Wednesday, 21 December 2011

Memes and the evolution of human hairlessness

Transcript:

Hi! I'm Tim Tyler and this is a video about how memes fit into the picture when it comes to the evolution of human hairlessness.

The evolution of human hairlessness has been a hot topic since long before Desmond Morris's "The Naked Ape" - which thrust the topic into the limelight. Humans have been hairless for over a million years - according to gene sequencing analysis. No other ape is hairless - why are we?

One thing practically everyone agrees on: sexual selection was involved. Women are less hairy than men, and many of them enthusiastically remove much of their remaining body hair using depilatory techniques. Men are hairier, have what appears to be pubic hair growing on their faces - and they don't normally attempt to remove their body hair.

Body hair offers many benefits - most of which are to do with insulation from environmental insults. In particular functionally-naked flesh is more vulnerable to scuffs, scrapes, bruises, stings, heat loss, heat gain and UV radiation. The substantial benefits of having hair mean that any hypothesis about its loss needs to propose fairly substantial benefits. This effectively rules out some of the existing proposals.

Lots of ideas have been proposed to account for the origin of human hairlessness - including the following ones:

  • Ectoparasites are easier to detect and remove without hair;
  • Signalling low parasite loads is easier if you are hairless;
  • Sweat-based thermoregulation is easier without hair;
  • Bathing is easier if you are hairless;
  • Swimming and diving is helped by hairlessness;
  • Hairlessness and fire don't mix very well;
  • Neoteny produced hairlessness as a side effect.
The traditional theory invokes the role of sweat-based thermoregulation. Ancestral male humans hunted in a manner that emphasised endurance running - and humans would badly need to sweat to cool down. No doubt this picture has some truth to it - but the selection pressure seems rather mild - few other hunting mammals have lost their hair - and the idea explains the greater hairlessness of women particularly badly. Modern female humans don't pluck out their body hairs to help them cool down. Thermoregulation is not really the answer.

I think parasite pressure is likely to be the main answer. Our ancestors lived in parasite-riddled environments - and parasites would have caused our ancestors problems on a large scale. Hairlessness deprives ectoparasites of homes and makes them easier to detect and remove. Many modern paraistes prefer the hairy areas on humans - for example, head lice and genital crabs. Clothes and bedding can also be infected by parasites - but these can more easily be washed and disinfected.

Also, hairlessness facilitates signalling of freedom from parasites. An unblemished skin signals a healthy immune system, good genes and freedom from parasites. People of both sexes are interested in partners with good disease resistance genes.

Successful males may be particularly interested in partners that are free from disease, since otherwise - if they rack up a lot of partners - they are more at risk of infection themselves. This is probably a factor in the male preference for virgins the and male preference for youth. If men seek unparasitsed women, women can be expected to want to signal their unparasitised state to men - and unblemished skin allows them to do that - and that's the reason why many women are so averse to their own body hairs. Though female choice rules throughout most of biology, mate choice by dominant males is a surprisingly significant force among humans.

Hairlessness also makes bathing easier. Bathing helps to wash away parasites, but it leaves hairy creatures soggy for extended periods of time. For hairless humans, bathing has become more practical.

Human hairs haven't gone away totally because they play the role of acting as combined trip wires and intruder detectors that help to deal with ectoparasites - according to a recent study from 2011.

The position that parasites caused hairlessness dates back to Darwin's writings. It has previously been articulated in modern times by Mark Pagel and Walter Bodmer. The idea was fairly dramatically endorsed by Richard Dawkins in his book "The Ancestor's Tale". I link to these works in my references.

Next: the role of memes. It is likely that cultural transmission buffered humans against the negative effects of the loss of their body hair. Clothes protect against U.V. light, scrapes, bruises and stings - though clothes may not have come along concurrently with hairlessness. Cushions can protect against the ground. Night time temperature losses can be compensated for by fires, shelter and good quality bedding. Culture increases human adaptiveness and gaves them the ability to colonise a wide range of environments. Culture is likely to have allowed humans to adapt to their own hairlessness with cultural apatations - ameliorating the costs associated with being naked.

Considering things from the meme's perspective, one thing that memes want to do is to push humans together into social groups - to facilitate their own reproduction. However, increased population density increases the potential for horizontal transmission of parasites. As the ancestral humans came together their parasites would have had a field day - and the human need to keep clean and hygenic would have intensified. Hairlessness would have helped them in this battle.

Another possible case of increased sociality resulting in hairlessness comes from the naked mole rats. Mole rats are among the most social mammals - unlike most other moles - they have lost their body hair. Their subterranean environment may play the same role for the rats as shelter does for humans - by forcing them to live together and by regulating the temperature. The mole rats also appear to have practically eliminated most of their ectoparasites. Ectoparasite elimination seems to be a plausible explanation for hairlessness in their case as well.

So, hairlessness may be linked to ultrasociality. In which case, in addition to ameliorating the costs of hairlessness, memes may have actually fairly directly caused the main selection pressure that led to hairlessness becoming more adaptive in the first place.

Enjoy,

References

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