Science Facts

Why Are We Right Or Left Handed? – Handedness Facts

Handedness

Evolution spent nearly 400 million years crafting these works of art, two of the most important pieces of the human evolutionary puzzle. Yet 99% of humans end up being good with one hand and not the other for everyday tasks like writing, high-fiving, and all-important one-handed texting. Even life itself seems to have chosen sides: amino acids are “left-handed,” DNA turns into a right-handed helix. Both hands seem fully capable, and both are connected to fully functional arms.

In the animal kingdom, where people find creatures with a preference for one paw, hoof, or wing, it’s usually 50/50. Yet on average, only 1 out of 10 humans are southpaws. Why are so few people left-handed? Maybe because it’s the world is conspiring against them. If only someone would open a chain of stores specifically stocked with products for left-handers. That’s a money-maker. Even language hates lefties. To be correct is to be right, and proper is also a word from “right.” Gods are full of righteousness. To be skillful is to have mastery.

Why are we right or left-handed?

The right lung is divided into three lobes, while the left lung has only two. Two areas for speech and language are found only on one lobe in the brain, usually the left. The starfish show similar internal asymmetry. The interesting thing isn’t that human bodies are asymmetrical in the first place, but that we all have the same asymmetry.

About 1 in 20,000 people on Earth have a genetic condition called situs inversus, where the internal organs are inverted left to right. These people usually show no negative effects from their reversed innards. Highly symmetrical bodies might signal quality genes, and looking for balanced bodies might make better offspring.

When humans pair up, many kinds of symmetry and a lot more than just balanced bodies. To define left from right is to determine which ends will be the head, tail, and back and belly. But that only sculpts our outer shell. The orientation of organs may have to go with the flow. Some reasons cause left or right-handed people:

  • For the brain.
  • For gene.
  • Natural selection.

For brain function

Left itself comes from Old English, meaning “weak.” Words for left give us sinister. But language turns out to be an essential part of the puzzle. Our brains are cross-wired, meaning the left side and vice versa control the right hand. It doesn’t mean our brain’s symmetrical. In the 1860s, French scientist Paul Broca discovered that a region of our brains used for speech processing is usually located on one side. Language is a complex process, and the move to one hemisphere probably helps the brain deal with it more efficiently.

In 99% of right-handers’ brains, Broca’s area is located only in the left hemisphere. And in left-handers’ brains, it’s also on the left, 70% of the time. Only 19% of lefties process speech in the right hemisphere, and 20% use both. So language and hands, not directly linked.

Two similar theories have been proposed to explain this. They suggest that as early humans evolved, a gene mutant popped up that threw a whole bunch of our brain’s functions to one side and allowed them to specialize.

For genetic behavior

Handedness is genetic, and technology has allowed us to explore the numerous genes associated with all types of human tendencies and behaviors with more accuracy. In 2007 researchers found a gene LRRTM1 which was involved in the development of handedness. There are two alleles of that gene: the “D” gene and the “C” gene.

  • The D gene is more frequent in all populations and promotes right-handed preference.
  • The C gene does not promote left-handedness but instead is a gene of chance.

It appears that 50% of people with this a left-handed, and 50% are right-handed. The discovery also helps explain some of the well-known phenomena regarding handedness. More specifically that you can teach someone to use their non-dominant hand and become ambidextrous. The leading theory is that it affects the asymmetry of the brain. The LRRTM1 gene also slightly increases a person’s risk of developing schizophrenia.

If both parents are leftists, then there’s a 26% chance that their baby will be, too, more than double the normal odds. Premature babies are more likely to be left-handed. There’s also some evidence that ultra-sonic scans increase the chances of a baby being a left-hander. All of which is an elaborate way of saying nobody knows.

Language and hand dominance move to the left hemisphere if someone has two copies of the so-called Right Shift mutation. That shift is slightly less if they carry one copy and if they don’t carry the Right Shift mutation at all. Language and handedness could go to the right or left, just by chance. It means instead of their being a gene for left-handedness, it’s the lack of a gene.

A genetic influence makes sense since parents sometimes seem to pass on handedness to their kids. While two right-handed parents only have a 9% chance of having a left-handed child, one parent being left-handed raises that to 19%, and two to 26%. Research has found evidence of right-hand preference in captive chimps.

For natural selection

There’s even another theory. Human bodies tell right from left when it comes to guts. The whole brain-hand sidedness can be traced back to embryonic days. In 2013, researchers reported that some of the same genes tell spleens and stomachs which way is which are linked to brain asymmetry.

If being right-handed and shifting our speech and coordination to one side was such an excellent thing for evolution, why do lefties still exist? A simpler theory is just that nature enjoys variety. Chris McManus believes that left-handers may have what he calls “random cerebral variation,” their brains are more shuffled up. It might lead to more unexpected connections, cross-talk between the right and left hemispheres, and maybe more creativity. What’s clear is that having a diverse set of brains has made humans a much more exciting species.

How humans become right and left-handed?

The brains of our primate relatives work the same way, but scientists believe that the human brain is more asymmetrical. So it may not be a coincidence that the side of our brain in charge of speech also controls the hand used to produce written language. It could be that natural selection made most righties because the left brain is telling our right hands how to write.

This theory is helped by studies that have found that many lefties tend to process language more evenly across both hemispheres of their brains. In some cases, entirely on the right side. So it could be that lefties don’t exactly inherit their hand dominance. Instead, they inherit a lack of neurological bias toward the dominant left brain.

Handedness, or hand dominance, is unique to humans among primates. Close relatives like chimps and gorillas do not consistently favor one hand over the other. While many cats and dogs will often favor one paw over another, the breakdown between righties and lefties is roughly equal. Though many believe one exists, scientists have yet to discover a gene that determines the dominant hand. Though handedness runs in families, that doesn’t necessarily mean it’s hereditary.

Identical twins, who share the same genes, don’t always have the same dominant hand. There are many theories, including the idea that our right-handed world is an evolutionary byproduct of cooperation among humans. According to one study released in 2012, cooperative acts, like sharing tools, have favored the presence of a dominant hand within a group. So everyone can use the same tools and stuff.

The more widely accepted theory is that hand dominance is connected to the asymmetry of our brains. Our brains are contralateral, which means that the left hemisphere of the cerebral cortex. Here, language and logical processing are usually localized and control the movement on our body’s right side. And the right hemisphere, where spatial recognition usually takes place, controls the left side of our body.


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Sources:

Holder MK. “What does Handedness have to do with Brain Lateralization (and who cares?)”.
Papadatou-Pastou, Marietta; Ntolka, Eleni; “Human handedness: A meta-analysis.” Psychological Bulletin. 146 (6): 481–524.
Santrock JW, “A Topical Approach to Life-Span Development.”

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