Handedness: Has Human Evolution Always Been Right?

By Gemma Tarlach | October 20, 2016 12:00 pm
An extreme preference for right-handedness is unique to our species. Credit: Working Title Films.

An extreme preference for right-handedness is unique to our species. Credit: Working Title Films.

Right-handedness is very much a human thing. About 90 percent of Homo sapiens are right-handed. On the other hand, literally, about 50 percent of apes are southpaws, similar to most other primates.

Researchers believe that right-handedness may be linked to other traits, such as the development of language. They have long sought to find the point in hominin evolution where the right hand became dominant for the vast majority of a species.

Previous research identified right hand dominance in Neanderthal fossils that were about 400,000 years old, but no one had found evidence of handedness older than that — until now.

University of Kansas paleoanthropologist David Frayer and colleagues were able to determine whether a 1.8-million-year old hominin was dexter or sinister thanks to its teeth. Yeah, its teeth. Stay with me now.

Hands-Off Approach

You might think well, just look at the bones of a hominin’s hands and compare right to left to see differences in size and shape based on preferential use of one side — which can in fact be detected. But only if you’ve got both hands to study. And quite frankly, despite a lot of exciting finds in recent years, hominin fossils tend to be fragmentary. We’ve got one hand, or maybe just the jawbone, most of the time.

That’s why researchers look to teeth to tell the story of handedness.

The technique has been used previously both by Frayer and other researchers looking at Neanderthal teeth in sites from Spain to Croatia. Hominins tended to use their mouths as a kind of third hand, particularly when cutting a bite-sized, or at least chewable, piece of meat.

Researchers believe the individual would hold a sharp-edged stone tool in its dominant hand and hold a big piece of meat on one end in its mouth, on the other end in its non-dominant hand. Then, with the stone tool, the hungry hominin would saw off a piece of meat small enough to chew with our comparatively puny teeth.

Every now and then, while sawing away at the meat, the hominin would miss and strike its teeth.

Ouch.

Bad for the hominin, but great for science, because over time the striations, those little cuts across the outer surface of the front teeth, formed a pattern: a pattern aligns one way if you’re right-handed, another if you’re left-handed.

Researchers believe hominins sawed off pieces of meat with a stone tool in their dominant hand (left), occasionally hitting their teeth and leaving a distinct pattern of striations (right) that indicates handedness. Credit: David Frayer.

Researchers believe hominins sawed off pieces of meat with a stone tool in their dominant hand (a), occasionally hitting their teeth and leaving a distinct pattern of striations on the surface (b) that indicates handedness. Credit: David Frayer.

 

The idea that striations on teeth might indicate handedness was systematically tested back in 1988, when a team found that most of the hominins from Spain’s Sima de los Huesos site — individuals we now know to be Neanderthals — were right-handed. Subsequent tests and re-tests of more Neanderthals, including at the Croatian site of Krapina, confirmed that about 90 percent of the individuals analyzed were right-handed, the same proportion as modern humans.

Frayer’s team applied the same analysis method to the teeth of a Homo habilis individual from the famed Olduvai Gorge in Tanzania. And, despite its age of 1.8 million years, they were able to determine that the individual was right-handed.

All Right, All Right, All Right

Frayer’s team, publishing their finding today in the Journal of Human Evolution, were quick to point out that you can’t make any grand statements about an entire species by sampling one individual. But the fact that they could determine handedness in a hominin 1.8 million years after the fact is pretty amazing, and it at least suggests that Homo habilis was not ambidextrous, like some primates.

How do we know? Well, if a hominin was ambidextrous, the striations would not have aligned in the same direction. Okay, so maybe now you’re thinking what if this particular hominin cut with its right hand but used its left hand for other tasks? And that may be the case. We need more samples from Homo habilis to know for sure whether this early member of the Homo genus had the same preference for right-handedness as Neanderthals and our own species.

Because there appears to be a link between right hand dominance and the development of language, it’s important to figure out when that evolved, and today’s study, ah, lends a hand to that line of research.

Left Behind?

Fellow sinister sorts, I know you’re thinking great, here’s another study suggesting that right-handedness is somehow superior to being a southpaw. That was my reaction, too. So I asked Frayer in an email to ‘fess up and come clean about any handedness bias.

