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Evolution - eggs, hair & breasts

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155368.  Fri Mar 09, 2007 11:55 am Reply with quote

Q: Why are/aren't women's breasts hairy?

Quite interestingly, the mammary gland (from which 'mammals' get their name) are exapted* hair follicles.

It's always been a bit of mystery as to how lactation occured, because it seems as if it requires two rather specialised and complex adaptations to happen simultaneously, which is rather unlikely in evolution. As with all explanations of this chick-egg variety, though, it only requires one to have led the way for the other to have followed suit.

A recent theory by Oftedal looks to our monotreme cousins, especially the platypus, which is quite an evolutionary 'throwback' - it still lays eggs, and has not developed a mammary gland in the way that other mammals have. It still gives milk, though, which the hatched pups drink by sucking on the fur around the lactation gland.

Reptile and bird eggs are hard-shelled, and so can keep their moisture. Proto-mammal eggs seem to have developed by keeping the eggs moist by holding them next to the body, and this is where Oftedal noticed that platypus eggs have another clacified layer on them - secreted by the lactating gland.

So maybe hair came first, to keep egg-laying proto-mammals warm, then to keep the eggs moist, with the aid of these beserk sebaceous hair glands, then as those glands developed nipples, the young fed directly off the nutrients, then the need for the hair to deliver the nutrients was gone.

So the platypuses still need their hairy glands, to coat their eggs, but we don't, as we've done away with eggs, and feed our littluns directly. Well, the wives do.


* Exaptation is the process whereby one feature of an organism that is no longer critical to its survival becomes vestigal, and is therefore available for use, in slightly adapted form, for some other function. The famous example is the panda's thumb, which exapted from a wrist bone, enabling it to have better grip on food.


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