Denseman on the Rattis

Formerly known as the Widmann Blog


A body that requires a brain

Originally uploaded by Ryan Somma

One thing about human evolution that has been puzzling me for years is that there are several areas where the human body seems to be dependent on a highly developed brain:

  • Human head hair (and to some extent, facial hair) just keeps growing, which means it will need to be cut, combed and/or washed. While this is hardly a problem for modern humans, I wonder what an animal with the intelligence of a chimpanzee would do if it developed human head hair.
  • The same applies to human nails. The ones on the hands might be worn down through use, or by biting one’s nails, but the nails on the feet tend to require some attention. Do apes have similar nails, and if so, how do they cut them?
  • Finally, the human buttocks are so big that fæces tends to get stuck, which is why we use toilet paper (in other cultures, water is used instead). Monkeys and apes tend to have smaller buttocks that are less likely to get filthy, but again I’m not certain what they would do if their buttocks evolved to look like their human counterparts.

In short, I can’t see how human head hair, nails and buttocks could have evolved before the human brain was sophisticated enough to invent combs, knives and toilet paper.

Perhaps it’s not really a problem – I could well be that these features turned up very recently.

I have just always assumed (perhaps wrongly) that the modern human brain was more recent that the human body, but perhaps it’s really the way round.

10 thoughts on “A body that requires a brain

  • You really are a deep thinker aren’t you Thomas ?
    Maybe you should consider a career as a Philosopher .,.,I’ve checked .,., there were Ancient Greeks, British, American, Buddhist, Chinese, Catholic, German, Indian, Russian, Iranian, .,., even Muslim philosophers ,, but so far NONE from Denmark .,., But does it pay well ?

  • Neglene, er det ikke fordi vi lever indendøre og klatrer for lidt?
    Har endnu ikke klippet Ulfs (9 måneder) negle. Lauge holdt også selv finger- og tånegle korte indtil han begyndte at gå.
    Hørte i UK at mange fårebønder er nødt til at få klippet fårenes negle nu til dags.

    Stort tillykke med bebsen i maven.

  • Dougie: No Danish philosophers? Come on, how hard did you look? What about Kirkegaard, just to mention the most famous one…

    Thomas: I think most of your questions could be answered with simple sexual selection. Probably not the nails – that is a left over from our evolutionary heritage and the reason you need to cut them is that you use them less than other apes.

    The body hair, the buttocks (and breasts and penis size probably as well) can easily be explained by sexual selection. I’m not saying that is the true explanation, but it is one that fits quite well.

    Sexual selection doesn’t need to make sense with respect to natural selection – it often goes against fitness, actually – but we do tend to select for even useless features just because they attract the other sex.

    How extreme those features can be depends on what the drawbacks are.

    As for extreme body hair, for example, it could be a major drawback if we still lived like the other great apes, but then we don’t know how long our hair got back then (probably not much longer than the apes). If it evolved on the Homo line, it might not have been much of a problem at all, so it would be selected for, not against. Probably getting more extreme over the years.

    Sexual selection is a very strong evolutionary force, so if something doesn’t make sense to you from a fitness point of view, keep it in mind as a null model 🙂

  • …and by the way, yes the human brain seems to be more recent than the human body – as we can see it in fossils.

    Anatomically modern humans appears much earlier than art, complex tools, etc.

    As for how much we know about soft tissue like body hair, breasts, penis size etc. (where we are really extreme among the apes) I don’t know. I don’t know if we have any paleontological knowledge here.

  • @Sara: Du har sikkert ret, hvad angaar fingernegle, men jeg har svært ved at se, hvordan man kan slide sine fodnegle ned naturligt. Og tak!!!
    @Mailund: While I totally agree that well-kempt hair and clean buttocks could be selected for by sexual selection, my problem is that head hair is likely to become a total mess without access to either a comb or a pair of scissors (or a knife), and not something that anybody would find even remotely attractive. The same goes for a pair of buttocks that have never been wiped or washed.

  • Sure, but cats don’t need any implements to do it, and they seem to do it instinctively. As anybody with small kids will tell you, teaching them how to wipe their bum is hard (at least if it’s supposed to be clean afterwards).

  • “Finally, the human buttocks are so big that fæces tends to get stuck, which is why we use toilet paper”

    Diet makes a huge difference in this. Eat properly (minimal or no starchy carbs) and there’s nothing to wipe.

  • IOW that was probably not a big issue for pre-agricultural humans.

  • It’s a good point that a hunter-gatherer diet will mean there’s nothing to wipe. I hadn’t thought of that. Thanks!


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