Most of the human body is reasonably symmetrical, either centrally located or, where pairs are involved, symmetrically located on either side of the body. However, for unknown reasons, some important internal organs are not centrally located. For example the heart, stomach and spleen are all situated on the left side of the body, and the liver and gall bladder are found on the right. The lungs are also slightly asymmetrical, with two lobes on the left side and three on the right.
The genetic mechanism by which a person's DNA can tell the developing fetus that, for example, the heart should go in the left side of the chest is still very poorly understood. Whether such a mechanism may be connected to the gene for handedness (it may, for example, also dictate whether the left or right hemisphere of the brain should become dominant) is also unknown.
In its very earliest days, the human embryo appears to develop completely symmetrically. The asymmetry that later develops is thought to be caused by microscopic hair-like monocilia within the developing embryo. While most monocilia tend to wave back and forth at random, rather like seaweed on the seabed, the monocilia in certain proteins all move in a constant right-to-left direction, creating a kind of spin or directional current, which acts as a lead-off mechanism for the body to “know” left from right.
A Japanese team led by Nobutaka Hirokawa has recently managed to identify the gene that causes this left-to-right monocilia movement, and have shown how mice bred without the gene have a 50-50 chance of having a heart on the left or the right. It is possible that such a gene may affect brain (as well as body) asymmetry, and thus may conceivably be linked to handedness. All this does not, however, explain why the monocilia flow from right-to-left and not left-to-right, but it is thought that this could have been caused by a purely random event at a very early stage of evolution (a so-called “spontaneous symmetry-breaking event”, such as occurs in nature all the time, analogous to the situation where a pencil balanced on its point has to fall one way or another).
There is a rare congenital condition known as situs inversus, affecting fewer than one in 10,000 people, in which most or all of the internal organs of the body are reversed, with the heart, stomach and spleen appearing on the right, and the liver and gall on the left. In all other respects, the person’s anatomy is unremarkable, and often the individuals may live a full and normal life. There are even rare cases of identical twins where one has a heart on the right and the other has a normal left-sided heart, showing that such an occurrence is genetically possible.
It was once thought that situs inversus was linked to left-handedness. Lodovico Richieri, in the late 15th Century, was the first to make this association, although Sir Thomas Brown pointed out in 1648 that situs inversus is a statistically much rarer condition than left-handedness and so the two are unlikely to be connected. In 1788, Matthew Baille produced definitive evidence that situs inversus sufferers may also be right-handed. But, regardless, the idea was resurrected by Andrew Buchanon in 1862, who "explained" the effect by noting that the right half of a normal human body is slightly heavier than the left side (as indeed it is, by about a pound, largely due to the arrangement of the internal organs), so that people tend to compensate by leaning slightly to the left, thus giving greater freedom of movement to the right arm. People with situs inversus, therefore, Buchanon argued, tend to lean slightly in the other direction and end up left-handed, an ingenious but scientifically specious rationale.
Today, it appears clear that situs inversus has no connection with handedness at all, and such individuals may be right-handed or left-handed.