Fat and the AAT/H

The feature that apparently started the AAT/H ball rolling is fat. Hardy claimed, and Morgan and others have echoed, that fat in humans is an aquatic adaptation, modified by convergent evolution to be similar to fat in seals and other aquatic mammals. They claim that it is bonded to the skin and not internally, as in our primate relatives. They're wrong.

On the subject of fat, Elaine Morgan has taken a route like that she's taken with salt and tears, finding a leading expert on the subject and then completely misrepresenting what that expert says. The expert Morgan uses here is Caroline Pond, who is accurately described as the leading authority on fat and its evolutionary significance in humans and other animals. (I've included excerpts from a few of Pond's works after this section; also check out her terrific 1998 book, The Fats of Life.) Morgan uses Pond's observations that humans are on the fat end of the scale compared to other mammals as her basis, but ignores Pond's observations that both quantity and amount of fat in humans is similar to that of captive monkeys if they aren't kept on a strict diet.

She also quotes Pond on the number of adipocytes (fat cells) but seems to not understand what Pond was getting at when she made this statement. This is the statement referred to when you see various AAT/H proponents talking about "10x as many fat cells" or, erroneously, "10 times fatter". The implication (sometimes said explicitly) is that we are 10 times fatter or necessarily have 10 times more fat cells than our primate relatives or some unspecified "savanna" animal. This is not what Pond was saying at all. Pond's article was aimed at pointing out that the usual models for human fat studies -- rats and mice -- are inappropriate because they have relatively large adipocytes which then expand and reduce as total fat goes up and down. Humans, as with other primates, have relatively small adipocytes and we tend to add more since they can't expand as much as those of rats and mice. This makes it harder to lose total fat compared to those rodents. Pond pointed out that this feature, relatively small and numerous adipocytes, is common to humans, fin whales, hedgehogs, monkeys, and badgers; Morgan ignored all but the "whales" part of that statement.

Pond has also pointed out that human fat distribution indicates that it was not part of an aquatic adaptation. Such an adaptation is seen in whales and seals, but not in humans. Morgan has said that fat was an adaptation for insulation in an aquatic environment. Pond points out that fat is not adapted as insulation, although she also points out that this idea is fairly well entrenched, even in many physiology texts. For one thing, the distribution of fat, even in arctic species, doesn't indicate insulation; the fat layer is quite thin in some areas of the body of all species, even ones, such as seals, which are around 50% fat. She also notes that the subcutaneous (just beneath the skin) fat is the first fat to be used up, even in winter, and even in arctic animals, which is the opposite of what would be expected if insulation were a major purpose for fat. It plays, she suggests, no more than a minor role. The major role of fat, she feels, is as a food supply; this fits with how it's used up. The reasons for the differences seen in fat distribution in different species seem to be for shaping. Humans have the shapes they have due to fat distribution due to sexual selection; this of course makes sense in any species where fat distribution differs between the sexes; females are also much fatter than males. These sex differences make no sense if both sexes are "using" that fat (quantity and distribution) in the same way, as we would have to be for it to be an aquatic adaptation shaped by natural selection via the principle of convergent evolution. In fat aquatic species like the whales and seals Hardy and Morgan incorrectly say we resemble, both sexes are shaped basically the same, and their fat distribution seems to be primarily for streamlining while swimming. Humans and these aquatic species are radically different in their fat distribution.

Another obvious and huge difference ignored by Hardy, Morgan, and other AAT/H proponents is that the aquatic mammals they say we resemble also have fat life histories which are radically from humans; they rapidly gain fat while very young, and at a very early age they are essentially like their parents in their fat distribution and quantity. We start off fairly fat, drop within a few years to the leanest condition of our lives as children, and then rapidly build up fat at puberty, with radical differences in quantity and distribution of fat between boys and girls, and to top it off, at middle age our fat distribution changes once again. These are classic telltale signs of a trait which has been shaped by sexual selection rather than by adaptation to our environment. All this is very unlike those aquatic mammals whose fat has developed in such a way as to help them deal with that environment. For these human fat characteristics to be due to an aquatic adaptation, we would have to be aquatic as babies, non-aquatic as children, aquatic again in puberty, and even more aquatic in our old age. And females would have to be far more aquatic than males, but only from puberty on. It just doesn't make sense as an aquatic adaptation, but it makes perfect sense as a feature developed as a result of sexual selection.

Also valid is the question of how likely it is that fat necessarily shows adaptation to an aquatic lifestyle. It does in seals and whales, animals which have been aquatic for tens of millions of years, where it shapes their bodies to streamline them for swimming speed. But does it necessarily show such an adaptation in aquatic animals versus their terrestrial relatives, as the AAT/H suggests? Studying bears answers that question, and Pond has studied a lot of bears. The polar bear is well enough adapted for aquatic life to be able to swim easily at enormous distances from land, diving and catching food underwater. They also show physical adaptation to aquatic life in their relatively small, streamlined head compared to their relatives (one of those ubiquitous aquatic adaptations absent in hominids I mention in the "Relevant Questions for the Aquatic Ape Theory" section). According to the AAT/H idea that fat differences between such animals are due to living in an aquatic environment either full or part time, one would expect polar bears' fat quantity and distribution to show differences from their terrestrial relatives. It doesn't. This is just one of many scientific tests of the AAT/H which it fails.

