Why do we have handedness?

23 February 2016

Question

Why aren't all human beings ambidextrous? Is there an evolutionary advantage to handedness?

Answer

Kat put this question to Ginny Smith...

Ginny - Well, the answer to this is we don't fully understand why we have handedness and why around 9 out of 10 of us worldwide are right-handed. From an evolutionary point of view, we know that it's been like this for a very long time because there are some cave paintings from between 10 and 30,000 years ago, where what the people did was they put one hand on the cave wall and they used the other hand to hold a pipe and they blew paint through it and they took their hand away, so they left like an outline of their hand. And if you're right handed, you do that by putting your left hand on the wall and blowing through the pipe in your right hand, and if you're left handed you naturally do it the other way round. And the proportions of right and left hands are exactly as you'd expect, if they had the same proportions of right and left handers as we do now. So it's obviously been around for a very long time. There may actually, interestingly, be some advantages to being a left hander in a right handed world. Because you see this nowadays in sport, there are some advantages to being unexpected, and it may have been that when there was a lot more hand-to-hand combat, the lefties actually had a bit of an advantage, but only if there weren't very many of them. So you get this interesting kind of evolution where there's a benefit to being the odd one out.

Why we even have handedness? It's likely to be down to the fact that we're not completely symmetrical on the inside. I mean we've only got one heart, which is on the left hand side for example, and our brains are equally non-symmetrical, and we think that might relate to handedness. We know that most right handers have their language areas in the left hand side of their brain. Whereas left handers - sometimes their brains are the other way round and sometimes they're just less lateralised, so they're more symmetrical, but their brains can be organised slightly differently. So it may be that actually, there's no advantage to the handedness itself, but there was an advantage to the way of organising your brain and then the handedness sort of comes out of it. So, if you think about language, that requires very precise muscle movements - moving your lips and tongue and throat in very specific ways. So perhaps having the same hemisphere controlling that, as controlling the hand that you do most of your stuff with, was kind of beneficial.

Kat - Thanks very much Ginny.

Chris - I think I also saw Kangaroos have handedness was announced recently, but they don't have language as far as I know, Ginny.

Ginny - That's true, and interestingly, they tend to be lefties.

Chris - Yes. I wonder why.

Ginny - Who knows...

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