An ape living on sea food would have been comparatively well placed to adjust to a saline environment. As a primate it possessed eccrine glands in greater numbers than non-primate species, and they were already structured to exude salt water. All that was needed was an increase in the output of the glands and a further increase in their number. The instinct for responding to ‘salt hunger’ could well have been lost at this stage. Sodium deficiency would not have been a danger that needed guarding against.
As the land-locked sea continued to shrink and become saltier, the aquatic apes would have been driven to retreat south along the water courses of the Rift Valley. By the time they were ready to return to terrestrial life, apocrine sweating was no longer an option for them: they had lost their apocrine glands. The eccrines were drafted to fill the gap, needing only two modifications. They have been adapted to respond to heat instead of salinity and to keep the leakage of sodium within more acceptable limits.
9
Fat
‘An exceptionally obese monkey resembles a lean human being.’
Caroline Pond
In 1930 a young marine biologist called Alister Hardy returned from an Antarctic expedition and read a book by the eminent anatomist Professor Frederick Wood Jones. In it he came across the following passage:
The peculiar relationship of the skin to the underlying fascia is a very real distinction, familiar enough to anyone who has repeatedly skinned human subjects and any other member of the Primates. The bed of subcutaneous fat adherent to the skin, so conspicuous in Man, is possibly related to his apparent hair reduction; though it is difficult to see why, if no other factor is involved, there should be such a basal difference between Man and the Chimpanzee.
Hardy was forcibly struck by what he read because he knew from first-hand experience that just such a layer of fat, bonded to the skin, was a common characteristic of most aquatic mammals.
These facts were, of course, familiar to biologists. But it took a giant leap of Hardy’s imagination to bridge the gap and arrive at the daring conclusion that the ancestors of Homo sapiens may have acquired this fat layer in the same way as the whale and the seal and the penguin and the dolphin and the hippopotamus – as an adaptation to an aquatic environment during some part of their evolutionary history. He felt that such an interlude would at the same time account for other unexplained features of the human anatomy. The fat layer was thus one of the earliest of the arguments advanced against the savannah hypothesis. It has never seemed very plausible that being fatter could be an advantage to hominids in a savannah habitat. Whether chasing prey or running away from predators, the extra weight would be bound to slow them down.
The fat layer, then, is a mystery, but a mystery made easier to ignore by the wide disparity between individual human beings. Some are fat and some are not fat. There is a tacit assumption that the leaner ones are the norm, and illustrate the way Nature always meant us to be. If some humans exceed the ‘norm’ it is assumed they are too greedy or too lazy or suffering from some metabolic disorder. Fatness is never acclaimed as a human attribute in the way the large brain is acclaimed. It is spoken of as if it were a pathological departure from the true human blueprint, as something to be treated and, if possible, ‘cured’.
First, therefore, it is necessary to establish that subcutaneous fat in Homo sapiens is a specific evolutionary development and not simply a punishment for eating too much. One way of doing this is to consider the case of a human who cannot be accused of gluttony, that is, a new-born baby.
Around the thirtieth week of gestation, the growth pattern in a human foetus begins to change. The hitherto rapid growth of the bones begins to slow down, while resources are concentrated on the accumulation of fat. Some of the fat is deep inside, in standard mammalian sites such as that around the kidneys. But it also accumulates under the skin all over the body, in a way uncharacteristic of land mammals.
Between weeks 30 and 40 the amount shoots up from 30 grams to 430 grams. At full term, fat constitutes sixteen per cent of body weight in a human baby, as compared with three per cent in a new-born baboon. The more fat the baby has acquired by the time it is born, the greater its chances of survival. Moreover – like brain size – the fat deposit sustains its rapid growth rate for several months after the birth. This – more than brain size and just as much as hairlessness – is what distinguishes the appearance of a human baby from the infants of the other primates. A week-old human baby is naked and plump; a week-old gorilla or chimpanzee is hairy and, by comparison, cadaverously thin.
The laying down of this fat layer in the baby imposes an extra strain on a human’s physical resources during late pregnancy and lactation which other primates do not have to sustain. The percentage of lipids (fat) in the mother’s blood goes up by more than 50 per cent to facilitate the transfer of food resources to the growing foetus. To replace these resources she needs to increase her own food intake by fourteen per cent in the last stages of gestation, and by up to 24 per cent while breast-feeding a growing infant. If she does not do so, the reserves of fat (and calcium, and whatever else is in short supply) will be drawn from her own body. Nature is almost always on the side of posterity, so that even if there is a food shortage during pregnancy, it is unlikely to reduce the birth weight of the baby by more than ten per cent.
However, if the woman were seriously undernourished, the chance of her giving birth to a viable infant, or herself surviving the pregnancy, would be infinitesimally small. This contingency is guarded against by ensuring that if a woman’s fat reserves are below a certain level she will not conceive. In a 16-year-old girl, fat tissue constitutes around 27 per cent of body weight; if it drops below 22 per cent, menstruation will not begin and, if it has begun, it will cease.
