We Might Have Been Wrong About What Makes People Left- or Right-Handed
Scientists have long been intrigued by why people tend to naturally favour the use of one hand over the other. We still don’t fully understand what causes people to be left or right handed, but for decades researchers have assumed that the origin lies inside our brains.
But a study published in 2017 provides early evidence that it’s not only the brain that determines handedness – the spinal cord could also play an important role.
An international team of biopsychologists led from the Ruhr University Bochum in Germany has now shown that genetic activity in the spinal cord is already asymmetrical in the womb, and could be linked to a preference for either the right or left hand.
“These results fundamentally change our understanding of the cause of hemispheric asymmetries,” the researchers wrote in the journal eLife.
To be clear, this is nothing more than a hypothesis in its very early stages for now – we need a lot more independently verified research to come out before we throw out decades of work on handedness and the brain.
But it suggests an intriguing possibility – what if handedness starts to be determined before our brains are even involved in controlling our movements?
Ultrasound scans back in the 1980s provided evidence that left- or right-handedness develops in the womb from as early as the eighth week of pregnancy, and can be easily detected by the 10th week.
Research has also shown that from the 13th week of pregnancy, unborn children in the womb tend to preference either sucking their left or right thumb.
Because arm and hand movements are initiated by the motor cortex in our brains, scientists had always looked to asymmetric gene expression in the motor cortex and other parts of the brain to explain why this preference happens so early on.
But in the developing embryo, the motor cortex isn’t always functionally connected to the spinal cord. In fact, when the earliest indications of hand preference appear, the spinal cord hasn’t yet formed a connection with the brain.
“Human foetuses already show considerable asymmetries in arm movements before the motor cortex is functionally linked to the spinal cord, making it more likely that spinal gene expression asymmetries form the molecular basis of handedness,” the team wrote.
Because of this, the team decided to investigate whether perhaps something happening independently in the spinal cord might influence handedness.
They looked at the gene expression in the spinal cords of five human foetuses between the eighth and 12th week of pregnancy.
The researchers detected differences between the amount of genes being expressed on the right or left side of the spinal cord in the eighth week. Interestingly, this difference was seen in the segments of the spinal cord that control the movements of arms and legs.
The team also looked into what was causing the asymmetric gene activity, and showed that it was environmental factors that seemed to be controlling whether spinal cord activity was greater on the left or right side.
Environmental factors can control gene expression through something known as epigenetics – a layer that sits above our genome and determines which genes are switched on and off.
The study suggests that it’s through epigenetics that environmental factors can cause more gene activity on one side of the spinal cord compared to the other.
“Our findings suggest that molecular mechanisms for epigenetic regulation within the spinal cord constitute the starting point for handedness,” the researchers concluded.
As we mentioned above, this is a very small and early study, and it’s too soon to throw out our current assumptions about handedness just yet. But it’s definitely intriguing new evidence that scientists will need to investigate further.
With so many questions remaining about how and why people are right- or left-handed – and how this affects them later in life – the more we can learn about why we favour certain limbs, the better.
The research was been published in eLife in February 2017.,