12 Adipose Tissue in Human Evolution
Caroline M. Pond (The Open University 27 pages)

SUMMARY

The sparse data on the ‘natural’ distribution and abundance of adipose tissue in primacy 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. Superficial  adipose tissue  appears to extend over a greater area of the body in humans than in of her terrestrial mammals because of changes in the proportions of the limbs and in the shape of the girdles, the dorso-ventral flattening of the thorax and abdomen and the bipedal posture of  the hip, knee and shoulder. The contrasts between humans and of her primates have parallels in of her mammals, and may be a direct  consequence  of the increased abundance of adipose tissue, which itself may be of very recent origin. Experimental and comparative data on the physical properties and natural functions of  adipose tissue in mammals are reviewed in this chapter. Superficial  adipose  tissue does not necessarily contribute significantly to thermal insulation except in specialised aquatic mammals 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 Of her explanations for the exceptional features of human adipose tissue are briefly discussed. The greater fatness and modifications in the distribution of adipose tissue in young women are more likely to be conspicuous in indicators of social and sexual status than an adaptation to energy storage for reproduction. The accumulation of mesenteric and omental adipose tissue in men and older woman has no parallels in wild mammals and may not be physiologically adaptive.  
INTRODUCTION
Adipose tissue is often cited in many theories about human evolution but the truth is little is known because few studies have been made.

SOURCES OF INFORMATION ABOUT ADIPOSE TISSUE
Not fossilised so we rely on comparative data. Skinfold thickness has traditionally been used but this is notoriously inaccurate. Bodies are usually ailing or elderly. Only recently has the quality of data improved. Also artefacts and drawings from early modern humans - eg Venus figurines.

THE COMPARATIVE ANATOMY OF ADIPOSE TISSUE IN MAMMALS
Mammalian deposits of adipose tissue are not well known but as data is being gathered they seem to fit a generally similar pattern.

THE EFFECTS OF ADIPOSE TISSUE ABUNDANCE ON ITS DISTRIBUTION
"The exceptional features of the anatomy of human adipose tissue are a direct consequence of the changes in body proportions and in its total abundance. The modern condition can be explained without postulating a major evolutionary shift in the distribution of adipose tissue, as required by the AAT. Three major features peculiar to humans are:
1) Avg fatness of humans greater than monkeys.
2)Superficial adipose tissue more conspicuous because of hair reductions.
3) There are substantial differences in both abundance and distribution that is associated with puberty.

 THE FUNCTION OF ADIPOSE TISSUE IN HUMANS AND OTHER MAMMALS
Long term energy store. Mass versus need to move quickly are important constraints.
Protection around delicate organs - a secondary role.
Superficial adipose tissue as a thermoregulatory device and to protect tissues from damage "are two of the most firmly established of all theories in biology."  -almost invariably without supporting evidence. 'Insulation from the cold' is often cited as a reason for fat.
Still air/stagnant water surrounding the surface are effective insulators. Blubber is overrated as an insulator - whales are leaner than we think. .Only pinnipeds have substantial amounts of fat which seem plausible for this purpose.
Many hibernating mammals accumulate fat - as energy store more than as thermo device.
Biochemical data is not supportive of the thermo explanation either.

Distribution of fat is not significantly different in humans and nhups - again suggests not adaptive to water. Eskimos are leaner than most Europeans and heat loss in cold water is more dependant on body shape and musculature than fatness. Infants, despite being particularly fat are still vulnerable to heat loss.
Fat as a protector - but why then mainly in children and women?

SEX DIFFERENCES IN THE ANATOMY OF HUMAN ADIPOSE TISSUE
Sex differences in abundance
Sex differences in distribution

EXPLANATIONS: SEXUAL SELECTION AND LIFE HISTORY STRATEGIES
Adipose tissue in women
Adipose tissue in men
 

 COUNTER-ARGUMENTS
1) Her assumption is that AAT needs sc fat as a thermoregulatory device. This is probably a misconception due to the analogy Hardy originally made with the blubber of fully aquatic mammals. Two points emerge here - firstly most of those aquatic mammals either live all their lives in the water - which is something the AAH has never claimed, or they live in Polar regions, a long way from where the aquatic ape might have thrived. With this in mind, going for a dip was more likely have been a way of keeping cool. Adipose tissue, when seen from this angle, is not so much a thermoreg device as a buoyancy aid.

2) Her attempt to equate human adipose tissue with that of other primates disguises the fact that human babies are very much fatter than any other mammal, let alone primates. 15% fat in infants is not something she has given an adequate explanation for. Bouyancy would seem a logical explanation for an ape that lived in a waterside habitat and regularly had a risk of drowning. This surely should have been one of her 'special' points.

3) An astonishing absence of discussion of fat as a buoyancy aid. 90% as dense as water. Surely a major topic in a paper on fat and the AAT.

4) Her discussion of sexual differences in adipose tissue, although interesting, does not have a great deal of bearing on the AAH debate.

Fat floats. It would have acted primarily as a buoyancy aid to semi-aquatic ancestors, particularly infants. I suggest females may have had more requirement than males because they were likely to be weighed down with the infant and so needed extra buoyancy themselves.