The Scars of Evolution

by Elaine Morgan

  He was able to cite examples of creatures where the females are more ornamented, but these are species in which there is total transposition of the sexual roles. An example is a bird called the phalarope, in which the female, beside being more brightly coloured, is also larger and more aggressive, engages in courtship behaviour to attract the male and fights to defend the territory, while the male tends the nest and hatches the eggs. No one suggests that this kind of transposition was manifested by the ancestors of Homo.

  Thirdly, adornments for purposes of sexual selection are usually seasonal, more especially when they are inconvenient or injurious. After mating the lyre bird’s tail and the buck’s antlers are shed; the colours of the bright feathers and the shining scales are dimmed. A feature that was inconvenient all the year round, and to both sexes, would surely be selected out.

  Fourthly, the characters which are sexually esteemed are usually, like the peacock’s tail, additions rather than deficiencies. They are often indicative or suggestive of good health, such as lustrous hair or plumage, whereas the first stages of hair loss would have seemed more indicative of ill-health (mange, or debility, or alopecia) rather than fitness to survive.

  Fifthly, there is in most people a rooted aesthetic prejudice against hairlessness except in the case of our own species. If we admired hairlessness in other animals, it would be easier to believe that our hominid ancestors yearned to find such a characteristic in their mates, but we find it almost universally repellent. We prefer the squirrel’s bushy tail to the rat’s naked leathery one, and the swan’s feathered neck to the vulture’s skinny one, the fluffy chick to the naked nestling, the velvet-coated European mole to the hideous naked Somalian mole-rat.

  For these and other reasons, the idea of an ancestral ape seeking out the females with the baldest bodies has fallen out of favour, even with Darwin’s illustrious name to commend it.

  3 The hunting hypothesis held the field for some time and was believed to explain many things about human physiology. The problem about hairlessness was why one savannah primate needed to go naked while all other species in the same habitat retained their fur. It was explained on the grounds that vegetarian primates do not need to move very fast, but a carnivorous primate would get hot while chasing its prey, and losing its hair would enable it to cool down.

  The weaknesses of this hypothesis are, firstly, that fur or hair is a protection against over-heating because it provides a barrier against the heat of the sun. Secondly, that in the scenario of the hunting male and the non-hunting female, it would be predicted that males would become more hairless than females, whereas the reverse is the case. Thirdly, that any protection against over-heating in the day would have been counterbalanced by increased danger of hypothermia in the night. Fourthly, that on a predator-hunted savannah, all primates and not merely carnivorous ones would need to be able to run fast if they wished to stay alive, yet no other primate has lost its hair.

  4 Neoteny is the concept that humans are a juvenilised form of ape. They are said to be characterised by a general retardation of the pace of development, so that they mature more slowly than the other primates and live longer, and this involves retaining some characteristics of a juvenile or foetal ape into adult life. The examples most often quoted when comparing humans with apes are the flatter face, the rapid pre-natal rate of brain growth persisting for some time after birth, and the hairlessness characteristic of a foetal ape. The theory suggests that the prolonged period of rapid brain growth was advantageous to the species, and some other characteristics were retained as part of the neotenic package. One of these was nakedness, described as ‘the embryonic distribution of body hair’.

  The foetalisation theory was first proposed by the Dutch anatomist Louis Bolk in the 1920s, and revived in the 1970s by Stephen Jay Gould of Havard. Many people were convinced that the question ‘Why naked?’ now had a simple and adequate answer: ‘Because of neoteny’.

  The weakness of the theory is that while some characteristics may be retained as part of a neotenic package, that only applies to characteristics which are either benign or neutral in their effect on fitness to survive. No one claims that all foetal characteristics are retained in a neotenic species. For example, a human foetus and a human baby both have very short bandy legs, but natural selection ensures that this feature is not retained in adult life.

