Why are humans (mostly) hairless?

Most of us are (mostly) hairless. That is very unusual among mammals and pretty much unique among primates.

Why have humans evolved this trait? This New Scientist article outlines some of theories offered over the years.

We still don’t have a conclusive answer.

Theory 1: Avoiding the heat

Perhaps hairlessness made us better running hunters?As humans became savanna hunters body hair was shed to avoid overheating during long runs.

I have blogged before about theories suggesting that the human combination of hairlessness and profuse sweating allows for persistence hunting.

The problem:

Why has no other mammal adopted this strategy?

Many other mammals would benefit from better cooling systems and they all have fur – what is so different about us? Fur actually insulates against heat and the sun as well as keeping us warm in the cold.

Theory 2: Side-effect of big brains

This theory suggests that nakedness is a side effect of growing larger brains. It was noted that:

  • Human brains grow very rapidly just before birth
  • Humans retain characteristics of juvenile apes (flatter faces, bigger heads and eyes and naked skin).

The idea is that the stage just before birth has been protracted to allow the brain to grow even larger.

The problem:

If having hair were an advantage then we would just grow hair later in life. For example, we grow teeth even though we aren’t born with teeth.

Theory 3: No ticks here

Another theory is that we lost our hair in order to avoid ticks and other parasites.

The problem:

Why only us? If this were really an advantage other mammals would have evolved the same thing.

Theory 4: Aquatic ape

Humans are hairless and have a thin layer of fat under their skin. Sounds like whales and dolphins right?

The idea is that our evolutionary ancestors went through a stage evolving in aquatic environments. This could also help to describe why we walk upright.

By otters have thick fur...The problems:

There are many problems with this theory (read about them on Wikipedia). For example, humans can’t hold their breath very well and are not efficient swimmers.

Most importantly, hairlessness is only an advantage in fully aquatic species like dolphins and whales. Even full time swimmers like otters and seals have thick fur and swim superbly. In short, the aquatic ape theory doesn’t work all that well…

  • Ken Fabos

    I’ve had a longstanding problem with most of the theorising about human ‘hairlessness’ – the failure to recognise and take into consideration one of the most basic functions of hair; it’s sensory function. We get tactile sensory information via our hairs – presence of insects, close encounters with solid objects, air movements. This tactile sense can extend well beyond the surface of the skin. Around the eyes and ears the hairs are extremely sensitive, even (especially?) the hairs so fine as to be near invisible. What many people perceive as direct skin contact is often contact with hairs; in my own case an insect has to struggle to reach skin on arms and legs and I feel them from the moment they disturb the hairs.
    With respect to it’s evolution, sparser, finer hairs are (all else being equal) more sensitive than denser, heavier hairs. This is basic physics, not biology; being sparser means vibration and movement of hairs is not dampened and dissipated so much by being laid up against other hairs. Being finer means they are moved and vibrated by smaller impulses. This gives a lower sensory threshold to sparse fine hairs than to hairs that are part of a thick pelt. I would also point out that goosebumps, by separating the individual hair shafts, would increase sensitivity as well as extend the sensory reach to it’s maximum. When our ancestors lost their thick coat, they gained greater tactile sensitivity.
    Until the scholars who study and theorize on the evolution of human hair recognise and take into considerations all of it’s existing functions I don’t believe their speculations should be taken too seriously.

  • http://alistairpott.com/ alistairpott

    Great comment and interesting point. I suspect that our remaining hairs do provide an advantage through tactile perception.

    The obvious question though is why only humans? If the thin coat were such an advantage why would (pretty much) only humans have evolved this approach?

    Since only humans evolved this trait there must be another factor that is not taken into account in your theory. Even in warm climates other land mammals have thick coats rather than taking advantage of your theory. Why?

    This suggests that the theory that humans evolved a thinner coat for increased tactile perception is at best incomplete.