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Why is almost everyone right-handed? A new study connects it to bipedalism

ox.ac.uk

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Re: Why is almost everyone right-handed? A new study connects it to bipedalism

#51

Post by throwway120385 »

yawpitch wrote: post #22
scythe wrote: post #16

Earlier quoted context omitted.

Here's my five minute lunchtime hypothesis: it's because the heart is on the left. As human behavior demanded increasing precision from the hands, being a little farther from the heartbeat was a slight advantage.

Here’s my multiple years of anatomy classes response: the heart isn’t on the left. The aorta is, sure, but the vena cava is on the right. Also people with situs inversus (essentially all organs flipped laterally from “normal”) aren’t obviously more prone to left-handedness.

It might be hard to eliminate confounding factors depending on when the research was done. A lot of people in my generation were still dissuaded pretty heavily from writing with their left hands. I'm not entirely convinced anymore as a lay person that "handedness" is a real, distinct phenomenon that's primarily genetically determined or a result of the organization of the brain. It's equally possible that it's a learned preference and that the way the brain organizes around it is as a result of the preference's impact on how you have to solve problems with your preferred hand in a society that preferences right-handedness.

Re: Why is almost everyone right-handed? A new study connects it to bipedalism

#52

Post by toast0 »

SoftTalker wrote: post #32
jvanderbot wrote: post #27

Earlier quoted context omitted.

You were probably a left-handed person who was taught to write/use tools with their right hand in kindergarten. I got this treatment too.

When was that? I know it used to happen, but I haven't heard of or seen that in my lifetime, I'm nearly 60.

My parents generation is maybe a bit older than you, one of my mom's siblings was forced to right handedness. My mom is left handed and says they tried a little with her, but it only took for some things.

Re: Why is almost everyone right-handed? A new study connects it to bipedalism

#53

Post by rolph »

stackghost wrote: post #49
yawpitch wrote: post #42

Earlier quoted context omitted.

The heart is asymmetrical, but it’s in roughly the center of the chest. The left auricle and ventricle are larger muscles because they’re pumping through the descending aorta to the extremities, that’s the systemic circulatory branch, the plumbing for which is also largely to the right, while the right are pumping into the lungs alone as part of the pulmonary circulatory branch. The left lung (right on those with sit…

>if you really want to kill someone you stab them through the sternum, kind of dead center, not where they hold their hand when performing patriotism. Noted, thanks.

even this is wrong, a penetrating weapon aimed for the heart is applied below the sternum at roughly the positionof the 3rd shirt button, and thrust upward at shallow angle topass behind the manubrium, and is then levered into a pommel upward position so as to lacerate the heart

Re: Why is almost everyone right-handed? A new study connects it to bipedalism

#54

Post by toast0 »

bkjelden wrote: post #28
jvanderbot wrote: post #27

Earlier quoted context omitted.

You were probably a left-handed person who was taught to write/use tools with their right hand in kindergarten. I got this treatment too.

I'm otherwise a lefty but I use computer mice right handed, because when I first started using a computer in elementary school all of the computer labs were set up right handed.

FWIW, I'm a righty, but relearned to use a mouse left handed for ergo benefits at my first real job; now I left mouse for work and right mouse for home. I prefer ambidextrous mice anyway, but it's really hard to find a left hand mouse if you want that. Even the ambidextrous mice often have thumb buttons for the right thumb. It's not to hard to learn to use a pointer with either hand; IMHO as someone who can't do a lot of complex motion with my non-dominant hand. I think there's a lot of convenience gained by accepting right mousing, although it is a longer reach if you have a keyboard with stuff to the right of your letters.

Re: Why is almost everyone right-handed? A new study connects it to bipedalism

#55

Post by cortesoft »

yawpitch wrote: post #22
scythe wrote: post #16

Earlier quoted context omitted.

Here's my five minute lunchtime hypothesis: it's because the heart is on the left. As human behavior demanded increasing precision from the hands, being a little farther from the heartbeat was a slight advantage.

Here’s my multiple years of anatomy classes response: the heart isn’t on the left. The aorta is, sure, but the vena cava is on the right. Also people with situs inversus (essentially all organs flipped laterally from “normal”) aren’t obviously more prone to left-handedness.

> Also people with situs inversus (essentially all organs flipped laterally from “normal”) aren’t obviously more prone to left-handedness.

I feel like this isn’t really an argument against the theory. If right handedness did evolve because of heart position, a later genetic mutation to have the heart on the opposite side wouldn’t suddenly undo the previous evolution towards right handedness.