Another thing Pond emphasizes is that studies on living mammals suggest that fat (contrary to AAT/H claims) isn't an especially good insulator; these mammals rely on building a layer of stagnant air or water around there bodies rather than on their fat layer. Earlier studies by Scholander et al. which Morgan has used for her "evidence" (ignoring the later studies) were conducted with the skin of dead seals and polar bears, and of course do not accurately reflect the way in which living tissue attached to a living animal works. Later evidence using temperatures taken at the skin and the body core of seals show only about a 5 degree C difference -- helpful but minor compared to other factors. This fact has been pointed out to her.

Morgan has professed to being perplexed as to why human babies are so fat; she can't see any advantage to it outside of water. However, as anthropologist Jane Lancaster has pointed out, the fat built up in women for nursing, and that seen in babies, is most likely correlated with the massive brain growth seen in human infants. This brain growth is pretty much off the scale compared to other animals. So rather than the AAT/H proponents' surprise at this feature of human babies, one would be surprised not to find such a helpful feature to aid in this postnatal brain growth. It's also not surprising that it's unique among animals, since we also find our infant postnatal brain growth is unique among animals.

Finally, note that accurate measurements of fat in humans have mostly been on people who have plenty of access to food at all times, and this tends to make them fatter. Pond found that even northern Canadian natives who live on high meat diets generally are less fatty than Canadians who live in cities. It seems likely that measuring the fat of modern humans in food rich situations does not give a accurate model for the rudimentary tool-using populations of 5 or more million years ago. Even early Homo sapiens and Neanderthals would likely have more steady, fat rich food supplies than those earliest ancestors. Using us to model them, as the AAT/H does, is very unlikely to be accurate when it comes to fatness. Using a combination of us and our similarities to other primates, which Pond documents, makes sense, but does not support the AAT/H.

So, why can we get fat? This feature is shared with our primate relatives. Why this is a shock to AAT/H proponents is, frankly, a mystery to me.

Why does our species get fat? It seems the answer to that is because we can. Having a relatively large amount of fat -- that is, the amount of fat which we see in normal, non-obese humans -- is useful in an evolutionary context, as long as it doesn't impair you too much. It gets you by in times of little food, and provides a cushion, so to speak, in times of illness. This is true for all animals. But too much fat is a hindrance, and one of the biggest problems influencing this is trying to avoid predators. It's been found that animals in places with fewer predators (some islands, for instance) tend to get fatter than their relatives in more predator-rich areas. (Predators have been a relatively minor problem for humans since we developed the controlled use of fire and effective weapons around a million years ago.)

Caroline Pond has shown this in studies on reindeer, which back up other studies showing the same thing. This excerpt from her university's site tells the story:

"Separated from the mainland perhaps 40,000 years ago by rising sea levels, and free from the need to gallop swiftly or be eaten, the Svalbard reindeer have evolved into a sub-species which is fatter, shorter and, frankly, dimmer, than the mainland reindeer. They need to get fat because it provides them with a fuel supply to carry them through the winter, when even the poisonous plants, which Svalbard reindeer have evolved to digest, are in short supply.
"Norwegian mainland reindeer would probably like to get fat too, but this would make them inefficient runners, a luxury they cannot afford. "Having to run away from wolves keeps them slim," explains Dr Pond, who has made several trips to north Norway and Svalbard to study the reindeer in collaboration with scientists from Tromso University."
The Open University Alumni On-line Community, Open Eye On-line, "Reindeer" December 2000

1987 "Fat and figures", by Caroline Pond (Ph.D., Senior Lecturer in Biology at the Open University in Milton Keynes) New Scientist 4:62-66.

pg. 63:
"More than half the 31 captive monkeys that we examined were less than 5 per cent fat, thinner than most laboratory rodents, although all of them had continuous access to food and little opportunity to exercise. However, a few of the monkeys kept in the same way became obese, and at more than 25 per cent fat, were fatter than normal rats. Males and females were equally fat. In contrast, a typical "reference man" is estimated to be 15 per cent by weight adipose tissue, and a "reference woman" 27 per cent."

pg. 63:
"Thus most human beings are not only much fatter than most wild and captive mammals, but women and girls are consistently fatter than men and boys."

pg. 63:
"Both "normal" humans and exceptionally obese monkeys have large quantities of adipose tissue inside the abdomen, and the superficial depots around the limbs spread out to form an almost continuous layer."

pg. 63:
"The main differences between ourselves and other primates arise from the quantity of adipose tissue in humans. The distribution of human adipose tissue is similar to that of exceptionally obese monkeys. We don't need to postulate that special evolutionary forces are behind more than minor changes in the shapes of certain depots, such as the breasts (see Box, page 64)."

pg. 64:
"But, however you compare them, Homo is clearly the odd man out. In proportion to body mass, we have at least 10 times as many adipocytes as expected from this comparison with wild and captive mammals. Humans easily surpass such notorious fatties as badgers, bears, pigs and camels, and are rivaled only by hedgehogs and fin whales, in their deviation from the general trend, indicated by the regression lines on the figure."

