I understand that 92% of us are right handed and 8% are left handed. This is a significant difference. I don’t see any evolutionary advantage to being either predominately right or left handed. I can see being able to use both well. Is handedness a remnant of a common ancestor? Are there any better theories out there?
As far as I know left-handedness is not genetic it is just some strange thing. Last year in high school I read a book whos author believed that left handedness was an abnormality caused by undue pressure on the brain during some part of development and or birth. He also claimed that left handers on average died earlier then righties. If anyone wants the title I could probably go find it.
See http://duke.usask.ca/~elias/left/ for something more than just unsubstantiated opinion.
(At least third generation left-handed with a family-genetic streak where everyone lives to triple digits!)
Having a favoured hand is an evolutionary advantage because if you tend to do complicated tasks with one hand, you will learn that task much faster than if you randomly used either hand. A google for “animal handedness” finds lots of articles from people studying the idea, though you’ll have to filter out all the self-congratulatory “we’re lefties and we’re great!” bullshit.
Being left-handed myself, I can’t see a huge advantage, though I see that it’s better to have some preference. There are only a few tools that are difficult or impossible to use left-handed, and my right hand is not some useless appendage that I can’t put to work when I need to.
Cecil’s article on that subject: Do left-handers die young?
Having a favored hand is an evolutionary advantage IF it aids in survivability and/or the odds of reproducing. I don’t see it affecting either.
A common trait (e.g., right-handedness) does not necessarily translate to it being an advantageous trait.
Also, according to this article from the Scientific American website, the genetic component, if it exists, does not appear to be Mendelian in nature. That would tend to rule out any possibility for a “common ancestor” theory.
so which percentile do the ambidextrious people fall into?
[ul]
[sup]Strange, I’ve noticed the same thing about my left hand.[/sup][/ul]
Sure, it does. I’ve seen primates doing tool-based tasks like jamming a long reed into an anthill and pulling it to eat the ants stuck to it. If the primate consistantly used a favoured hand, his skill would improve much more rapidly (and he would get more ants and be less likely to starve) than if he used a randomly-chosen hand. The same applies to peeling fruit or shelling nuts or doing any number of tasks requiring some dexterity. It certainly helped our forebears when it came time to use primitive weapons on each other.
I’m not sure this is a critical advantage, but evolution loves minor improvements that give you better odds of surviving to adulthood. Being able to eat or fight without having to think about it (i.e. through practice you can do these things quickly and easily) can give an animal just enough of an edge over his ambidextrous rivals.
Actually according to some book I read on the hand, called, I think “the hand” the real evolutionary advantage is that each hand is “specialized” (not just that one is favored over the other).
ie. (for right handers) the left hand will tend to be specialized for support or orienting an object for use (like positioning a paper while you write, or holding a nail while you hammer), while the right hand is specialized for more fine motor movements.
I’m not so sure offhand how true that is. I mean I’m sure I’d rather be writing or hammering with my right hand, but I’m not sure my left hand is much better at holding a nail or moving a paper around than my right.
Still it’s easier to see an obvious evolutionary advantage in having each side be specialized than it is just that one hand is dominant. Considering that most tasks require each hand to be doing something different.
Which, however, does not address the high incidence of right-handedness. If handedness were favored, either hand should be favored equally; being right-handed, in and of itself, presents no clear advantage over being left-handed.
And learning through mimicry is not ruled out - if primate B learns how to dig for ants from primate A, B might well use the same hand as A did simply because that’s how he saw it done, not because of any genetic programming to focus on using one hand or the other for a task.
I would consider tool-using primates to be little different than early spear-hurling hunters in this respect. The skill with which one uses the tool is not selected for; the ability to use it at all is. The focused primate (the one which chooses a hand and sticks with it) may get more ants, just as the focused hunter can throw a spear better; however, the advantage gained in each case is not wholly genetic, thus does not necessarily represent a selective advantage.
Something that must be practiced is not itself a selectable trait - it’s a learned one.
Are you saying that any behavior that is not present from birth in full flower is not truly heritable and therefore cannot be selected for?
Where does that put bipedal motion, or practically every other behavior humans engage in? Almost all of which require lengthy practice in order to use?
A tendency to practice or focus or divide labor between the hands can certainly be inherited and selected for.
I remember something from Anthropology and I’m sorry I don’t have a cite but I’ll try to dig it up.
But some Anthropologists believe that originally we were 50/50 born right or left handed, this assumption comes from the discovery of ancient tools that could be used with both hands. Then it was some 10,000 years later that all the tools were right-handed only.
Not to mention, for hundreds of years left handedness was seen as “bad” and rooted out, children who expressed left-handedness were forced to use their right.
So it may be that genetically hand dominance is unimportant, but enviromental factors lead to right-handedness becoming dominant or even learned.
What ugly said.
Humans are not born knowing how to chuck spears, of course, but a human who has some minor neurological advantage that lets him learn how to chuck spears better is more likely to beat out his less skilled neighbor.
Somewhere along the neurological evolution of humans, the left brain picked up a tendency to finer motor control, translating to right-handedness. Whatever mechanism this is, it can’t be that specific, because left-handed people live and thrive among us. Why is right handedness more common? Who knows? Why do we have ten fingers instead of twelve or eight? Whatever the reason, it works well enough.
As such, it clearly doesn’t matter which hand is the preferred one, but a preference does offer an advantage.
I’m not sure what the ratio of ambidextrous-ers is. I think its very small, though. Less than 1%.
There’s just something sinister about left-handers.
(No, this is not a flame, look up the derivation of the word. 
)
Hey SC_Wolf, that is neat!
From dictionary.com definition 4:
On the left side; left
also:
Now, to add to the discussion… I’m right-handed but left-legged. How does the left-brain/right-brain theory work in such a case ?
No, I am saying that any behavior that is not present at all from birth is not heritable. Digging ants with sticks or throwing spears fall into this category. The capacity to learn (in a very general sense) those things, on the other hand, is heritable.
This, I agree with. But note, in your example, it is the underlying neurological control which is being selected for, not handedness in and of itself. Handedness may well be a side-effect of selection for a separate trait.
Which is, rather, my more general point. Not every trait we possess, whether behavioral or physical, is necessarily the result of direct selection. As such, not every trait is necessarily the result of some vague “evolutionary advantage”. Many modifications during development, for example, no matter how much advantage they may confer, cannot be passed on. Similarly, purely learned behaviors fall outside the realm of natural selection. And, of course, some traits may be selectively neutral.
The truth is, we don’t understand fully understand handedness in general, much less the extreme statistical dominance of right-handedness. As such, proclamations for evolutionary advantage (when it has not even been proven conclusively to be the direct result of genetics) are speculative at best.
Completely unencumbered by the facts, I humbly offer a point of view:
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What previous posters have said re: favoring a hand leads to quicker mastery of skills - makes sense
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Hypothesis - isn’t the Right brain focused on other stuff besides fine motor control, so the default of “handedness” tends to be right-handed because that’s what the Left brain normally does, and it control right hand skills? I thought that that hypothesis is what led to the conclusion that left handers suffered some birth trauma - the default wiring is for right-handedness, unless something happens, in which case left-handedness emerges.
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Having said that, I have a right handed child and a left handed child, both quite young, and there is no obvious indication that nurture had anything to do with it - they appear to have shown their preferences on their own.
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Hypothesis - can same-handedness make it easier to train others, leading to increased survivability? Another thought as to how handedness may be evolutionarily desirable.
Carry on.