4 mars 2016

Quelle est l'utilité d'un menton?

Je ne m'étais jamais posé la question, mais il s'agit apparemment d'un des mystères de l'évolution humaine!

Extrait de l'article de la BBC:

(...) It becomes even stranger when you consider that among the all primates – including our extinct relatives – only we have chins. Nobody seems to know why – although over the last century several theories as to its purpose have been offered.

(...) Even our closest extinct relatives such as Neanderthals did not have them. 

(...) "It implies that there was some sort of behavioural or dietary shift between Neanderthals and anatomically modern humans that caused the chin to form," says Zaneta Thayer of the University of Colorado, Denver, another researcher who has studied the human chin.

Although nobody can quite agree why the chin exists, there are three prominent theories that have been around for decades.

To start with it has long been proposed that our chin may help us chew food. The theory goes that we need the extra bone to deal with the stresses involved with chewing. However, this idea falls flat when you compare us to other great apes with similar-shaped jaws.

(...) Others have argued that our chin helps us to speak, that our tongue needs reinforcements from extra bone below our jaw. We are the primates with the most extensive speech repertoire after all.

The issue here is that we don't need much force to speak, so it’s not at all obvious why we would need extra bone to help with the process.

(...) The third idea is that the chin doesn't have an immediate function, but that it has been chosen by sexual selection. 

(...) Again there is a problem here, Pampush says. In all other mammals only one sex will have a sexually selected trait. Chins on the other hand are found on men and women. "If it’s an adaptation for sexual selection then we are the only mammal that has the same in both sexes," he says.

The three hypotheses mentioned all therefore fall flat, says Pampush. (...) But if we look at it another way it might become more apparent how it came to sit on our faces so prominently, despite having no functional use.

It could simply be what's called a "non-adaptive trait" that arises as a by-product of something else. (...) The chin, they said, is a "spandrel". (...) spandrels – both biological and architectural – are a by-product of a change happening elsewhere.

Our faces getting smaller may be what caused this particular spandrel to show, according to Nathan Holton of the University of Iowa. He says the chin may simply be a by-product of the reduction of the human skull.

(...) Other features changed too. We lack a prominent brow bridge and we have a hollow point below our cheek bones (technically called the "canine fossa"). These have also been linked to our smaller faces, Holton says. 

(...) Perhaps surprisingly, it's also rare to find a trait that is uniquely human. Many traits that humans have, other animals do too. The chin on the other hand, literally sticks out, and looking at how it did so may help us understand another step in the process that led to us. 

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