13 Body Hair Reduction and Tract Orientation in Man: Hydrodynamics or Thermoregulatory Aerodynamics?
Peter E. Wheeler (16 pages)

SUMMARY

The alleged hydrodynamic advantages of both the sparsity and the alignment of human body hair are frequently cited in favour of the theory that some early hominids lived in water. This hypothesis can be challenged on thermoregulatory grounds alone. It is extremely unlikely that naked-skinned hominids  would be energetically viable as aquatic mammals, because of the large surface area to volume ratio resulting  from their relatively small size and elongate body form. Alternatively, both the sparsity and the orientation of human body hair can be satisfactorily explained as thermoregulatory adaptations evolved by a large savannah primate to facilitate essential evaporative cooling, In this context, by opposing the direction of natural convection current rising up the body of  a bipedal hominid, the hair tracts may have maximised  airflow over the wetter skin surface during the evolutionary period when body hair was being progressively reduced.

INTRODUCTION
Two ways we differ from nhups: sparsity of hair and the way the follicles are arranged. Most clearly seen in the covering of the body hair, or lanugo, developed by the human foetus in the 6th month but shed before birth.
Both key factors in Hardy's original theory. Although it is not clear now whether the aquatic ape was fresh water or marine.
The idea that hair loss would have helped in hydrodynamics only works in relatively fast swimming. "However, despite these potential hydrodynamic advantages, there would have been other physical factors in operation which would make it extremely unlikely that the human naked skin could have evolved in water." (p 223)

THERMOREGULATORY PROBLEMS OF AQUATIC MAMMALS
Animals get cold in water. The longer the stay there the more likely they are to suffer. The more energy they will need. The bigger the surface to volume ratio the more this is true.

ENERGETICS OF HOMINIDS IN WATER
Humans die quickly in cold water. Juveniles and infants more so.
Assuming there was an aquatic ape - adapted to water - so thick sc fat and short limbed - how would they have adapted to the savannah.

THERMOREGULATORY HYPOTHESIS
Naked skin + sweat glands provides humans with the most effective whole-body cooling system known.
Hominids particularly sensitive because of their large brains but lack of carotid rete.
Defends counter-arguments - savannah animals have not lost their fur and that shaving a sheep causes it to overheat in the sunshine. - the second point answered by bipedalism.
Full benefit of nakedness only comes with sweat glands and bipedal posture.
This also might explain the hair tract orientation too.

CONCLUSION
AAT not needed to explain sparsity of hair or the orientation of them. Ends by speculating that hair loss probably came after bipedalism because the orientation appears to benefit cooling whilst standing.
 

COUNTER-ARGUMENTS
1) Wheeler makes the classic mistake of assuming that the AAH propose a water-living mammal. ("some early hominids lived in water") The whole basis of his argument is therefore flawed from the start. The AAH never claimed that and certainly I do not. In fact, a limited amount of aquatacism is a far, far better means of thermoregulation than the very expensive notion that we were savannah animals designed to sweat to keep cool. Sweating only makes sense if you happen to live by a guaranteed supply of fresh drinking water.

2) He assumes also that even AAH proponents accept that humans evolved on the savannah in the later stages. This would indeed have provided an obstacle to the 'aquatic ape.' But what if humans never generally exploited the savannah at all? What if it were only a few isolated races that did so, later?

3) His thermoreg hypothesis of bipedalism is self-defeating. It relies on grassland, specifically. No trees to block the breeze at shoulder height but some grass to block the breeze at knee height - otherwise why go bipedal. The sweat cooling must be fuelled by water but next to water you find trees. Plus what were they doing, wandering around in the mid-day sun? Looking to pick carrots?