  In 1977 Stephen Jay Gould published a definitive account of neoteny. The book was called Ontogeny and Phylogeny, and was rightly hailed as a classic. In it the author stressed that he had ‘… no intention of falling into the Bolkian trap of all or nothing’. He was concerned to give a clear and scrupulous account of his ideas on the subject which would stand the test of time. It is perhaps significant that in that volume the matter of hair loss is not discussed. The only fleeting reference to it is in a list quoted from a book by Louis Bolk. In summarising his own views, Gould stated his belief that neoteny does not inevitably result in the retention of juvenile characteristics, ‘… but it certainly provides a mechanism for such a result, if this result be of selective value.’

  The last seven words are crucial. If nakedness increased the hominid’s chances of survival in its particular habitat, neoteny would provide a mechanism by which it could be acquired. If it decreased the chances of survival in that habitat, then natural selection would countermand it and we would have remained hairy. Neoteny in no way absolves us of the need to identify the type of environment in which hairlessness is at a premium.

  5 A more recent theory is that of the ‘noonday ape’, already discussed in connection with bipedalism. It hinges on the proposition, so far unproved, that an ape’s brain may be more susceptible to over-heating than the brains of other primates like the baboon, and that Homo’s ancestors were therefore forced to adopt unique strategies to cope with the heat of the savannah. Its weakness lies in the idea that such an animal would opt to specialise in foraging during the heat of noon, a niche for which it would seem spectacularly ill suited.

  The other speculative element is the widespread assumption that hairless means cooler. In its earlier and simpler form this was held to be self-evident: if a man feels cooler on taking his coat off, the same must apply to an animal. This had been disproved, so it is now replaced by a subtler proposition which states, (a) that some animals use sweating as a way of keeping cool, (b) that Homo is a prime example of a species making use of this strategy, and (c) that it is self-evident that the efficiency of sweating must be enhanced by hairlessness.

  The weak link here is (c). It sounds so much like pure common sense that it is seldom questioned. One famous fossil-hunter on a radio programme was asked by a listener about human hairlessness, and he explained that when the hominids began to sweat, they were forced to become naked. He reminded listeners of the discomfort they feel when sweating while wearing a woolly cardigan, and told them that if the perspiring hominids had retained their fur it ‘would have caused their skin to go mouldy’.

  Human sweating is a phenomenon requiring a separate chapter, but one quotation may help to correct the mouldiness hypothesis. In 1974 a major survey of mammalian skin was published by V. E. Sokolov in the USSR, and an English edition by the University of California appeared in 1982. It described in detail the macro-and micro-structure of the skin of around 500 mammal species from all the orders and most of the genera, and followed that with a section on comparative morphology, and another on the way in which mammal skin is adapted for life in different environments. On page 578 Sokolov points out: ‘It has been found that moisture evaporates twice as fast from fur as from a smooth surface, proving that fur does not prevent the evaporation of sweat.’

  Many species combine sweat-cooling with a hairy pelt. None of them suffers from mouldy skin.

  6 A new approach suggested in 1981 was based on the allometric analysis of hair density in primates. Allometry is the phenomenon by which, when species become larger in the course of evolution, not all organs of their bodies increase in the same
ratio as their overall mass.

  Allometric analysis shows that a monkey which is twice the size of another monkey does not have twice as many hairs to cover the increased surface of its body. In small monkeys like marmosets the hairs grow very close together, but increasingly massive primates have systematically fewer hairs per unit of body surface.

  The allometric theory proposes that since the first hominids were descended from apes and apes are larger than monkeys, Lucy’s ancestors would have had sparser hair and been less well protected against the direct heat of the sun than small savannah species like baboons and patas monkeys. Therefore, it is argued, they had to evolve an alternative system of keeping cool – that is, sweating – and that system ultimately became so effective that they were able to dispense with body hair almost entirely.

