Freethought & Rationalism Archive

ps418 · 06-10-2003, 07:54 AM

A new paper in the online journal Biology Letters argues that human hairlessness was selected as an adaptation to ectoparasite load rather than for thermoregulation.

Quote:

Unusually among the mammals, humans lack an outer layer of protective fur or hair. We propose the hypothesis that humans evolved hairlessness to reduce parasite loads, especially ectoparasites that may carry disease. We suggest that hairlessness is maintained by these naturally selected benefits and by sexual selection operating on both sexes. Hairlessness is made possible in humans owing to their unique abilities to regulate their environment via fire, shelter and clothing. Clothes and shelters allow a more flexible response to the external environment than a permanent layer of fur and can be changed or cleaned if infested with parasites. Naked mole-rats, another hairless and non-aquatic mammal species, also inhabit environments in which ectoparasite transmission is expected to be high, but in which temperatures are closely regulated. Our hypothesis explains features of human hairlessness - such as the marked sex difference in body hair, and its retention in the pubic regions - that are not explained by other theories.

Pagel and Bodmer, 2003. A naked ape would have fewer parasites. Proceedings of the Royal Society: Biological Sciences

From the NewScientist news story:

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Humans may have lost their body hair to reduce their vulnerability to fur-loving parasites and therefore attract the opposite sex, a new evolutionary theory proposes.

The nakedness of the human species is extremely rare among the 3000 or so living mammal species. Other naked mammals include elephants, walruses, pigs, whales and the bizarre naked mole-rat.

A widely accepted view is that humans lost their hair to help control their body temperature as they evolved into upright creatures on the warm plains of the African savannah. But this theory has problems that researchers believe the new theory can solve.

"The body cooling hypothesis is interesting, but some of the advantages in not having fur in the Sun become disadvantages at night," says Pagel, an evolutionary biologist at the University of Reading, UK. Humans would lose too much heat at night, he says.

"In animals, ectoparasites like biting flies, exert tremendous fitness costs - they really affect our health," he told New Scientist. "Our view is that hairlessness is an adaptation for reducing the ectoparasite load."

Warmth and shelter

However, if humans evolved to beat parasites by losing the hair they hide in, why did our hirsute ape cousins not do the same? The reason, says Pagel, is that we also developed our own culture. We are the only ones who learned to build fires and shelter and to make clothes, he says, all of which helped us keep warm while shedding our fur.

"It's one of those nice cases of gene/culture co-evolution," says Pagel. "It's the culture which helped us acquire the means to lose our hair."

"It does sound quite plausible," agrees Christophe Soligo, a researcher in human origins at the Natural History Museum in London, UK. Soligo calls the theory "elegant" as it addresses a key problem.

"The question we always have in explaining unique human traits is: why didn't other animals evolve them as well if they are so advantageous?" he told New Scientist.

Sexual selection

The only other animal that has achieved something similar is the naked mole rat. These strange sausage-like creatures live underground in large social colonies in arid regions. In their densely populated networks they should suffer a huge parasite load, but they do not. Their nakedness is possible as the temperature of their underground tunnels is very even.

Sexual selection for desirable mates may be another pressure which fuelled hair loss in humans, suggest Pagel and his colleague Walter Bodmer at the University of Oxford, UK.

Pagel says natural selection might initially favour less hairy individuals, as they have fewer parasites. But sexual selection could accelerate the loss of fur, as more naked early humans could be fitter and therefore more attractive as mates.

The researchers say one way to test their theory would be to see whether humans who live in areas with large parasite populations are less hairy than those who do not.

Patrick

MrDarwin · 06-10-2003, 08:16 AM

An interesting hypothesis and I agree that fire almost certainly played an important role, but hairlessness raises some other questions. It may be adaptive with respect to ectoparasites like fleas and lice, but it gives much less protection from biting insects like mosquitoes and flies, which are also important vectors of human disease.

On the other hand, the invention of clothing (even something as crude as an animal skin) would provide protection from both cold and biting insects, with the added benefit of hairless skin harboring fewer ectoparasites.

pz · 06-10-2003, 08:36 AM

Ugh. Another instance of blatant adaptationism. The authors have no evidence for their claims -- it's armchair speculation, nothing more, and calling it "plausible" is pathetic praise for a scientific theory.

This is the kind of fact-free just-so story that does deep harm to the science of evolutionary biology.

