The purpose of sweat is to liberate excess heat. That much is obvious, since we sweat when we are hot. But why is it that no other primate builds up heat in its body to the point of necessitating such an efficient cooling mechanism? Nor do other mammals sweat for that matter, the horse being the only other mammal species that produces large amounts of sweat to cool down.
The most widely held explanation for human sweating is decades old: it is part of the Savannah Hypothesis, which proposes that major physical adaptations occurred about 4-6 million years ago when hominins moved from their forest-dwelling environment to the open savannah on the plains of Africa. This transition, so the theory goes, introduced a major heat stress due to the action of the sun and encouraged three major evolutionary responses to it: bipedalism, loss of body hair, and the emergence of sweat glands. Bipedalism is thought to have reduced the amount of body surface area exposed to the noon-day sun, while the loss of body hair made evaporative cooling from the skin possible, and favoured the emergence of sweat glands. The problem with the Savannah Hypothesis is that there is no explanation of why we were unable to contend with this heat stress – without tremendous physical adaptations – while other animals on the savannah appear to get by just fine without them.
Around 2008, I came up with the hypothesis that it was the evolution of a subcutaneous fat layer that triggered the evolution of sweat glands in humans. (This is the first time however, that I am sharing this hypothesis). A layer of subcutaneous fat, I propose, acts in essence like a blanket over the body’s heat generators – our muscles – thereby effectively containing the heat we then need to get rid of through sweating. But why do we have subcutaneous fat in the first place? No other primate does. The only mammals that have appreciable quantities of subcutaneous fat tend to be those that are in a sufficiently cold environment where insulation is required: aquatic and semi-aquatic mammals, mammals that live in northern regions, and those that live at high altitudes. Two notable exceptions exist, humans and members of the pig family. And it is noteworthy that pigs have a coat of hair as well as subcutaneous fat – two potent insulators – but ineffective sweat glands, leaving them with relatively poor heat tolerance. As a result, wild pigs are either nocturnal, or tend to move about during the day under the shelter of brush or tree cover. All other species of mammals store fat viscerally (ie. within the abdominal cavity, around organs like the kidneys, and the intestines). The primary function of this fat is as a source of energy, and to some extent cushioning, but not as a means of insulation. Storing fat internally allows them to dispel heat sufficiently well through a combination of panting, and heat radiation through either the fur, or thick skin layer they might possess. In mammals possessing fur, it can seasonally adjust in length to compensate for temperature variations. It is clear to me that given that the subcutaneous fat layer functions as an insulation mechanism, it goes completely against the Savannah Hypothesis’ argument that we actually lost our body hair because we were too hot.
Why did humans evolve this subcutaneously fat layer, and at what stage of our evolution? The answer to this question is that the subcutaneous fat layer’s emergence was intimately tied to the evolution of bipedalism – but in a most surprising way that is linked to the sexual evolution of our species.
At the point of the hominid line’s divergence from an ancestor common to chimpanzees, we possessed a sexuality much like theirs. Our VNO (vomeronasal organ) deep in the nasal cavity, responsible for sensing sexual pheromones and triggering a sexual instinct, had been declining in size and strength of operation for several million years prior. This much is known, since Old World primates do show a reduction in VNO size and functioning. In step with this decline in pheromonal reliance, had been the evolution of trichromatic vision and a new kind of reproductive sexual signalling stimuli that it had made possible, namely the visible estrus (the red swelling of a female’s sexual skin that correlates to hormone changes and the probable time of ovulation). The advantage of the visible estrus is that it made a fertile female identifiable from a further distance. The visible estrus thus worked in tandem with the VNO (though reduced,) to trigger the sexual instinct to reproduce. It is important to note that both were required to operate together to induce an instinctive mating response – much like they function synchronously in chimpanzees today. Hence, the visible estrus was vital to reproduction.
The evolution of bipedalism however, would have made the retention of the visible estrus virtually impossible, because it is bulky and can swell up to impractical size, making bipedal locomotion difficult. The loss of visible estrus – a visual stimuli for sexual attraction and intercourse – was, I suggest, replaced by the loss of body hair, which made clothing necessary and thus artificially generated a sex drive by creating sexual curiosity by concealment of the sex organs.
When we lost our body hair, we still had a sexual instinct that was at least partially mediated by sexual pheromones, as in evidenced by the retention of armpit and genital hair, two areas which mature at puberty – a time of sexual maturation – and which emit chemical substances which at one time presumably acted like pheromones. The sexual curiosity provided by clothing I suggest worked in tandem with the sexual instinct mediated by the VNO, much in the same way that the visible estrus had worked in tandem with it up to that point. The fact that ovulation would from now on be concealed, strongly worked to favour pair bonding – a shift from the promiscuous sexuality of chimps.
Hence, we can see that there were four evolutionary changes that were linked in a cascading fashion: bipedalism arises first, which then triggers the loss of body hair, which in turn creates a need for an insulating subcutaneous fat layer, which in turn necessitates the evolution of sweat glands.
This blog is part of the blog portion of my site www.humansexualevolution.com.