As you scroll through these words with a swipe of the finger or click of the mouse, you're likely doing it with the same hand you use to draw or swing a bat. But why? Science has told us that our favoring of our right or left hands could be traced back to our roots as thumb-sucking fetuses, where the brain directs such movements, but new research out of Germany is casting doubt on this thinking, suggesting that it all begins in the spinal cord instead.
Ultrasound scans in the 1980s revealed that we develop a preference for the right or left hand as early as the eighth week of pregnancy and begin sucking the left or right thumb from the 13th week. It is known that arm and hand movements are initiated by the brain's motor cortex, which sends a signal via the spinal cord to turn commands into motion.
But for researchers at Ruhr-Universität Bochum, a public university in Germany, something didn't quite add up with this line of thinking: the motor cortex is not connected to the spinal cord when we begin to exhibit this early asymmetry. So with the precursors for handedness evident before this connection has formed, the researchers suspected that its roots could be found in the spinal cord instead.
They set out to explore this theory by studying the gene expression in the spinal cord during the eight to twelfth week of pregnancy. Sebastian Ocklenburg, who headed up the research, tells us that they did this by first drawing spinal cord tissue samples from the fetus. They then used a product called INVIEW Transcriptome Discover to analyze the extracted mRNA, the family of RNA molecules that transfer genetic information from the DNA to protein-making structures called ribosomes.
He and his team found clear right and left differences in the spinal cord segments of the eight-week fetuses that control arm and leg movement. Interestingly, the team traced the cause of this asymmetric gene activity and believe that epigenetic factors may be behind it. That is to say, that environmental influences may be at play. One example of this could be enzymes bonding methyl groups to DNA which in turn inhibits reading of the genes. When this process plays out to a different extent on the left and right side of the spinal cord, it would mean there is a difference in gene activity on either side.
"These results fundamentally change our understanding of the cause of hemispheric asymmetries," the researchers write.
Their work was published in the journal eLife.
Source: Ruhr-Universität Bochum