“I am right handed,” Frayer admitted, “Although my parents told me I started out as a lefty, but they made me switch as a young baby. I have always maintained this is why my penmanship sucks.”

We give Frayer and his team the thumbs-up for taking on this research. With both hands.

 

 

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  • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

    Strike dinner or an enemy with your right hand, putting your heart out of harms way. Then, swords and shields, One presumes Greek Spartans, almost 100% of whose males were warriors, had no lefties. That is easily verified.

    • Erik Bosma

      Could also have been a female preference over a long period of time or a genetic bottle-neck when the homo population was reduced to a few thousand, most of them coincidentally right-handed or a dominant ruler who executed lefties which then caught on with the rest of them. Probably a combination of all theories.

    • OWilson

      All other things being equal, a southpaw has a slight advantage in hand to hand combat or sporting competition.

      Enough so for the trait to survive and be passed along.

  • Erik Bosma

    My brother is left-handed and both of my kids are also lefties. I’m right-handed however I do all my eye coordination stuff like aiming as if I’m a leftie ie: with my left eye. My brother tried to claim a partial victory by stating that I’m “left-eyed” and was possibly born a leftie but changed over later in life. He was overjoyed when he discovered both of my kids were lefties. Any comments?

    • GemmaTarlach

      Hi Erik. I am left-handed but right-eyed, which was a problem for me at first as an archer. Over time I went with what felt more comfortable overall (shooting left-handed). I have good eyesight in general so it wasn’t a big deal, but I can tell you eye dominance is definitely a thing and has nothing to do with handedness (for me, for example, my right eye is stronger because my left eye has a nerve palsy issue. Undetectable by the naked eye, no pun intended, but just off enough to show up during an eye exam).

      • OWilson

        I’m right handed, but left eyed, and “right toothed”.

        My preference for aiming and chewing has more to do with comparative visual impairment and tooth quality, than my hand preference.

        Even hand preference could also be a result of some long standing wound or impairment like nerve damage in a cut finger.

        I agree strongly with the author that to assign traits to entire populations, based on just one, or a limited number of specimens, while tempting, can be very misleading.

      • Erik Bosma

        Only question, I think, is how to determine if these “handedness” AND eye-dominance traits are ‘nurture’ or ‘nature’. We would need to follow several babies for the first 4 or 5 years of their life and devise some set of tests to discover the answer to whether we are born with it or if it is foisted upon us or if we have chosen our preferences. One thing, though, which would concern me with testing of this nature is if the testing by itself wouldn’t somehow influence the subject’s choices and preferences. Just like when we first study a newly-discovered primitive tribe of people in a deep jungle somewhere. Our actions and just our presence influence and skew the outcome of any ‘objective’ findings. Or taking a measurement in Quantum Physics…

        Suggestions?

  • Jenny H

    I have read that in humans hand preference is equal between committed lefts and rights — but society favours right handedness so children with no particular preference are encouraged to be right handed.
    I apparently started out ambidextrous so my Mum used to simply take the pencil/chalk/crayon out of my left hand and put it in my right hand. (She has some weird idea that ambidexterity was a signs of mental retardation :-( Unfortunately I STILL (73 years later) still find it difficult to tell right from left :-(
    Personally I am peeved I would have preferred to be ambidextrous, and am now trying to recover that skill properly.

    • Erik Bosma

      Hope you’re successful with the ambidexterity. And I’ll bet your idea re: left/right is probably also a part of the reason. I would imagine there are more reasons as well. That’s not something you could pin on just one thing.

      • Jenny H

        But I have known quite a few people who have told me that as a child they were ambidextrous and their parents of teachers encouraged them to be right handed. I know a man who tells me that as a child her was ambidextrous, and his music teacher told him that he had to decide which ‘hand;’ he was going to be :-( Nobody ever told Leonardo da Vinci that!

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  • okiejoe

    So they have one example of H Habilis who is known to be right-handed but we still don’t know if that was normal or if that individual was known as “old righty,” the one who was different. Get a few dozen more examples so we can see what was normal.

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