This sounds like a cogent argument but, like the theoretical proof that it is impossible for a bee to become airborne, it does not work in practice. If it did, then no warm-blooded species could ever have adapted to life in water, and a large number of mammals have, in fact, done so.
It does not hold true of modern humans, either. Diving women off the shores of Korea and southern Japan earn their living by diving for shellfish and edible seaweed. In summer they work for four hours a day wearing loin cloths, sometimes descending to depths of 80 feet. In winter, when the water temperature sinks to 10°C they work for shorter periods in light cotton bathing suits. Kirsten Schuitema, a zoologist from the University of Lund, reports that ‘… adults in one group in Indonesia spent a daily average of 6 hours in water, and the children 4–5 hours.’ Individuals spent up to ten hours daily in the sea in periods of 20–90 minutes with short breaks for bringing the catch home.
In 1987 the American Lynn Cox swam from Alaska to the USSR across the Bering Strait, without wet-suit or covering of lanolin. She swam for four hours in water of 7°C maximum, dropping to a low of 3°C – that is, 20 degrees lower than the allegedly fatal figure. She did not die of cardiac arrest. There must be some factor or factors which are being left out of account in the abstract calculations about putative loss of body heat. One obvious factor is the efficiency of the fat layer as an insulator in water, but that is clearly not the whole story.
Donald W. Rennie conducted an experiment with diving women lying in a tank of water for three hours with only the face above the surface. Other Korean women from the same community – who were not divers but had the same thickness of subcutaneous fat – were immersed in the same way. Rennie found that the divers suffered less heat loss than non-divers with an equivalent fat layer. He, and other researchers in the same field, have deduced that there must be some latent human mechanism of adaptation to cold stress. Rennie suggests that ‘… the divers’ fatty insulation is supplemented by some kind of vascular adaptation that restricts the loss of heat from the blood vessels to the skin, particularly in the arms and legs.’
A mammal living in water would probably need to eat more than a land mammal of the same size. The Korean amas (the diving women) consume on average 50 per cent more calories a day than a non-diving Korean woman of comparable age.
But this would hardly have posed a problem to the aquatic ape. Some of the richest food sources in the world are found in tropical wetlands and off-shore waters. The food supply, in quantity, variety and ease of procurement, would far exceed what the savannah could offer to a small unspecialised primate.
The resources available to baboons, for example, consist mainly of leaves, seeds and seed pods, grubbed-up roots and corms, and occasionally berries. Foraging methods may vary – the baboon pulls up grass rhizomes with its teeth, and the gelada tugs them out with its hands – but the savannah menu offers such a meagre and monotonous selection that foraging can be a virtually non-stop occupation. For an early hominid like Australopithecus the savannah would have been equally inhospitable. In time they would have learned to supplement their diet by scavenging or opportunist predation on small animals, as some baboons have now learned to do. But as scavengers they would have been in competition with species both fiercer and more specialised, from hyenas to vultures. In the early stages of adaptation to bipedalism they would have been without the speed or the natural or artificial weapons to equip them for the job.
By contrast, coastal food resources include an abundance of edible aquatic plants, fish, crustaceans, bivalves, molluscs and other invertebrates, and the eggs of sea birds, apart from occasional flotsam such as the carcases of dugongs or turtles. Most of these items are available all the year round, and would have enabled an aquatic primate to move from a vegetarian diet to an omnivorous one without waiting to develop the skills and weapons necessary for hunting game on land. For example, some chimpanzees already crack open nuts: an aquatic ape would have had no difficulty cracking open small shellfish, as crab-eating macaques already do.
A change of diet can have far-reaching effects on a species. In this instance it may have been a factor in the development of the one unique feature we are most inclined to pride ourselves on – the human brain.
14
Brains and Baboons
‘The surest and best characteristic of
a well-rounded and extensive deduction
is when verification of it springs up, as
it were spontaneously, into notice from
quarters whence they might be least
expected.’
J. F. W. Herschel
At one time it was assumed that the crucial change in man’s cerebral evolution began with Homo habilis – that is, much later than the fossil gap. However, Robert Martin in 1982 re-examined the evidence from fossils and comparative anatomy. With the use of scaling analysis he demonstrated that hominid brain growth relative to that of other anthropoids began at least five million years ago. It was not a sudden event but a progressive phenomenon, though it apparently accelerated about the time of H. habilis.
Robert Martin’s approach was an original one. Instead of asking why brain growth might have been adaptive or desirable, he asked what made it more attainable in one particular primate than in others. In energy terms brain tissue, as compared with other body tissues, is uniquely expensive to produce and maintain, making heavy demands on the mother during pregnancy and lactation. That may be one reason why there is in many species a correlation between relative brain size and different diets. For example, fruit bats’ brains are twice the size of the brains of insect-eating bats. Similarly, leaf-eating monkeys have relatively smaller brains than fruit-eating ones of closely related species. A move from a forest diet to a savannah one does not seem to facilitate brain growth. Among cercopithecine monkey species, for example – the family that includes baboons, mandrills and patas and rhesus monkeys – there is no distinction in brain size between forest-dwelling and savannah-dwelling species.