This does not only apply to women and girls who are anorexic, or have become ill and debilitated through severe malnutrition; it also applies to healthy, active females if they happen to be ballerinas or athletes on a regime of eliminating all ‘surplus’ fat. If and when their weight is allowed to go up, the periods normally recommence.
It seems clear, then, that a percentage of fat well in excess of the primate norm is not merely non-pathological in our species: it is essential to the survival of the human race.
Today in the developed countries few people need to worry about the minimum fat level which natural selection has laid down for our species. But much anguish is caused by its apparent failure to lay down a maximum level. If a horse, a wolf, a cheetah or a kangaroo is kept in captivity with little or no opportunity for exercise and given more food than it needs, it may put on some fat. But it will not treble or quadruple its body weight. A non-human primate kept in captivity may develop a rounder abdomen, but it will not develop bloated cheeks, fat buttocks, flabby arms, a large bust and fleshy thighs.
Humans are liable to develop these characteristics because one of the main depots in Homo sapiens is just under the skin. In most mammals the main depots are internal, so their expansion is limited in extent by the body wall or the rib cage; the result is also less visible. But our skin is so elastic that it imposes virtually no limit on the amount of fat that can accumulate there.
Another notable difference is the number of adipocytes we are endowed with. Adipocytes are the cells which contain the fat. When they are empty they are virtually flat, but each one is capable of swelling up, becoming spherical, and expanding to three times its original size without bursting. Hence a major factor governing how fat we can get is the number of adipocytes we possess.
A survey was carried out of 191 different mammal species. It was found that carnivorous mammals in general have more adipocytes in proportion to body mass then herbivores. But the most remarkable feature of the final report was summarised by Caroline Pond, senior lecturer in Biology at the Open University in Milton Keynes:
However we compare them, Homo is clearly the odd one out. In proportion to body mass, we have at least 10 times as many adipocytes as expected from this co
mparison with wild and captive animals. Humans easily surpass such notorious fatties as badgers, bears, pigs and camels, and are rivalled only by hedgehogs and fin whales in their deviation from the general trend.
The hedgehog and the fin whale exemplify two kinds of mammal which need extra adipocytes, namely, the hibernators and the aquatics. No one would seriously argue that we acquire our ten-fold allowance of those cells because our ancestors went through a stage of being hibernators. Hibernators only get fat seasonally, just before retiring for their winter sleep, whereas fat is with us all the year round. And if we cut out the hibernators, the picture is clear. An excess number of adipocytes is among the characteristics which man shares with aquatic mammals.
There must be a reason for the existence of these cells – about twenty-five billion of them, ten times as many as in other land animals of our size, and ten times as many as we presently need. They must have been acquired because at some stage of our evolution they were useful or necessary. The unavoidable conclusion is that at some stage in the past our ancestors were considerably fatter than we are today.
Nowadays people who are ‘overweight’ are often made to feel a kind of guilt, as if they were degenerate heirs of an endless line of lithe and slender ancestors – as if they had somehow betrayed that inheritance by allowing themselves to go to seed.
They have not betrayed their inheritance. Their inheritance has betrayed them. They come into the world with a capacity to become obese not shared by other primates. If the excess adipocytes were not present in their bodies they could not be filled up with fat. If we had only one-tenth of that number, then once those receptacles were full any further excess calories would perforce be burned away by a rise in temperature, or excreted as fat in the faeces or as sugar in the urine, and not added to the deposits on the belly or buttocks.
Cheerleaders in slimming clubs are apt to prescribe for their clients a certain number of kilograms as representing the amount they ‘ought’ or ‘were meant’ to weigh. They urge weight watchers to aim at the target and become slim as Nature intended. But the sad fact is that ‘Nature’ is not on the side of the slimmers. Nature decrees that when they have lost the first half-stone, their metabolism slows down and calories are burned up at a slower rate, which makes it that much harder to lose the second half-stone. If they achieve their target they are entitled to take credit for having succeeded not with Nature’s aid, but by dogged determination in spite of their unhelpful genetic blueprint.
No one can yet be sure why some people put on weight while others on a similar regimen do not. All we can say is that in our own species the mechanism for controlling weight is unreliable in its operation, in much the same way as the mechanisms controlling salt balance, and regulating temperature. In these respects our bodies do not respond as promptly and as appropriately to our present-day needs as the bodies of most animals do.
Another penalty we pay for the possession of our subcutaneous fat layer was described in 1955 by Peter Medawar who referred to it as ‘… the appalling ineptitude of wound healing in the human skin’.
In mammals generally the skin is very loosely connected to the body wall. Anyone patting a spaniel is aware of how easily its coat slides to and fro over the underlying flesh. Mammals also have muscles in the skin so that they can move it independently, as a horse does when it twitches its skin while pestered with flies. Humans have almost entirely lost these intrinsic skin muscles – except in the face where they enable us to change our expression.
