June 10, 2010 | David F. Coppedge

Darwin’s Sweatshop: Why Ethiopia Made People Hairless

Five scientists think they have figured out why people walk upright and don’t have fur like other mammals.  They had to evolve in Ethiopia, where it is hot.  This led to the loss of body hair, and the evolution of sweat glands and other adaptations to deal with the heat.
    It’s not that the scientists from Caltech, Johns Hopkins and University of Utah actually found evidence for this.  It’s just that they studied rocks from the Turkana Basin, where some important fossils of alleged human ancestors have been found.  According to their analysis of carbonate rocks, the temperature has always been hot and arid in this area – for 4 million years, they claim.  They published their results in PNAS,1 and the story was picked up by Science Daily and PhysOrg.
    Although their paper primarily concerned deducing climate and temperature from the rock record, they considered implications for human thermophysiology:

This temperature record is relevant to the evolutionary origin or maintenance of a unique suite of adaptations that permit humans to remain active under high ambient heat loads.  For example, upright posture in hot, open environments confers thermophysiological advantages to bipedal hominins owing to reduced interception of direct solar radiation and to displacement of the body away from the near-surface environment, which may be excessively hot due to solar heating.  Derived human traits such as very little body hair, high sweating capacity, and high surface area to volume ratio are also advantageous for daytime activity in hot, arid climates, and temperature is a central variable in hypotheses of behaviors such as long-distance scavenging and persistence hunting.  However, the thermoregulatory advantages of these adaptations arise primarily under very hot, sunny conditions.  Our results suggest that such conditions were relevant to human ecology in the Turkana Basin, either directly within or at the spatial or temporal margins of human-preferred habitats….
    Whereas our data are silent on the importance of ambient temperature in shaping human evolution, they comprise a necessary prerequisite for beginning to evaluate temperature-related hypotheses.
[italics in original].

If this is so, then it should also be a necessary prerequisite for beginning to evaluate the null hypothesis, or for evaluating why such conditions failed to generate similar physiological traits in the other mammals living alongside the humans in the same ecological environment.  It would also make one question why the hominids they believe inhabited South Africa, Europe and Asia for millions of years and during long ice ages did not quickly gain all that body hair right back.  The authors seemed to overlook those parts of the evolutionary logic.
    The popular press swallowed it all without question, though.  “The need to stay cool in that cradle of human evolution may relate, at least in part, to why pre-humans learned to walk upright, lost the fur that covered the bodies of their predecessors and became able to sweat more, Johns Hopkins University earth scientist Benjamin Passey said.”  Perhaps they need to consider another uniquely human trait: blushing (see 12/19/2007 commentary).


1.  Passey, Levin, Cerling, Brown, and Eiler, “High-temperature environments of human evolution in East Africa based on bond ordering in paleosol carbonates,” Proceedings of the National Academy of Sciences, published online before print June 8, 2010, doi: 10.1073/pnas.1001824107.

This has the makings of a great cartoon: the Turkana Gymnasium, where all the camels, wildebeests, zebras, giraffes, oryx, lions, cheetahs, and gerbils all strip down to the skin, stand upright, and work up a sweat under the hot sun, dancing to the beat of “Do the Evolution” (08/31/2006).

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