Why do humans have so little body hair?

November 30, 2012

Dear Cecil:

Assuming evolution didn’t anticipate the invention of clothing, why is the human race relatively hairless? What hair we have provides minimal protection against the elements. Were we only supposed to live in tropical climates where such protection was unnecessary? On the assumption that we descend from hairy apes, and that evolution occurs due to need, what was the need for us to lose our body hair?

Cecil replies:

Nobody really knows, and in fact human nakedness remains one of the great mysteries — the author of Genesis, no less, felt compelled to work a partial explanation into the biblical creation myth. We’re the only essentially hairless primate species, and one of few hairless mammals. Considering how helpful a fur coat can be, not only in keeping warm but also in protecting against the sun, you’d think losing our ancestral hair would have been a sure route to extinction. Yet here we are, masters of the planet.

It’s only natural to wonder whether these circumstances are related. Although too little is known to permit any definite statement, it seems clear nakedness was closely tied to human progress. No one would say we needed to develop big brains to compensate for our lack of hair; the development of technology, on the other hand, is a different story. At minimum we can say this: once we started down the road to civilization, nakedness prevented our going back.

Before we get into that, let’s run through a few of the explanations for human hairlessness proposed up till now:

We’re sexier with no hair. Charles Darwin was one of the first to suggest this, although he didn’t put it so bluntly. He merely noted that hairlessness may have been a factor in sexual selection and that women, historically the object rather than the initiator of pursuit, have less hair than men. Many later scientists have suggested variations on this theme. However, it can’t be the entire explanation. While nakedness may increase lust, a fat lot of good that does you if the other party has frozen to death.

Lack of hair makes it easier to cool off. Since it’s generally agreed humankind originated in tropical Africa, this is plausible — indigenous inhabitants of tropical regions typically wore minimal clothing before being overtaken by modernity. Zoologist Desmond Morris, author of the 1967 best seller The Naked Ape, offers the twist that hairlessness prevented hominid hunters from overheating when chasing game, which also makes sense; as distance runners we have few equals among mammals. But again, that surely isn’t the whole story, as we shall see.

Humans are descended from aquatic apes. The idea is that hairlessness made our seafood-loving forebears more streamlined in the water. There’s little evidence supporting this much-promoted notion, and scientists have roundly rejected it.

Less hair = fewer bugs, or to put it more formally, hairlessness reduces “parasite load.” Another unpersuasive claim: notwithstanding their paucity of hair, humans have largely been infested with lice, fleas, and other parasites until recently.

If none of the above explanations will cut it, what does? Here we have to guess, since the timeline of human development is poorly understood. Our hominid ancestors began walking on two legs at least 4 million years ago, and the trend toward bigger brains began about 2 million years ago. Genetic analysis suggests hominids have been hairless for at least 1.2 million years. Clothing is much more recent — the earliest evidence for hide scraping, the most primitive form of couture, dates back just 300,000 years.

The wild card is fire, needed not just to keep the cave warm but for cooking, another critical step. Recent archaeological analysis suggests hominids were using fire as of a million years ago.

If it turns out hairlessness and mastery of fire occurred around the same time, we have a plausible sequence of events. Once they were no longer at the mercy of the elements, hominids could indulge a yen for less hirsute mates without jeopardizing their offspring.

If the tendency to hairlessness long preceded fire we have more of a puzzle, although not necessarily an insoluble one: the random genetic mutation that made hairlessness possible might have occurred in the ancient past but not expressed itself till conditions were favorable. We know, for example, that Homo erectus lived 1.8 million years ago in the Caucasus region, which had cold winters then as now. Without fire, these protohumans must have had hairy coats to survive.

Once our ancestors had acquired both fire and clothing, there was nothing to prevent nakedness from becoming dominant, and at some point the capacity to grow abundant body hair evidently was lost. When the ancestors of modern east Asians were trapped in Siberia by glaciers during the last ice age, 25,000 to 50,000 years ago, they evolved eyefolds and flatter facial features to protect against the cold. But body hair didn’t re-emerge.

Technology, in this reading, made nakedness possible, and nakedness in turn made technology indispensable. We’ve been the prisoners of our own cleverness ever since.

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References

Alexander, Richard D. How Did Humans Evolve Ann Arbor: Museum of Zoology, University of Michigan, 1990.

Berman, Judith C. “Bad Hair Days in the Paleolithic: Modern (Re)Constructions of the Cave Man” American Anthropologist, New Series 101.2 (June, 1999): 288-304.

Clemens, Jordan L. et al. “Death and Hairless Creatures? Elucidating Existential Factors in the Evolution of Human Body Hair” Eureka 3.1 (2012) 7-8.

Darwin, Charles. The Descent of Man and Selection in Relation to Sex, Volume 1, Second Edition. 1874.
Harris, Judith Rich. “Parental Selection: A Third Selection Process in the Evolution of Human Hairlessness and Skin Color” Medical Hypotheses 66 (2006): 1053-1059.

Hardy, Sir. Alister “Was Man More Aquatic in the Past?” The New Scientist 17 March, 1960.

Ibraimov, A.I. “The Evolution of Body Heat Conductivity, Skin and Brain Size in Human” J. Hum. Ecol. 21.2 (2007): 95-103.

Jablonski, Nina G. “The Evolution of Human Skin and Skin Color” Annual Review of Anthropology 33 (2004): 585-623.

Kushlan, James A. “The Evolution of Hairlessness in Man” The American Naturalist 116.5 (November, 1980): 727-729.

Pagel, Mark and Bodmer, Walter “A Naked Ape Would Have Fewer Parasites” Proceedings: Biological Sciences 270 (August 7, 2003): S117-S119.

Rantala, Markus J. “Human nakedness: adaptation against ectoparasites?” International Journal for Parasitology 29 (1999): 1987-1989.

Rebora, Alfredo “Lucy’s pelt: when we became hairless and how we managed to survive” The International Society of Dermatology 49 (2010): 17-20.

Rogers, Alan R. et al. “Genetic Variation at the MC1R Locus and the Time since Loss of Human Body Hair” Current Anthropology 45.1 (February, 2004): 105-108.

Straight Dope Staff Report: Did Humans Descend from Aquatic Apes? http://www.straightdope.com/columns/read/1991/did-humans-descend-from-aquatic-apes

Wheeler, P.E. “The Influence of the Loss of Functional Body Hair on the Water Budgets of Early Hominids” Journal of Human Evolution 23 (1992): 379-388.”

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