When tom cats retire to lick their wounds after a fight, they may have numerous lesions in the skin, but, once the bleeding has stopped, the edges of the cuts lie side by side, a thin film of skin epithelial cells migrates inwards and the skin edges are drawn together by a process of contracture until they meet. Healing occurs very quickly, and without permanent scarring. In a rabbit, for example, an area of missing skin up to a hundred square centimetres, which in humans would necessitate a skin graft, will be naturally repaired and heal so that the scar is almost invisible.
In humans the skin is bonded to the layer of fat, which often prevents the edges of a cut from uniting. We speak of a man’s cheek being ‘laid open’ or of a ‘gaping wound’ in the thigh, and the thicker the fat the wider the gape. This is the phenomenon which impressed Professor Wood Jones when he dissected cadavers, and Alister Hardy when he watched a seal being cut up. To many a surgeon obliged to operate on an obese patient, the comparison with blubber must seem uncomfortably apt. Medawar comments:
The upshot of this new anatomical arrangement is that contracture, far from being an efficient mechanism of wound closure, has become something of a menace; it constricts, disfigures and distorts, and yet may very well fail to bring the edges of the wound together. As a mechanism of wound healing, the contracture of human skin is therefore as archaic as the vermiform appendix. We only become aware of it when it leads to harm.
Nowadays we are seldom aware of it because it is standard practice to go to a doctor to have some stitches put in or, in severe cases, for a skin graft, but for primitive man an untreated wound could cause a seriously disabling injury by failing to heal, or by gathering up and scarring in such a way as to constrict the blood supply or immobilise a limb. Marine animals would seem to suffer similar disadvantage. Manatees in Florida have had their habitats invaded by pleasure boats, and their skins are sometimes sliced into by the blades of underwater propellers. The ones who manage to survive the experience show, not the narrow trace of a healed cut, but a broad white scar where the pigmented outer layer of the skin has failed to come together.
Medawar’s final comments strike a familiar note: ‘What compensatory advantage the human being gets from the novel structure of his skin is far from obvious, though it is hard to believe there is none.’ The compensations on land are indeed obscure, but for aquatic mammals at least two possible advantages of the fat layer can be readily detected – insulation and buoyancy.
The subcutaneous fat layer, while in air it provides less efficient insulation than a coat of fur, is extremely efficient as a protection against heat loss in water. P.F. Scholander and his colleagues published a study of body insulation in some arctic mammals in 1950. They studied a seal (Phoca hispida) and a polar bear. The seal, which spends most of its time in the water, has a thin hair covering and a thick layer of blubber. The bear, which spends most of its time on land, has a thick fur coat but, except when preparing for hibernation, not much fat under its skin. When the polar bear moved from zero degree air into the water, it lost body heat 50 per cent more rapidly than on land. When the seal did the same, it lost heat only five per cent more rapidly than on land.
In order to serve this purpose of insulation in water, the adipose tissue in aquatic animals not only constitutes a higher proportion of the body tissue, it is also differently distributed. In aquatic species, by comparison with land animals, the tendency is for fat at the internal sites – around the kidneys and intestines – to be diminished, while the superficial (that is, subcutaneous) fat deposits are disproportionately enlarged. For example, in some land mammals such as the horse, 50 per cent of the fat in the body is found in the abdomen. In some species of seal the fat at this particular site – the mesenteral deposit – has virtually vanished, whereas the blubber under the skin is very thick.
In humans the redistribution has not gone as far as in whales and seals, but it appears to have moved quite a long way in that direction. We retain a supply of internal adipose tissue, but the subcutaneous deposits are greatly expanded. In species like cats and dogs the ‘fat’ under the skin amounts to little more than a thin film of cells serving as a lubricant, enabling the skin to slide easily over the muscular tissues beneath. Thus, an attacking enemy or predator is more liable to end up with a mouthful of fur and skin rather than inflicting deeper damage.
In humans, by contrast, it has been calculated that on average, between 30 and 40 per cent of all the fat in our bodies is located just under the skin. Caroli
ne Pond, who has added greatly to our understanding of the anatomical distribution of adipose tissues in mammals, once wrote: ‘It is difficult to see what mechanical or thermoregulatory function the subcutaneous fat on human limbs could serve. By increasing the inertia of the limb, it could actually hinder movement.’ In water, as an insulator, it would make sense.
Another advantage of fat to an aquatic animal is that it gives buoyancy. Of our body tissue, fat and lean have different densities, so that while they weigh the same in air, they do not weigh the same in water. Drop a piece of lean meat into water and it sinks; a piece of fat will float. Swimmers are not among those athletes who strive to shed every surplus pound in order to excel at their particular sport. Among aquatic mammals it is notable that a surface feeder like the white whale has fifty times as much blubber as it would need solely for insulation. The walrus does not need so much buoyancy because it gets its food from the sea bed, so though it lives in the same chilly latitudes its fat layer is much thinner, and varies with the seasons and the food supply. That is why a dead whale floats and a dead walrus sinks.
Thus, the same fat which could be a burden to a land ape, hindering locomotion and incurring extra energy costs, would be a benign development in an aquatic one. It would convey a saving of energy costs both by reducing heat loss and by enabling the animal to stay afloat more effortlessly.
The Scars of Evolution Page 11
