ZOOL 304, Class Notes

Evolution of human hairlessness.  

Hair (or fur) is a characteristic feature of mammals, although several species -- including not only people but also hippotomuses, modern elephants, and naked mole rats -- are noted for hairlessness.  (Also see recent article here.)

The relative lack of hair is such a remarkably distinctive feature of human beings, especially when compared with all other primates, that Homo sapiens has been called "the naked ape".

We deduce from the phylogenetic distribution of hairiness in most major mammal groups that hairiness is the ancestral mammalian condition and that hairlessness is a derived condition.  

To address the evolutionary derivation of hairlessness, we should consider both phylogeny and ecology.

• Does phylogeny suggest that hairlessless evolved once or more than once?
  • Do the several hairless species belong to a single taxon of hairless species, which would indicate a hairless is derived from a common ancestor?  
  • Or are these individuals unrelated, suggesting that hairless evolved several times independently?
• Does ecology suggest that one single adaptive function can explain all examples of derived hairlessness, or are different explanations needed in different cases?  
• Do all hairless species share some common ecological circumstance, which would suggest one common advantage for hairlessness?  
• Or do these several species live in markedly different niches, suggesting quite different advantages for different species?

Basic engineering considerations suggest that hair is common among mammals not only because it was the ancestral mammalian condition but also because hair continues to function adaptively for most mammals.  Hair (fur) serves a number advantageous roles, including insulation from cold (and heat), protection against injury, protection against radiation (sunburn), and camouflage.  

Comparative analysis (especially correlation of variation in coat thickness and coat color with the relevant environmental variation) supports a general hypothesis that for most mammals hairiness is indeed an adaptation that continues to be maintained by natural selection.  [Sexual dimorphism (e.g., the mane of male lions) also suggests sexual selection as well as natural selection may also influence the degree of hairiness.]

Thus the loss of hair during the evolution of any particular species, such as human beings, presents a puzzle.  Any explanation for loss of hair must address the general adaptive advantages associated with hairiness.  Loss of hair must proceed under more-or-less special circumstances where at least one of the following applies.

• The advantages have become irrelevant or insignificant.
• Hairlessness conveys some advantage which outweighs the loss.
• Hairlessness is somehow linked to some other trait which carries an overriding advantage.

So, for each example of hairlessness (e.g., elephants / naked mole rats / human beings), we should ask the following questions.

• Does niche ecology obviate the utility of hair for insulation or protection or camouflage?  (Especially for humans, consider not only current ecology but also circumstances during the evolution of the species.)
• Can hairlessness confer some alternative advantage that outweighs the costs of losing insulation and protection?  If so, what advantage, under what circumstances?
• Or is there some other explanation?

For more, see Analysis of Adaptation.

Notes for week 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10 / 11 / 12 / 13 / 14 / 15

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