Re: Why is almost everyone right-handed? A new study connects it to bipedalism

#56

Post by Someone »

NickC25 wrote: post #14

What does it say for mixed-handed folks like myself (different skillsets per hand - in other words, throw and write with different hands)? What about cross-dominance (different body parts differ on dominant side - in other words, a right-handed person being left-foot dominant)? I've been told that it's effectively a mental illness if discovered during childhood (as is ambidexterity). Yet I can't help but think that i…

> I've been told that it's effectively a mental illness if discovered during childhood (as is ambidexterity). Yet I can't help but think that it is not a mental illness, but rather something else.

https://en.wikipedia.org/wiki/Handedness#Types: “Mixed-handedness or cross-dominance is the change of hand preference between different tasks. This is about as widespread as left-handedness.”

⇒ about 20% of the population is not strictly right-handed. That’s not a majority, but I think the word to use for that is “normal”.

Re: Why is almost everyone right-handed? A new study connects it to bipedalism

#57

Post by scythe »

gherkinnn wrote: post #23
scythe wrote: post #16

Earlier quoted context omitted.

Here's my five minute lunchtime hypothesis: it's because the heart is on the left. As human behavior demanded increasing precision from the hands, being a little farther from the heartbeat was a slight advantage.

Wikipedia on Situs Inversus (visceral organs are mirrored, heart on the right, liver on left) [0], mentions mixed results regarding handedness. There would be a load of other confounding factors here and I know nothing about medicine. 0 - https://en.wikipedia.org/wiki/Situs_inversus

Situs inversus ("dextrocardia") is a rare disorder. What I postulated is a (very) small selective advantage leading to a neurological mechanism evolving over generations, not a direct line from the heart to handedness during development. Anyway, the effect would be very slight, and even if it did exist, it could have gone away later, but dexterity would have been baked in at that point (see also the ocular blind spot).

Re: Why is almost everyone right-handed? A new study connects it to bipedalism

#58

Post by scythe »

booleandilemma wrote: post #50
scythe wrote: post #16

Earlier quoted context omitted.

Here's my five minute lunchtime hypothesis: it's because the heart is on the left. As human behavior demanded increasing precision from the hands, being a little farther from the heartbeat was a slight advantage.

I wonder why you're getting downvoted? Even if it turns out you're completely wrong it's still an interesting point and something I never even considered before.

Sometimes I think people downvote me because they're frustrated that I didn't engage further. After twenty years of Internet discussions, I'm a little burned out and I tend to fire and forget.

Re: Why is almost everyone right-handed? A new study connects it to bipedalism

#59

Post by Technolithic »

taeric wrote: post #37

I am curious at what age hand preference develops. And can you exert any influence on that development? In particular, I would expect the influences to be somewhat counter intuitive. With things like having to use the left hand to hold a caregiver's hand in early walking preferencing the right for accessory use. At infant ages, it would be neat to see if preference of holding a baby on a side influences things.

The introduction of this article makes reference to a couple of papers (e.g. https://doi.org/10.1038/s41598-017-16827-y ) that handedness is observable in utero but cautions small sample sizes for these studies.

Re: Why is almost everyone right-handed? A new study connects it to bipedalism

#60

Post by adrian_b »

It is a very bad choice of words to say that "bipedalism" is a cause for hand specialization.

For hands, it is completely irrelevant how many legs a human has, regardless if a human had used 2, 4, 8, 14 or any other number of legs for walking, the hands would have become specialized.

The reason why the hands acquired specialized roles was that they were no longer used for locomotion, i.e. for brachiation in the trees, like in orangutans or gibbons, but their purpose became holding, controlling and moving various objects from the environment.

It is wrong to say that bipedalism has freed the hands to be used for other activities than locomotion, because the causality was reverse, locomotion became restricted to the hind legs, because the hands were used for other activities, like throwing sticks and stones, so they were no longer available for locomotion.

The strong specialization of the 2 hands has appeared because in most cases when something is transformed with the hands, e.g. bones are broken to get the marrow or stones are knapped to get a cutting edge, one hand must be used to fix in place the object that is processed, while the other hand must move against it, normally with some tool.

For the former role, the left hand became specialized, while for the latter role, the right hand became specialized.

Similar specialization is also seen at other animals where a pair of legs is no longer used for locomotion, but it is used for manipulation, for instance at crabs and lobsters.

So there is no doubt that the specialization of the hands was a necessity when they stopped being used for locomotion. However, it is not known why the right hand became the moving hand and the left hand became the holding hand, and not vice-versa. It could have been a random event or it could have been related to the asymmetry in the locations of the unpaired internal organs, like heart, liver, stomach and so on.

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