. The likelihood of a gorilla, for example, bringing food to a female or juvenile is as improbable as a cow bringing grass to a calf and for the same reason: once the young are weaned their food is all around them and they can help themselves.
The other weakness is the unquestioning assumption that an ape on the ground carrying things would automatically do it on two legs. Graham Richards published a paper in 1986 challenging the long-held theory that the transition from trees to the grassland brought with it a sudden freeing of the hands for manipulation and tool-making. He concluded that ‘Talking about “freed hands” when there is little indication as to whether they were either enslaved in the first place or in any genuine sense “liberated” subsequently, helps nobody understand what really happened.’
In practice, knuckle-walking chimps and gorillas can free one hand instantly whenever they need to.
In 1985 Russell Tuttle and David Watts, of Chicago University’s Department of Anthropology, analysed the result of 1,700 hours of observation of mountain gorillas in Ruanda and Zaire. The animals spent 98 per cent of their time on the ground, most of it sitting or squatting to feed. When they needed to reach up or out for food they used a tripedal posture thirteen times more often than a bipedal one. Out on the savannah, with the need for faster and more sustained locomotion, bipedalism would be even less likely. A chimpanzee running off with a stolen banana runs on three legs and carries it. Stealing a blanket, it runs on three legs and drags it. The types of food-load which would force it to resort to bipedalism are very rare in the wild. An experiment with Japanese macaques shows that when co
nfronted with a pile of potatoes they may grab an armful and hobble away with them on two legs to a quiet spot. But it is not a good model for primates in Africa, least of all on the savannah.
Perhaps the unlikeliest aspect of Lovejoy’s thesis is the idea of females and young being left unprotected in the middle of the plain while the male wanders off to forage at a distance. It would be risky and untypical primate behaviour. In the case of baboons there is an elaborate strategy for ensuring that the females and young are always kept in the middle of the troop and whenever they move on they are flanked by male outriders. The hunter theorists were fond of the phrase ‘back to the lair’, but primates do not dwell in lairs, and even if they wished to adopt the habit, the open savannah is the last place they would be likely to find one without burrowing underground.
The sunshine theory was touched on in 1970 by R. W. Newman, and elaborated in 1984 by Peter Wheeler. Feeding habits had dominated the discussion for some decades, but this theory dropped the topic of food and returned to considering the nature of the terrain.
The last time that approach had been employed was in the horizon-scanning theory modelled on the patas monkey. But when an animal bred in the forest moves out onto the open plain, it may encounter one problem even more urgent than the fear of predators. In tropical Africa it is very much hotter away from the shade of the trees, especially in the middle of the day when the sun is almost vertically overhead. High levels of direct solar radiation can cause considerable stress, so, other things being equal, any method of lessening this stress would be favoured by natural selection.
Wheeler gives figures, derived from experiments with models, showing that at midday in the tropics an ape on four legs exposes seventeen per cent of its total body surface area to the rays of the sun, while an upright hominid exposes only seven per cent, thus absorbing less than half as much heat by direct solar radiation. The seven per cent consists of the top of the hominid’s head and shoulders, suggesting that the hominid stood up to keep cool, and scalp hair was retained as a shield over the exposed parts, reflecting and reradiating much of the heat before it could reach the skin.
One difficulty the theory does not address is the fact that an ape has to expend muscular energy – far more than we do – in order to maintain the upright posture, and the expenditure of muscular energy tends to raise body temperature. This fact would certainly diminish, if it did not cancel out, the cooling effect of standing up. Another snag is that erect posture would only work as a partial cooling device while the sun was at or near the highest point of the sky. Seeking to exploit a niche as a noonday forager would be an unrewarding tactic for a mammal allegedly so ill equipped to resist heat stress.
But the greatest weakness is the one that haunts every variant of the savannah hypothesis, namely: if this was such a good solution of a common problem, why was only one species driven to resort to it?
None of the suggested answers is good enough. Wheeler argues that most savannah animals, such as lions and zebras and hyenas, are protected when exposed to heat stress by a mechanism (the carotid rete) which protects the brain from overheating by maintaining it at a lower temperature than the rest of the body. Primates spent most of their evolutionary history in shady places, so they never evolved this protective device.
This explanation does not work because several primate species have at various times made the transition from trees to savannah, including baboons and geladas, patas monkeys and vervet monkeys, and they have all flourished there without being forced into walking on their hind legs. Some escape the midday heat in the same way as many other mammals, by taking a siesta in the shade of a rock or thorn bush; others continue foraging, and they too appear to come to no harm. The most savannah-adapted of primates is the baboon, and its mode of locomotion has evolved in the opposite direction from bipedalism, namely, towards resembling ever more closely the standard quadrupedal gait of ground-dwellers such as the dog.
