How is dominance/non-dominance of hands explained in scientific terms? What happens in the brain?

By Ken Saladin (Former professor of histology (microscopic anatomy)

Handedness and dominance raise fascinating questions on multiple levels—a good excuse for me to postpone going to the office and write this up instead.

There is a hypothesis that the predominance of right-handedness in humans evolved out of a tendency of females, maybe even as far back as our australopithecine ancestors, to hold their infants in the left arm. The human heart tilts to the left, and if a baby is held in the left arm with its ear against the mother’s chest, this is where it would be best able to hear its mother’s soothing heartbeat.

[1] It’s not hard to imagine a reproductive, and hence genetic and evolutionary benefit in this aspect of infant calmness, comfort, and parent-offspring bonding.

Holding the baby that way means the mother would predominantly use her free, right hand for manipulating things, including feeding solid food to the baby when it’s old enough. In this way, right-handedness may have gotten entrenched as the dominant phenotype in the human species to follow. For this to be passed down and spread through the population almost certainly means it is genetically influenced. In the alternative, the behavior could have spread culturally, or by a mixture of genetic and cultural transmission.

[2] Hand-dominance in the genera Australopithecus and Homo is reflected even in skeletal asymmetry.

Hand dominance is found throughout a broad range of vertebrates from frogs and turtles to parrots and apes.

[3] Some authorities describe great apes like gorillas and orangutans as right- or left-handed,

[4] whereas others say hand dominance is not as well developed in the other great apes as it is in Australopithecus and Homo.

[5] The nonhuman apes seldom walk around cradling their infants in their arms, so from the standpoint of the aforesaid evolutionary hypothesis, there’s seemingly less reason to expect them to develop a strong right-left bias.

A purely cultural basis for handedness is a less powerful hypothesis than the genetic–evolutionary one. It would not explain right-handedness in most males, for example, whereas genetics would, since both sexes inherit the same autosomal (non-sex) chromosomes and genes. We find hand dominance not just in humans but in other mammals as well, including mice where it has been easy to run breeding experiments and confirm that it is hereditary. Other species, though, tend to be right-hand-dominant for one task and left-hand-dominant for another—not so consistently dominant across multiple tasks like humans are. If other mammals exhibit hand-dominance and pass it on genetically, as we know from experimentation they do, it would make little or no sense to think humans are an exception to the rule. Actually finding and identifying genes for human handedness hasn’t been fruitful, however;

[6] it’s likely that multiple genes influence right- or left-dominance.

In reference to your question about the brain, motor skills are established by the creation and facilitation of synapses in neural circuits, which I’ll explain below. There is a region of cerebral cortex on each side of the brain, within a fold called the precentral gyrus called the primary motor cortex

[7] (in dark blue in these figures). It has subregions dedicated to the control of muscles on the opposite side of the body; the right motor cortex controls the left side and vice versa.

The right precental gyrus, which controls muscles on the left side of the body.

Part of that motor cortex is dedicated to the hand on the opposite side. In the next figure, body regions are shown in distorted sizes in proportion to how much cerebral tissue is employed in their control—a high degree of fine motor control in the tongue, face, and hand, and lower degree of control, with less precision, in the arm, torso, and lower limb. In my previous post (link 7), I show this depicted in amusing three-dimensional sculptures of a little man, or homunculus.

Schematic of a frontal section through that gyrus, showing regions of motor control.

If we take a tissue section through this gyrus, we find on the microscopic level an incredibly dense web of connections between the cerebral neurons, as shown below.

Photomicrograph of a thin slice of cerebral cortex. These neurons have been stained with fluorescent dyes and photographed through a fluorescence light microscope.

With a different, more conventional staining method (silver nitrate) and microscope, and at higher magnification, those neurons look like the next figure.

All these fine branches from the dark, globose cell bodies of the neurons form complex and exquisitely precise networks of connections we call neural networks or neural circuits. The points where two neurons meet, called synapses,

[8] are the decision-making devices in the brain. To explain how they make decisions would require another essay as long as this one or more. I won’t get into that here, but I’ve discussed it before.

[9] For present purposes, suffice it to say the chemical transmission of signals across these synapses, and excitation of the next cell in the circuit, is easier and quicker at some synapses and a little harder and slower at others.

Throughout life, the brain continually remodels its circuitry, partially in response to genetic influences and even more, in response to experience and environmental influences. This constant remodeling is called synaptic plasticity. Basically, that means that throughout life, our neural circuits can be constantly remodeled, unlike the hard-wired circuits of electronic devices.

No synaptic plasticity here; not a good model for the human brain.

In the brain, old synapses can be broken down and removed and new ones added, even in the space of a few hours. Adding new synapses is called synaptogenesis.

[10] Also, existing synapses can be structurally remodeled to make signal transmission easier—we call this synaptic facilitation—or to make transmission harder. Short-term and working memory are achieved by nonstructural, chemical means of synaptic facilitation; long-term memory is based on circuits of structurally modified synapses.

What is happening when we learn and practice motor skills—such as learning to tie your shoes, learning handwriting or keyboarding, or excelling in some sport or mental skill like chess playing—is that we build circuits of new and facilitated synapses that make that skill easier and easier with experience.

So coming full circle to your original question—“What happens in the brain?”—the answer is that as we develop our handedness in infancy, childhood, and beyond (even in fetal life), it seems that genes influence synaptogenesis and facilitation in the primary motor cortex opposite from the side with the dominant hand, and this is reinforced with experience.

Culled: quora.com