  The argument is ingenious, but it ignores two facts. Firstly, a covering of hair is a multi-functional mammalian asset; it would not become redundant just because one function (heat regulation) was being otherwise catered for. And more to the point, decreased density of hairs to the square inch does not necessarily involve decreased coverage. A gorilla is larger than a man and its hairs are farther apart, but it is certainly not more naked. The mountain gorilla in particular has a deep and luxurious fur coat. If a savannah ape had needed a better shield against the sun’s rays, its body hairs would have grown longer instead of shorter.

  7 It was Sherlock Holmes’s favourite axiom that when you have eliminated the impossible, whatever remains, however improbable, must be the truth. It is not to be expected that adherents of any of the preceding hypotheses will agree that their pet theories can be ‘eliminated’ from the enquiry, but the fact that there are so many of them proves at least that the question is still wide open.

  The basic question is Desmond Morris’s: ‘In what environment is nakedness at a premium?’ There would seem to be a simple and logical answer: it has to be the environment in which nakedness is known most frequently to have evolved. That environment is water.

  Charles Darwin was saying the same thing when he wrote: ‘Whales and porpoises, dugongs and the hippopotamus are naked, and this may be advantageous to them for gliding through the water; nor would it be injurious to them from the loss of warmth, as the species which inhabit the colder regions are protected by a thick layer of blubber.’ The possibility that this could have any relevance to the human condition was not raised by him.

  When Sokolov compiled his magnum opus on mammalian skin, Homo sapiens was one mammal not included among his hundreds of species, and when he wrote about adaptations to an aquatic environment he never counted how many of them could have applied to that illustrious absentee.

  Fur depends for its efficiency as an insulating agency on its ability to trap a layer of air next to the skin. In land mammals, when the fur becomes wet, that ability is normally impaired and its value as an insulator is lost. Small aquatic mammals like the water shrew depend on especially oily fur, so waterproof that in effect it traps a bubble of air around the body and its insulative value is retained. Fur seals and sea otters have specialised pelts – an inner coat very dense and soft, capable of trapping tiny bubbles of air, and an outer layer of longer, glossy guard hair.

  Larger and totally aquatic mammals, like whales, manatees and dolphins, have shed all their fur and replaced it by the fat layer more appropriate to an aquatic existence. Large and partially aquatic animals like seals and sea-lions which go ashore to breed, often in very cold latitudes, have of necessity kept the fur, which is the best insulator in air, and supplemented it with the layer of blubber which is the best insulator in water. But even among seals – especially those which have been aquatic for the longest time – the balance of benefit seems to tilt in the end towards hairlessness.

  The largest species, the elephant seals (bulls measure 17–19 feet and weigh up to 8,500 pounds) spend about forty days out of every year completing their annual moult. Their skin is thick, and the process of shedding the dead cells which continuously flake off from it is hampered by the fur. The hair comes off in large strips and patches still attached to the outer skin, giving them an exceedingly moth-eaten appearance. R. M. Martin described them as looking at these times ‘… as if they were coming apart at the seams’.

  For the second largest seal, the walrus, the attempt to hang on to a mammalian fur coat was obviously not worth the candle. It is completely hairless except for a rather splendid moustache.

  Because the largest aquatic mammals are the most naked, it is sometimes urged that our own ancestors would have been below the body size at which hair loss becomes the best strategy in water. But there are several examples which counter that argument. One or two species of river dolphins are comparable in size to humans. So is the marsh-dwelling tapir – the nearest thing we have to an aquatic horse. It is an excellent swimmer and diver, has a proboscis like an incipient elephant’s trunk, and its hair is very sparse. The most aquatic of the pig family, the babirusa – also smaller than a man – has no hair at all.

  Only two kinds of environment are known to be conducive to nakedness in mammals – a totally subterranean one, like that of the naked Somalian mole-rat, and an aquatic one. No one suggests that our ancestors ever lived in a hundred per cent subterranean habitat.