Calzaer · 06-10-2003, 08:47 AM

Newbie here...

Um, pz, do we actually have evidence that the loss of hair was for thermoregulation? How do you find evidence for "whys" in evolutionary biology?

pz · 06-10-2003, 08:56 AM

Quote:

Originally posted by Calzaer
Newbie here...

Um, pz, do we actually have evidence that the loss of hair was for thermoregulation?

Not really. There is a lot of physiological evidence on the effects of insulation on thermoregulation, though...but then, I'm not on that bandwagon, either.

Quote:

How do you find evidence for "whys" in evolutionary biology?

You make a hypothesis, and you do experiments to collect data to support or demolish your idea. Just like any other science. One of the distressing things about some kinds of evolutionary biology is that the last steps are neglected, and people go straight from speculation to publication. Plausibility is subjective, and is not sufficient.

RufusAtticus · 06-10-2003, 08:57 AM

Given that humans are neotenic I suspect that hair loss is simply a pleotropic effect of selection for brain power, along with much of our other human characteristics.

pz · 06-10-2003, 09:06 AM

Quote:

Originally posted by RufusAtticus
Given that humans are neotenic I suspect that hair loss is simply a pleotropic effect of selection for brain power, along with much of our other human characteristics.

That's my bias, too. However, the evidence for neoteny is also weak -- there is a tragic dearth of comparative embryology in the primates, for obvious reasons.

But yeah, I favor the spandrel idea. This parasite hypothesis doesn't explain anything -- if it were as plausible as claimed, why aren't chimpanzees, baboons, and big cats naked? (the speculation about fire and clothes is just more ad hoc rationalization to make the hypothesis fit, and just doesn't work for me.)

MrDarwin · 06-10-2003, 09:12 AM

The problem is that we have no idea whatsoever when hairlessness arose, or to what degree, other than that it was sometime between the divergence of humans from chimpanzees, and the very recent past (say, 5,000 years). In other words, we have no idea if it happened before or after humans harnessed fire.

It's also a bit disingenuous to call humans "hairless" when we all have body hair (and some people quite a bit). All reconstructions of human ancestors (from australopithecines right up to early Homo sapiens) showing them with varying degrees of body hair are likewise sheer speculation. (However, an unusual mutation that produces hyptertrichosis (extreme hairiness) in humans suggests that human "hairlessness" may have originated in a single individual, by a mutation of a single gene.)

ps418 · 06-10-2003, 09:49 AM

Quote:

Calzaer:
How do you find evidence for "whys" in evolutionary biology?

Quote:

pz:
You make a hypothesis, and you do experiments to collect data to support or demolish your idea. Just like any other science.

I'm interested in learning about how adaptationist hypotheses are properly tested. Could you list some examples of properly tested adaptationist hypotheses? Are there any features of H. sapiens that you'd regard as adaptations? If so, what are they, and what evidence suggests that they indeed adaptations rather than spandrels? Please don't mistake this for a rhetorical question, because I'm genuinely curious.

Regarding the hair topic in the OP, I have no idea what the state of the evidence is, since I haven't read the paper yet, but I have no reason to question that ectoparasites do in fact exert a fitness cost, or that ectoparasite load is reduced in when hair cover is reduced.

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pz:
But yeah, I favor the spandrel idea.

Why do you favor either spandrel or adaptation as an explanation? Do you just prefer spandrels by default, or is a byproduct hypothesis better in accord with the evidence in some way?

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Rufus:
Given that humans are neotenic I suspect that hair loss is simply a pleotropic effect of selection for brain power, along with much of our other human characteristics.

Couldn't multiple effects (say, brain power and reduced hair cover) of neoteny be selected at the same time? In other words, why need there be a dichotomy between pleotropic effect and adaptation? Also, how do you know that there has in fact been selection for brain power, rather than that brain power is a pleotropic effect of selection for some other feature?

Patrick

Oolon Colluphid · 06-10-2003, 09:57 AM

Quote:

Originally posted by pz
Ugh. Another instance of blatant adaptationism. The authors have no evidence for their claims -- it's armchair speculation, nothing more, and calling it "plausible" is pathetic praise for a scientific theory.

This is the kind of fact-free just-so story that does deep harm to the science of evolutionary biology.

Erm, pz...

Quote:

We propose the hypothesis that [...] Our hypothesis explains [stuff] that are not explained by other theories.

