HAIR / Hairless Why? / plus several articles

SciAm: The Naked Truth

Our nearly hairless skin was a key factor in the emergence of other human traits

EXCERPT: “In marine mammals that never venture ashore, such as whales, naked skin facilitates long-distance swimming and diving by reducing drag on the skin’s surface. To compensate for the lack of external insulation, these animals have blubber under the skin. In contrast, semiaquatic mammals—otters, for example—have dense, waterproof fur that traps air to provide positive buoyancy, thus decreasing the effort needed to float. This fur also protects their skin on land.”

“The largest terrestrial mammals—namely, elephants, rhinoceroses and hippopotamuses—also evolved naked skin because they are at constant risk of overheating. The larger an animal is, the less surface area it has relative to overall body mass and the harder it is for the creature to rid its body of excess heat. (On the flip side, mice and other small animals, which have a high surface-to-volume ratio, often struggle to retain sufficient heat.) During the Pleistocene epoch, which spans the time between two million and 10,000 years ago, the mammoths and other relatives of modern elephants and rhinoceroses were “woolly” because they lived in cold environments, and external insulation helped them conserve body heat and lower their food intake. But all of today’s mega herbivores live in sweltering conditions, where a fur coat would be deadly for beasts of such immense proportions.”

“Human hairlessness is not an evolutionary adaptation to living underground or in the water—the popular embrace of the so-called aquatic ape hypothesis notwithstanding [see box on page 26]. Neither is it the result of large body size. But our bare skin is related to staying cool, as our superior sweating abilities suggest.”

EXCERPT: “Studies conducted independently by Lieberman and Christopher Ruff of Johns Hopkins University have shown that by about 1.6 million years ago an early member of our genus called Homo ergaster (label applied to Homo erectus in Africa) had evolved essentially modern body proportions, which would have permitted prolonged walking and running. Moreover, details of the joint surfaces of the ankle, knee and hip make clear that these hominids actually exerted themselves in this way. Thus, according to the fossil evidence, the transition to naked skin and an eccrine-based sweating system must have been well under way by 1.6 million years ago to offset the greater heat loads that accompanied our predecessors’ newly strenuous way of life.”

“Another clue to when hominids evolved naked skin has come from investigations into the genetics of skin color. In an ingenious study published in 2004, Alan R. Rogers of the University of Utah and his colleagues examined sequences of the human MC1R gene, which is among the genes responsible for producing skin pigmentation. The team showed that a specific gene variant always found in Africans with dark pigmentation originated as many as 1.2 million years ago. Early human ancestors are believed to have had pinkish skin covered with black fur, much as chimpanzees do, so the evolution of permanently dark skin was presumably a requisite evolutionary follow-up to the loss of our sun-shielding body hair. Rogers’s estimate thus provides a minimum age for the dawn of nakedness.”

Much more: Full Article


RELATED: Not Exactly Rocket Science, A Blog by Ed Yong, 10/23/12

Q: Why don’t apes have bigger brains? A: They can’t eat enough to afford them

More on brain evolution:





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