Most of the human race is right-handed, and its been that way for 500,000 years. Dr Naomi Lavelle finds out more about handedness.
SCIENTISTS tell us that as far back as 500,000 years ago, our ancestors were predominantly right handed, just like today. Yet, the argument about what causes our handedness is still up for debate.
The most widely held theories are that handedness is a genetic trait and is determined by which hemisphere (side) of our brain is the most dominant. A staggering 85-90% of people are right handed.
Right handedness is thought to be controlled by the left side of the brain and visa-versa. So left-handed people get to joke that they are the only ones in their right minds; until now.
A recent study has thrown a lot of these solid theories on their heads. It was known that handedness was controlled by a network of genes, not just one; and that a left or right handed bias begins within the womb. This new research, however, reports that the process begins within the developing spinal cord, not the brain.
The study examined embryo development from eight to 12 weeks (post conception) and reported that gene activity in the spinal cord is asymmetrical in the womb.
It was previously thought that a difference between gene activity in the left and right hemispheres of the brain was the most likely cause of handedness. Limb movement is initiated in the brain where signals are sent to the spinal cord via the motor cortex. However, the results from this study are reported from a stage of embryonic development when the motor cortex is not yet functionally connected to the spinal cord.
The study looked at the region of the spine that controls limb movement, particularly the hands, fingers and arms. Previous research has shown that embryos at this stage of development will show a preference for sucking one thumb over another.
Nature versus nurture
This new study shows that gene expression is asymmetric in these specific (limb movement controlling) regions of the spine. There are differences between the expression of certain genes when comparing left and right side of the spinal cord, mimicking what is expected of handedness. These changes in the levels of genes expressed in either hemisphere of the spine are a result of local, environmental factors (epigenetic) within the embryo. This means that although the same genetic coding is found in certain genes within both sides of the spinal cord, the levels of the expression of these genes can vary greatly, due to these epigenetic factors.
So are lefties still in their right mind?
Though these results suggest the cause of handedness can be linked to gene activity in the spinal cord, it is assumed other factors contribute to handedness later in embryonic development and after birth. Some of these factors are associated with hemispheric dominance within the brain. Movement of the left hand is still under the control of the right brain.
Are we alone in our handedness?
Do other animals show a preference for handedness as well? It is harder to determine handedness in other species as most use all four limbs for movement or do not carry out very dextrous tasks that would indicate a preference for one hand over another. Some studies have reported interesting results though; there are two species of Australian kangaroos (the red and the eastern grey) that show a preference for left handedness, across the whole population. This is easier to observe as these kangaroos walk on two legs.
Parrots apparently show a population level bias of handedness too, with 90% of parrots preferring to use their left foot for a variety of tasks.
A recent study carried out at Queens University Belfast reported that, when it came to complex tasks such as removing a piece of food from a jar, cats show a distinctive preference for one paw over another. 95% of the female cats studied showed a preference for their left paw, 95% of male cats preferred to use their right. This gender preference for handedness has also been reported in opossums.
A bit of ancient history
So how do we know that right handedness was dominant 500,000 years ago? Anthropologists have come to this conclusion by looking at the teeth of cavemen (and women). Our ancestors did not spend all their time hunting and gathering; once the meat was eaten they would process the hide. To do this they would stretch the hide, holding one side in one hand and the other between their teeth.
Their dominant hand would work a stone scraping tool across the hide and sometimes the stone would slip and scrape off their front teeth. These scrape marks have allowed scientists to determine which hand was holding the stone and apparently 90% of the time it was the right.
While this new research changes the cause of handedness in humans, it doesn’t change the fact that only about 10 to 15% of the population have ever been left-handed. In case you are wondering, I’m one of them.
- Sebastian Ocklenburg et al., ‘Epigenetic regulation of lateralized fetal spinal gene expression underlies hemispheric asymmetries’ eLife 2017;6:e22784
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