I can�t get at the whole article, but the New Scientist item says:

Quote:

The researchers say one way to test their theory would be to see whether humans who live in areas with large parasite populations are less hairy than those who do not.

All they are suggesting, it seems, is a hypothesis. Since when does proposing a hypothesis do any harm to any science? I�m at a loss as to how this is a �fact-free just-so story�. The facts do not have to be new facts. Darwin collated a lot of evidence himself, but these days we can just look it up, �standing on the shoulders of giants�, as it were. And Darwin was not averse to �armchair speculation�, like the swimming bear; if he�d said swimming wolf-shaped ungulate -- which he could not have known -- he�d have been pretty well right.

I see no sign of Pagel and Bodmer proposing it as Theory�; it�s a hypothesis to be investigated. So by all means criticise it on the basis of the evidence, old and new. But you seem to be against them even proposing it, because... it�s adaptionist. Well gosh darn if we�ve not been here before. Maybe hairlessness is a spandrel. That�s worth investigating. But it might instead be an actual adaptation, and that�s worth investigating too.... by proposing hypotheses.

For what it�s worth, I�m sceptical of it. Are we really that deficient of ectoparasites compared to chimps? We do, after all, have our own personal species of flea (Pulex irritans) and louse (Pediculus humanus), which have presumably been living alongside us long enough to have become separate species. And there�s plenty of bot-flies, sand fleas and so on that are just as able to feed on us as they are on mammals with hair.... perhaps more so as they don�t have a pelt to get past to get at their lunch.

(Aw shit, that last was armchair speculation on my part. [Basil Fawlty] <spanks self> You�re a naughty boy Oolon! Don�t. Do it. Again. [/BF])

And as MrD has pointed out, we aren�t that hairless, especially men. Ask any primary-school parent about whether their kid having little body hair prevents the spread of P humanus capitis.

What about races that tend to have less hair: some Africans and Asians? Are they less encumbered with parasites? Well that could be looked into, to test the hypothesis... oh. That�s just what Pagel and Bodmer suggest. Um...

TTFN, Oolon

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06-10-2003, 08:16 AM	#2
MrDarwin Veteran Member Join Date: May 2001 Location: Washington, DC Posts: 4,140	An interesting hypothesis and I agree that fire almost certainly played an important role, but hairlessness raises some other questions. It may be adaptive with respect to ectoparasites like fleas and lice, but it gives much less protection from biting insects like mosquitoes and flies, which are also important vectors of human disease. On the other hand, the invention of clothing (even something as crude as an animal skin) would provide protection from both cold and biting insects, with the added benefit of hairless skin harboring fewer ectoparasites.

06-10-2003, 08:36 AM	#3
pz Veteran Member Join Date: Aug 2001 Location: Morris, MN Posts: 3,341	Ugh. Another instance of blatant adaptationism. The authors have no evidence for their claims -- it's armchair speculation, nothing more, and calling it "plausible" is pathetic praise for a scientific theory. This is the kind of fact-free just-so story that does deep harm to the science of evolutionary biology.

06-10-2003, 08:47 AM	#4
Calzaer Veteran Member Join Date: Mar 2003 Location: Ohio Posts: 2,762	Newbie here... Um, pz, do we actually have evidence that the loss of hair was for thermoregulation? How do you find evidence for "whys" in evolutionary biology?

06-10-2003, 08:57 AM	#6
RufusAtticus Veteran Member Join Date: Nov 2001 Location: NCSU Posts: 5,853	Given that humans are neotenic I suspect that hair loss is simply a pleotropic effect of selection for brain power, along with much of our other human characteristics.

06-10-2003, 09:12 AM	#8
MrDarwin Veteran Member Join Date: May 2001 Location: Washington, DC Posts: 4,140	The problem is that we have no idea whatsoever when hairlessness arose, or to what degree, other than that it was sometime between the divergence of humans from chimpanzees, and the very recent past (say, 5,000 years). In other words, we have no idea if it happened before or after humans harnessed fire. It's also a bit disingenuous to call humans "hairless" when we all have body hair (and some people quite a bit). All reconstructions of human ancestors (from australopithecines right up to early Homo sapiens) showing them with varying degrees of body hair are likewise sheer speculation. (However, an unusual mutation that produces hyptertrichosis (extreme hairiness) in humans suggests that human "hairlessness" may have originated in a single individual, by a mutation of a single gene.)

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