There remains the water theory. The nearest primate model here is the proboscis monkey, which often resorts to bipedalism not from choice but by necessity. In the award-winning documentary film Siarau (Partridge Films, Ltd. 1984), there is remarkably vivid footage of a band of these animals walking along on their hind legs, up to their chests in water. Leading the way in this shot is a female carrying an infant in her arms, the clasp and the posture strongly reminiscent of a woman carrying a baby.
The proboscis monkey has never in the classic sense made the transition from the trees to the ground, because it lives in mangrove trees in the coastal swamps of Borneo. More often than not when it climbs down a tree, what it encounters is water. If the water is deep the monkeys swim. They are excellent swimmers and can swim for miles, and may be seen diving into the water from the tree tops. If the water is shallow they wade.
Occasionally, and especially at low tide, they are seen on relatively dry land – on a mud flat or a sandbank – and their mode of locomotion is sometimes on four legs and sometimes on two. Local fishermen speak of family groups of these monkeys ‘going for a walk’, and by this they mean walking on two legs. Film of proboscis monkeys on the ground in the wild is rare because of the terrain, but some shots were captured by a Japanese cameraman up to his thighs in squelchy mud. They feature in a documentary film entitled Long Nose, Long Tail, directed for Japan Broadcasting Corporation by Yuiche Naka, and they include a sequence in long-shot of a group of proboscis monkeys walking bipedally.
The difference between the bipedalism of the proboscis monkeys and the African apes is striking. Gorillas and chimps make short energetic dashes on two legs, often moving sideways, and accompanied by hoots or squeals of excitement. In the case of gorillas it is invariably part of a display ritual, showing off or threatening, somewhat analogous to a human war dance. (In such a context the human dancer may dance sideways on one leg while emitting ritual noises, but this is unlikely to lead to monopedalism in daily life.) By contrast, the wild proboscis monkeys in the Japanese film, having acquired their bipedal gait in water, are seen walking calmly on the ground through the trees in single file.
Inundation of the habitat is their incentive for bipedalism. For proboscis monkeys crossing a stretch of water a couple of feet deep, walking upright offers only one single advantage, but it is an offer they cannot refuse. It enables them to breathe, whereas if they walked on four legs, their heads would be under water.
In all the savannah scenarios the disadvantages of bipedalism – unstable equilibrium, disruption to skeletal, muscular, circulatory and hormonal systems – would be incurred in their most extreme form immediately, and would only ease off in successive generations as the bones gradually changed their shape and the new muscles developed. On the other hand, the supposed advantages would be non-existent or minimal in the first generation while the ape was still an ape. They would only accrue to its distant posterity.
In other words, terrestrial bipedalism would only become advantageous provided it had been diligently practised for thousands of years while it was still awkward and laborious and the behavioural rewards were infinitesimal or nil.
In the aquatic scenario the position is reversed. Walking erect in flooded terrain was less an option than a necessity. The behavioural reward – being able to walk and breathe at the same time – was instantly available. And most of the disadvantages of bipedalism were cancelled out.
Erect posture imposes no strain on the spine under conditions of head-out immersion in water. There is no added weight on the lumbar vertebrae. The discs are not vertically compressed. (An astronaut in zero gravity gains an inch in height in the first days in space, and immersion in water is the nearest thing to zero gravity on planet Earth.)
In water, walking on two legs incurs no more danger of tripping over and crashing to the ground than walking on four. There is no distension of the veins because immersion prevents the blood from pooling in the lower limbs.
Standing up in water does not trigger secretion of aldosterone, salt retention or hig
her blood pressure. The reverse is the case: head-out immersion causes a prompt and marked fall in systolic and diastolic blood pressure, plus increased excretion of salt in the urine. This effect is so marked that in patients suffering from high blood pressure and sodium imbalance, such as children with nephrotic syndrome and women with late pregnancy toxaemia, head-out immersion is effective in relieving the symptoms. Some women suffer side-effects from the orthodox drug treatment, and in these cases spells of immersion therapy can reduce the amount of drugs they have to take.
Water thus seems to be the only element in which bipedalism for the beginner may have been at the same time compulsory and relatively free of unwelcome physical consequences.
An early objection to the aquatic solution was that primates in general, and apes in particular, generally have an aversion to water, so it is hard to believe that any population of African apes could have chosen to go into the sea. A much more probable explanation is that the apes stayed where they were and the sea came in to them. The vicinity of Hadar was one of the most unstable spots on the surface of the earth at the time of the ape/man split.
More than twenty years before Darwin published The Origin of Species the eminent geologist Sir Charles Lyell was working on three volumes of his classic work Principles of Geology, published between 1830 and 1835. Lyell was a friend of Darwin’s, and his ideas about the history of the earth helped to blaze the trail for acceptance of Darwin’s ideas about the evolution of living things.
The Scars of Evolution Page 5
