The Straight Dope on the aquatic ape hypothesis, courtesy of bibliophage.
Well, all righty then. Thanks!
Guess I should stick to Rudyard Kipling’s stories of “how the leopard got its spots” and such if I want to post more accurate info… 
I’m a grad student in evolutionary anthropology and a long-time reader of the Straight Dope, but a brand-new member of this discussion board. From what I’ve read, Cecil’s right about why we stayed hairless even when we moved into cold northern climates: by then, we had fire and clothing. But I think he missed the most logical reason why we became hairless in the first place: thermal regulation on the tropical steppes of Africa.
Heat, as we all know, rises upward. Quadrupeds, due to their horizontal posture, have a much larger percentage of their body’s surface area pointed upward than do bipeds like us; thus, they cool down more quickly on chilly nights (even in Africa, it drops into the 40s at night, especially in open country). Quadrupeds are also more efficient at dissipating excess heat on hot days, for the same reason. From this standpoint, it makes sense that human-sized (or even larger) quadrupeds like big cats and antelopes would still have fur. Eventually though, even for quadrupeds, fur becomes a liability; elephants and rhinos don’t have it. For a biped, the size at which fur becomes a liability would be much smaller because we dissipate most of our heat from only the tops of our heads, not our whole back-sides. So, we have really thick “fur” on the tops of our heads for insulation, but are otherwise naked for efficient dissipation.
Interestingly, the loss of fur also explains why the pale white skin of chimpanzees (under their fur) initially became dark brown in African humans. Melanin, in addition to reducing sunburn, prevents overproduction of vitamin D by making skin less translucent. When humans migrated into colder, cloudier latitudes with a lower sun angle, we re-evolved pale skin so we could make just as much vitamin D with much less solar radiation.
Some confused points here. First off, hot fluids (i.e. air, water) do rise upward, due to the nature of convection and gravity. Hot fluids are less dense than their corresponding cooler elements at the same pressure, ergo, gravity pulls the denser fluid downward, which pushes the less dense fluid upward. That is called “free convection”. So an animal is putting out heat into the air, that hotter air is slightly less dense, and thus rises away from the body. But that is not the same thing at all as saying that heat leaves the body preferentially from the top surface, or the upper part of the body.
Heat transfer occurs through three methods: convection, conduction, and radiation. Convection happens via conduction to the surrounding fluid (i.e. air) and then the migration of that fluid layer away from the body, bringing additional fluid in contact with the skin, to get heated. This process is not confined to the top portion of the body. Rather, it is a function of blood vessel size and distrubution, and blood pressure. It is also a function of metabolic process of the tissues in the area, how much energy is being generated by the body by activity level.
Conduction occurs via direct contact, typically with the ground. When upright, most animals including humans have fairly small contact points with the ground. Lying down is a substantial increase in surface contact. Thus the utility of things like piles of leaves and grass to form insulating layers in bedding.
Radiation is a factor of the surface temperature of the object in question, and surface properties. IIRC radiation is pretty consistent around the human body, again depending upon blood flow to the region (i.e. surface temp). In cold climates, the body constricts blood flow to extremeties to conserve it at the core. This is not really an issue in equitorial Africa during the day.
Second, I am unaware of humans that sleep standing up. Ergo, at night when the temperatures even in Africa drop, humans are not oriented with their heads up and their torsos upright. Rather, they tend to be prone, or curled on their sides.
I dispute this. For reasons stated above, please justify how body position provides better heat dissipation.
Why is it important for humans to have insulation on the tops of our heads, where supposedly we dissipate most of our heat? Wouldn’t we want less insulation there, in order to dissipate better, since that is where the heat is supposedly easiest to dissipate? Why do we need better dissipation in the areas where it is supposedly least effective?
Remember that insulation protects you against heat as well as cold. Tropical animals aren’t furred to just to keep warm, the fur also protects them against the sun.
Well, sure, it’s not “mainstream” and it’s not very Politically Correct. But the thing about Harris is that she has a history of coming up with ideas that go wildly against conventional wisdom, but turn out to be right. Her landmark book “The Nurture Assumption” turned the whole field of developmental psychology upside down by demolishing the research purporting to show that parenting has much effect on how kids turn out.
If this field interests you, you really must read her book (second edition). But the unfairly short version is: roughly half of your adult personality is determined by your genes. The other half is due to something else, but not how your parents raised you. Harris makes a strong case that the “something else” is the child’s peer group. Fascinating.
Anyway, the Mormons tell us that the Garden of Eden is in Missouri (pronounced “Misery” by the old timers), and if you can’t take the word of the Mormons . . .
So little body hair, eh? Two words: Ron Jeremy.
Here’s Judith Rich Harris’s very interesting article on her idea about parental selection, hairlessness, and skin color: Judith Rich Harris: Parental Selection: A Third Selection Process in the Evolution of Human Hairlessness and Skin Color