  There is one major barrier to acceptance of the idea that they may have lived at one time in an aquatic environment. The barrier is psychological rather than logical. It consists of the fact that the concept is new and bizarre and overturns at a stroke too many of the preconceptions we have grown accustomed to living with. In attempting to overcome that kind of resistance, reasoning alone in seldom effective in the short run. But the passage of time can work wonders.

  7

  Keeping Cool

  ‘Sweating is an enigma that amounts to a major

  biological blunder: it depletes the body not only

  of water but also of sodium and other essential

  electrolytes that are carried off with the water.’

  William Montagna

  In the above quotation Montagna was referring specifically to human sweating, which is in some respects unique. Some commentators speak of it as if it were an inexplicable burden we have to bear. Others praise it as an evolutionary innovation so brilliant that it was well worth losing our fur in order to improve its efficiency still further.

  Sweating appeared at a relatively late stage of mammalian evolution. The first mammals – which appeared during the age of the dinosaurs – were small, furtive creatures about the size of shrews, and animals as small as that do not need to sweat. If the temperature gets uncomfortably high, they pant, and the moisture evaporating from their lungs and the tongue and the lining of the mouth provides an adequate cooling system. A large number of present-day species, including small arboreal primates like the loris and the potto, do not sweat. Some animals in hot countries supplement sweating by spreading saliva over themselves to moisten their skin, and this cools the body surface as it evaporates.

  Homo sapiens appears to be the only land mammal which does not pant on getting heated. Gorillas and chimpanzees do it – not as spectacularly as a dog, with noisy breathing and lolling tongue, but their rate of respiration certainly increases. People pant during exercise in order to inhale more oxygen, but they do not pant to reduce their temperature when lying down in the sun. We resort to it only in situations where sweating would not work – for instance, when sitting in a hot bath.

  Later, as the mammals increased in numbers and diversified, some needed a special cooling system to supplement panting. For example, grazing animals have to spend long hours feeding in the open with no trees to shade them, so horses, sheep, cattle and donkeys all acquired the ability to sweat.

  Some water can seep out through an animal’s skin by diffusion, even when there are no sweat glands. But the amounts involved are very small, and inadequate for controlling body temperature. Water can pass out much more readily through tiny openings in the skin called pores. When th
e hooved animals began to sweat, they did not need to evolve special custom-made pores for the purpose: instead, they made use of the pores which had evolved to fulfil different functions. The history of evolution is full of such examples of physiological make-do-and-mend.

  These openings already served three purposes. They were the holes through which the body hair emerged. They were also used as channels for the secretion of sebum for lubricating the hairs: sebaceous glands are found in association with the hair follicles. Thirdly, they were used as channels for the secretion of the apocrine glands, tiny structures situated near the base of the hair follicles. It is generally agreed that their original purpose was scent-signalling. They were designed to exude an oily or waxy substance which in various species and various bodily sites is often odorous, sometimes pigmented, and may contain proteins as well as oily substances.

  Apocrines are highly adaptable. It is thought that even the milk glands of mammals are simply highly modified apocrine glands. In virtually all sweating mammals, with the exception of man, the so-called sweat glands are modified apocrines. The oily or waxy component has been diluted to a thin, watery emulsion, the consistency of skimmed milk. The glands are found all over the body in association with the hair follicles, and they respond to a rise in temperature by exuding this fluid onto the surface of the skin.

  No one has described this kind of sweating as a biological blunder. The loss of fluid is not excessive, and is finely attuned to the animal’s needs. In the case of the camel, for instance, only a thin film of moisture is exuded, to conserve water. In cattle, which use both panting and sweating to control their temperature, the evaporative heat loss through the skin is six times as great as the loss by panting, but at first this was not recognised because the sweat is not in excess of requirements and is not seen streaming down as it does in humans. Horses do sweat profusely when, for example, they are ridden at high speed, but that may be largely an emotional reaction. When at rest in a field even at high temperatures they do not stream with sweat.