pp. 64-65:
"But our recent observations on captive monkeys suggest that adipose tissue may grow in very different ways in rodents and in primates. The obese monkeys had more than 10 times as many adipocytes as slim monkeys of the same age, but the average sizes of the cells were similar in specimens of 5 per cent fat and those of 25 per cent fat. The fatter monkeys simply had more adipocytes; increases in the volume of cells contribute little to the growth of adipose tissue. The cellular mechanism of growth of adipose tissue in monkeys is thus fundamentally different from that of young laboratory rodents. Adipocytes probably do not normally expand more than about fourfold, but there is no obvious upper limit to the numbers of new cells that can be formed. Perhaps that is why the fatness of well-fed monkeys and humans is so much more variable than that of laboratory rodents, and why we primates are more susceptible to massive obesity than rats and guinea pigs."

pg. 65:
"On present knowledge, fin whales, hedgehogs, monkeys or possibly badgers would be better than rats as animal models of human obesity, because they develop relatively large numbers of adipocytes."

pg. 66:
"The study of other naturally fat mammals suggest that the anatomy of human adipose tissue, which contributes so much to the figures of both men and women, is not unique, nor necessarily a relic from an aquatic past (see Box)."

1987 "Not an aquatic ape -- just an exceptionally fat mammal", by Caroline Pond (Ph.D., Senior Lecturer in Biology at the Open University in Milton Keynes) New Scientist 4:64-65.

pg. 65:
"They think that the hair, skin and superficial adipose tissue of humans evolved into "blubber", similar in function to that of seals and whales. Let's consider the facts about the quantity and arrangement of adipose tissue in some aquatic mammals. In specialized aquatic mammals, such as whales, seals and manatees, the limbs are reduced or absent and the trunk is smooth and tapered. But whales and seals are not always exceptionally fat. Massively thick limbs or a bulging abdomen would spoil the streamlining. Adipose tissue around the guts and kidneys is greatly reduced. In all but the most emaciated terrestrial mammals, the mesentery that holds the gut in place is clouded with adipocytes. But in seals, even those that are 50 per cent fat, there is so little adipose tissue in the mesentery that you can read a newspaper through it. The superficial adipose tissue spreads over the trunk forming a continuous layer of blubber that may facilitate rapid swimming by acting as a shock absorber in turbulent water."

pg. 65:
"Less specialized aquatic mammals, such as otters, have elongated bodies and webbed feet, and but the limbs are relatively long limbs [sic] and they can travel some distance over land. The distribution of adipose tissue in otters is almost identical to that of their terrestrial relatives. No one can claim that the limbs and trunk of humans have evolved further towards fully aquatic habits than those of the otter. Why should humans have adipose tissue like that of a highly specialized aquatic mammal? Anyhow, as most of us know only too well, there is plenty of fat inside the human abdomen. Badgers, hedgehogs and hamsters are among the few common wild mammals that normally accumulate large quantities of fat. In such species, the superficial depots enlarge disproportionately as they fatten, so that specimens above about 15 per cent fat -- the same fatness as "slim" humans -- seem to have an almost continuous layer of adipose tissue between the skin and the muscles. In other words, humans are just one example among several naturally obese mammals in which the superficial adipose depots are relatively massive. We can explain the contrasts between the arrangement of adipose tissue in humans and rats without postulating an aquatic ancestry for humans. The apparent shift in distribution is a direct consequence of the much greater abundance of adipose tissue in modern human beings. We would expect women and girls to have proportionately more of their adipose tissue in superficial depots than boys and men because, unlike most wild mammals, human females are usually fatter than males of the same age and lifestyle."

Pond, Caroline M.
1991 "Adipose Tissue in Human Evolution", pp. 193-220. The Aquatic Ape: Fact or Fiction? Edited by Machteld Roede, Jan Wind, John M. Patrick and Vernon Reynolds. Souvenir Press: London.

pg. 193:
"The sparse data on the 'natural' distribution and abundance of adipose tissue in primates show that the basic anatomy of human adipose tissue is similar to that of terrestrial monkeys, and so was probably inherited directly from their primate ancestors."

"Anatomical, ecological and biochemical information provides no evidence that the distribution of adipose tissue in modern humans has evolved as an adaptation to thermal insulation, as required by the Aquatic Ape Theory, or as protection from mechanical damage."

pg. 206:
"These data do not support the hypothesis that the large quantities of adipose tissue in superficial depots are adapted to function as thermal insulation. The biochemical properties of the subcutaneous adipose tissue also suggest that they contribute little to passive insulation in the arctic species (Pond et al., 1991b)."

1992 "Allometry of the distribution of adipose tissue in Carnivora", by Caroline M. Pond (Ph.D., Department of Biology, The Open University, Milton Keynes) and Malcolm A. Ramsay (Department of Biology, University of Saskatchewan, Saskatoon). Canadian Journal of Zoology 70:342-347.

pg. 342 (from abstract):
"There is no evidence for adaptation of the gross anatomy of the adipose tissue of polar bears to their semi-aquatic habits or arctic habitat."