Martin concluded: ‘Homo sapiens must have evolved in response to a rather unusual combination of environmental factors which made available a relatively steady, predictable supply of food lacking in significant toxic levels.’
Michael Crawford in 1989 speculated that a seashore diet could have had special advantages in relation to brain growth in addition to those of abundance, variety and year-round availability. He pointed out that the building of brain tissue needs a consistent one-to-one balance between Omega–6 and Omega–3 fatty acids. These two types of polyunsaturated fatty acids are both essential for health, and the body cannot use one in place of the other. The latter type (Omega–3) are relatively scarce in the land food chain, but predominate in the marine food chain.
The most popular attitude to the question of brain size has always been the idea that having more room inside his skull enabled Man to become more intelligent. This may be described as the Red Riding Hood approach: ‘Oh, Homo sapiens, what a big head you’ve got.’ ‘All the better to think with, my child.’
This is a questionable assumption. Within our own species intelligence bears no relation to the size of the skull – either its absolute size or its relative size. Even in today’s complex civilisation we only utilise a small fraction of the brain’s potential, and brain scans have shown that people with up to half of the brain destroyed can lead a normal life.
As between species, there is no correlation between skull size and intelligence. Recent researches into the intelligence of parrots have led some people to re-class them as ‘honorary primates’. In the bird’s small brain there is an ability to categorise objects and grasp abstract concepts like similarity of colour, shape and texture, which rivals a chimpanzee’s. People have sometimes speculated that the dolphin’s large brain is necessary to house the intricate mental processes necessary for interpreting its surroundings by sonar. But a bat with a brain no larger than that of a mouse has a miniaturised sonar processor which works at least as well
as a dolphin’s.
Even within the evolutionary history of our own species the correlation between brain size and intelligence does not hold good. Most of us are happy to believe that as man evolved and his skull expanded he grew ever more and more intelligent. But most of us would be reluctant to believe that Neanderthal man was more intelligent than modern man simply because there were more cubic centimetres inside his skull. We might also find difficulty in accepting that the increase in human intelligence stopped dead when the increase in human skull size came to a halt.
So the Red Riding Hood approach is now less in evidence. It is the neoteny model which remains most influential – the concept that we are juvenilised versions of a more ape-like ancestor. The tempo of our growth rates has in some respects slowed down; we spend longer in the various stages of development between birth and death, and that includes spending longer in the infantile stage during which the brain is expanding very rapidly. The extended period of rapid brain growth means that we end up with relatively large skulls – a head/body ratio more typical of juvenile anthropoids than adult ones. (The neoteny factor and the ecological one are not, of course, mutually exclusive. Neoteny is a mechanism of evolutionary change; environmental conditions, such as changes in the food supply, could either facilitate the working of the mechanism or inhibit it.)
In 1989 C. P. Groves published a book entitled A Theory of Human and Primate Evolution, a detailed and meticulously documented account ranging from the earliest primate fossils to modern man. Near the end of it he declares himself ready to ‘… defend the following proposition: brain enlargement is an epiphenomenon of neoteny.’
This is the reverse of the more usual neoteny scenario, which suggests that the large brain somehow became so desirable and adaptive that other (disadvantageous) features such as hairlessness had to be tolerated as part of the package. Groves is suggesting that brain growth was ‘… not selected for as such’, but happened fortuitously as a result of the changing tempo of our patterns of growth.
He then asks two very good questions. Most accounts of neoteny are content to compare modern H. sapiens with modern or primitive hominids. Groves raises the question of exactly when the neotenous trend began. He is inclined to regard it rather as Martin regards brain expansion – as a progressive phenomenon. A few neotenous features – such as the shortened face – appear very early; they are detectable, for example, in Lucy.
The second question he asks is why it happened. The probability is that a species does not become neotenous for no reason. The star example is that famous amphibian, the axolotl. It turns into a salamander when climatic conditions around its pond are sufficiently damp and shady to keep its skin moist. But if the surrounding land becomes too dry it retains its neotenous form, remains a tadpole all its life, and is capable of reproducing without becoming in other respects ‘mature’.
Why, then, apparently at some time during the fossil gap, did man’s ancestors take the first steps along the neotenous path? Groves does not attempt to answer the question. He merely points out that it exists, and he indicates the extreme complexity of the issues involved.
But in the course of his discussion he makes some interesting points, such as: ‘The most advanced example of neoteny among mammals appears to be the order Cetacea’ (that is, whales and dolphins). He cites a long list of resemblances between the general body form of an adult whale or dolphin and the form of the embryo of a land mammal such as a cow, pig or deer at the stage when the limb buds are forming. Like such an embryo, the dolphin has a hairless skin, a torpedo-shaped body with no neck, no external appendages such as ears, poorly formed ribs with no breast bone, hardly any hind-limb skeleton, and compared to most adult land mammals a very large brain in relation to body size – a characteristic of most mammal foetuses.
Groves speculated that the dolphin’s large brain was neither adaptive nor maladaptive but ‘exaptive’ – a side-effect of the prolongation of foetal growth rates; it happened because in any mammal foetus at that stage the organ that is growing fastest happens to be the brain. He adds parenthetically: ‘Doubtless all marine mammals are neotenous to some degree.’
