I was cooling down from a Pilates workout when my virtual instructor said that the right side of her body is stronger and more flexible than her left. I, on the other hand, am stronger on the left side of my body (neither side is flexible though…I’m stiff as an old lady!). I prefer to use my left arm for heavy lifting, I open jars with my left hand and I balance better on my left foot. That was when I begin to wonder: why is our body asymmetrical? And why do we prefer to use one hand over the other?
Have you ever wondered how our internal organs are arranged? It all starts at the node on the embryo’s midline. The interior of the node is lined with tiny hairs called cilia that whirl round and round in synchronized motion to push the fluid from the right to the left of the embryo. This then triggers a genetic activation on the node’s left hand rim, causing the release of calcium atoms. The surrounding cells respond by making proteins called Nodal, which lead both sides of the embryo to be chemically different.
While our bodies start out symmetrical, left-right asymmetry begins at around six weeks. The first organ to show asymmetry is the heart. A simple tube loops to the left and different structures start to form on each side of the heart. At the same time, other organs such as the stomach and liver begin to move clockwise away from the midline of the embryo; an appendix appears on the right side of the large intestine; the right lung grows three lobes and two on the left.
Researchers indicate that it takes three or four hours for the left and right to be determined but there’s still limited understanding of what happens in between. Below are three other questions that require further investigation by the scientific community:
- Apparently two cilia is enough to start an embryo on its proper development. So what is the function of the rest of the cilia?
- How exactly does Nodal help determine the anatomy of each side of the body?
- As most research on this topic is performed on animals such as zebra fish and mice, it’s unclear if humans develop in the same way.
Did you know…
1 in 20,000 people suffer from a condition known as situs inversus, whereby the internal organs are inverted left to right! (nytimes)
Now that we have some idea of how asymmetry happens inside the human body, let us explore the idea of lateral preference (predominant use of either side of the body for carrying out specific actions). Cognitive scientist Stephanie Braccini states that “a strengthening of individual asymmetry [may have] started as soon as early hominins assumed a habitual upright posture during tool use or foraging”. Our ancestors, Homo habilis and Homo erectus, demonstrated some evidence of right handedness through the stone tools made about 1.5 million years ago in Koobi Fora, Kenya. It wasn’t until the Homo heidelbergensis appeared (some 600,000 years ago) that a clear right handed preference emerged. For example, the wear on the preserved teeth of Homo heidelbergensis suggests that food was usually brought to the mouth with the right hand.
The division of neurological labor has been an important feature in evolution. Both hemispheres of the brain control motor action on the opposite sides of the body, but they’re not equal in their control of different types of behaviors. This causes a bias of one hand over the other for certain tasks. The dominance of one hemisphere over the other for certain behaviors is known as cerebral lateralization. Having only one hemisphere control a response lowers competition, which enables different processes such as language and attention to function at the same time across two hemispheres. The left hemisphere may be dominant for speech but the same region also controls hand actions. Scientists conclude that this leads the majority of the human population to use their right hand for tools or gestures.
At the start of the development of motor skills, children may use both hands equally for simple actions such as reaching for objects. But when a complex task such as writing is performed, the specialized processing of the left hemisphere is activated. Of course, not all of us are right handed and genetics as well as personal experience both contribute to that difference. We don’t know which hand will be dominant when we’re infants, but through trial and error, we find out which one works better and is more comfortable.
There are plenty of other theories, speculations and arguments on this topic but I won’t be doing an in-depth analysis. But this is pretty interesting, don’t you agree? Also, the fact that left handers who adapt and do things with their right hand is pretty cool. My brother used to be amazed that I could operate the mouse with my right hand even though I’m left-handed. If you’re interested, here’s an interesting read: Left Preference for Sport Tasks Does Not Necessarily Indicate Left-Handedness: Sport-Specific Lateral Preferences, Relationship with Handedness and Implications for Laterality Research in Behavioural Sciences.
And before I sign off, I want those of you who identify as ambidextrous to know this – YOU ARE FREAKING AWESOME.