created by FractalParadigm on 29/06/2017 at 11:54 UTC
5 upvotes, 2 top-level comments (showing 2)
Comment by FrenchFry_Frosty at 29/06/2017 at 13:28 UTC
5 upvotes, 2 direct replies
Most mammals like dogs and cats do show paw dominance, but it seems to be about a 50/50 split in their case. Whereas in humans about 90% of humans are right handed. Twin studies have show that it is most likely genetic however the specific genes have not been pinned down. Two right handed parents can have a left handed child, but if a child has a first degree relative who is left handed they are more likely to be left handed. This suggests it's a recessive trait.
The reason why right handedness seems to dominate humans probabaly has to do with tool use. As we adapted to use tools, we built tools to be used with a specific had. Over time, one hand randomly became selected over the other. That just happened to be the right hand. There's no reason the right hand is better, but having most people in your species able to use the same tools with the same hand is advantageous. The right hand just happened to be randomly selected. In an alternate timeline it could just have well been the left hand and we'd be living in a world where 90% of humans are lefties.
But hand/paw dominance is a trait seen in nearly all animals. This is due to reinforcing neuro circuits. It's easier for the brain to focus on making on hand/paw extremely dexterous and having the other just be the helper, because for most two handed tasks you only need one main working hand and one stabilizing hand. It's simply saving brain power.
Comment by bioentropy at 29/06/2017 at 21:39 UTC*
1 upvotes, 0 direct replies
So there was a recent review article published in Neuron that examines the literature on this question. I'll paraphrase it and link it at the bottom:
First off some background, left-right asymmetries in the CNS used to be thought of as solely in humans. This was because of handiness but more importantly the discovery of language being mostly processed by the left cerebral hemisphere.
So in the 1970s it was discovered that some other animals, like rats and chicks, had functional brain asymmetries. To date, this has more generally been shown in worms, insects, and all sorts of vertebrates. Birds and fish have been shown to use one eye to pay attention to food while using the other eye to scan for predators while feeding. This seems to be a clear example of how functional brain asymmetries could interact with selective pressures.
Now, left-right asymmetry (think internal organs) begins early in embryonic development. During neurulation, when the neural tube forms, the leftmost mesoderm begins a genetic patterning process that leads to these left-right asymmetries. Scientists have done genetic studies on, most notably, zebrafish to better understand how this might affect CNS development.
Some important points...
1. primary ciliary dyskinesia is a genetic condition where half the people affected have a mirrored distribution of their internal organs (think liver on the left side) but they are mostly right handed.
2. tons of genome studies have found no single gene to correlate well with left handedness, a recent paper proposed that handedness might be determined by 40 genes interacting with the environment. Some of these papers implicate genes involved with schizophrenia or dyslexia.
3. covariation analysis with identical twins say that most of handedness must be determined through environmental interactions.
4. a recent paper has said that language lateralization and handedness have a complex relationship, with left handed people correlating with more VARIATION in the lateralization of language processing.
5. when babies lie on their stomach, they tend to turn to the hand that later becomes dominant.
Tl,Dr: its complicated, not determined by genes that do the general body plan. But if you watch how a baby sleeps you can generally predict which hand they will write with.