It is not known why in mammals a move to an aquatic environment seems to be one of the triggers which sets in train a trend to neoteny. It is tempting to envisage it as an instance of reculer pour mieux sauter. Conceivably, a species finding itself in a radically new environment (such as water) begins to shed the more advanced features which fitted it for its old environment. It back-tracks to a more unspecialised foetus-like form, before re-adapting to the new habitat. If that were the case, then our own ancestors, having moved from the land to the water and subsequently from water to land, would have been subjected to an impetus towards neoteny on two successive occasions. It would explain why in our case the trend was unusually powerful.
All the hypotheses in this field are highly speculative; the problem is intensely complex and remains unsolved. The two possible connections between AAT and brain growth are those already indicated: (a) that a coastal environment, unlike a savannah one, would have provided the food resources without which brain growth could not have been afforded, and (b) that brain growth appears to be related to neoteny, and a trend towards neoteny is common among aquatic mammals but not among savannah ones.
Derek Ellis of the University of Victoria, Canada, has therefore analysed the ecology of all the various types of marine ecosystems found in tropical latitudes in relation to the observed behaviour of extant primates, with the object of assessing the viability of an aquatic ape. He examined the resources available in such environments as mangrove forests, salt-marsh lagoons, archipelagos, estuaries, reefs and rock shores, and collected data about swimming and other water activities in 26 primate species.
His starting point was a succinct encapsulation of the minimal AAT premise: ‘We need a habitat other than a forest-savannah boundary ecotone for the separation of ape and human stocks.’ His conclusion was that ‘… the tropical coastal environment provides productive ecosystems exploitable by a range of extant monkeys. There is no evident reason to believe that an ape could not converge to successful adaptation there.’
Yet this is precisely what many people find it hard to accept. Most of the attempts to refute AAT consist of alleged reasons why an aquatic ape could never have survived. It would, they imply, have been doomed to perish. It was not stream-lined; it was quite the wrong shape for a swimmer. It was warm blooded; once its fur was wet, the water would have drained it of its body heat. Its movements would be slow and inefficient; it could never move fast enough under water to catch a fish, and it would have starved. In the meantime its helpless young would have been greedily devoured by crocodiles and sharks, and it would have left no descendants.
These are all reasons why it would have been impossible for any land mammal to survive in water. They apply with equal force to the dog-like animal which went into the water and in the course of millions of years turned into a seal. Yet that primitive dog survived. Probably only some kind of duress would have driven it to resort to such a desperate measure, and doubtless many of the pioneers did perish. But enough of them survived, and today their descendants are found in oceans all over the world. It happened again and again – to the bear-like creature that turned into a walrus, and the mole-like creature that turned into a platypus, and the elephant’s cousin that turned into a manatee, and the flying sea bird that turned into a penguin, and the ancient unknown quadrupeds that turned into whales and dolphins.
If the sea of Afar had not been cut off from the Indian Ocean we might have been able to add a primate to the list; some of the aquatic apes might have ventured into the open sea and stayed there while our own ancestors returned to the African continent.
It would have been fascinating to find cousins in the sea as genetically close to us as our cousins in the forest. But when the evaporating inland sea became untenably briny, any aquatic apes would perforce have gravitated to the inland shore where the water
was fresher, and southward along the waterways, until ultimately the dependence on water as a food source was weakened and became dispensable.
Alister Hardy lived and died in the hope that some kind of proof might be dug up by the fossil hunters which would settle the matter. But it is hard to conceive of any fossil evidence that would be regarded as conclusive one way or the other. To take a parallel situation, if fossil hunters in five million years’ time unearthed the fossilised skeletons of an otter and a stoat, they would be able to deduce that two closely related mustelids co-existed in Scotland in the twentieth century. They would find it very difficult to establish whether one of them was aquatic.
For a long time it appeared impossible to prove or disprove either the savannah theory or AAT. It seemed that belief in one or the other must always hinge on the balance of probability rather than anything more definite.
And when, finally, a new and quite unambiguous piece of evidence came to light it was widely ignored.
When the molecular biologists entered the field of evolutionary research, their first claim was that they could supply evidence about the timing of certain evolutionary events – the dates at which one species split off from another. During the 1970s their techniques were refined and developed, and they began to make statements about place as well as time. As G. J. Todaro declared: ‘There is in fact a record of our history, however tenuous, that is recorded in all of us in our genes, and we are beginning to read that history.’
The best known example of a genetic marker giving evidence about our geographical origins is that of the haemoglobin S gene, which is of medical importance because it can sometimes cause sickle cell anaemia. This property is obviously maladaptive, and the normal expectation would be that it would have been bred out by natural selection. But it was discovered that it had another property which meant that in parts of Africa it increased the chances of survival: it provides a degree of resistance to malarial infection. Since the gene is not found in all human beings, it has obviously been acquired since the races of mankind dispersed and became differentiated. It is now fully accepted that the presence of the sickle cell gene is a marker which proves that its possessor must have had at least one African ancestor, though the family may have lived outside Africa for hundreds of years.
The Scars of Evolution Page 17
