Many molecules, including amino acids, have isomers that are left-handed (levorotatory) or right-handed (dextrorotatory). When amino acids are formed in the laboratory, about half are right-handed and half are left-handed. But in living things, this is not the case. Almost all amino acids used by living things are left- handed. Perhaps there is some as yet unidentified process in living things that favours the formation of these molecules over their right-handed counterparts. Or perhaps the very formation of living things was fostered by left-handed molecules that formed when our solar system formed.

Many of the organic compounds on Earth are believed to have derived from meteorite impacts. Some meteorites are rich in organic compounds. The Murchison meteorite is the most recently observed and studied of the organic meteorites known to have hit Earth. A study of the organic compounds in this meteorite has produced some surprising results. More of the amino acids in the Murchison meteorite are left-handed than right-handed. Some of the amino acids in the meteorite show a preponderance of left-handedness, whereas others show just a slight increase in left-handedness.

What does this mean? Some scientists hypothesize that life could not have originated on Earth without an abundance of left- handed amino acids. Because there are more left- handed than right-handed compounds in living things as well as in organically rich meteorites, some scientists suggest that life on Earth may have derived from the compounds that were formed in space. It is possible that the molecular cloud from which the solar system condensed produced a preference for left-handed compounds. How this occurred is still unknown. Some scientists hypothesize that a preference for left-handed compounds was imposed on the solar system by a nearby neutron star that caused light to be polarized in a circular pattern (causing compounds to form with distinct handedness). If this happened, then scientists looking for life elsewhere in the solar system may examine rocks from other planets to determine if they, too, have greater numbers of left- handed chemical compounds.

A preference for the left or right hand is shown by humans and many other species, including mammals, birds, and amphibia. In the case of the human, the preference first appears several months before birth and can be recognised on ultrasound scanning.

Approximately 5-8% of babies are born left- handed and the remainder right-handed. How these preferences arise is a topic of continuing discussion, but clues have been found in studies of embryos and even in the structure of biological molecules such as proteins.

Vertebrate embryos are considered to have a symmetrical arrangement when they begin development, so the question could be expanded into how and when embryonic symmetry is broken and then how preferential handedness arises somewhat later in development. The first physical signs of asymmetry in the embryo have been detected at the primitive streak stage - for example, the node region at the future cranial end of the streak in chick embryos is deviated to the left, and in mouse embryos the cilia of cells lining the node beat in an anticlockwise pattern. It is proposed that this early asymmetry may be a result of the ‘handedness’ (chirality) of the molecules that drive the cilia, and result in a cascade of events which produce asymmetric development of the internal organs. Subtle asymmetries in gene expression have also been detected in early embryos before any morphological differences become apparent: two genes called nodal and lefty are expressed only on the left side of the midline.

Although the human body has a symmetrical appearance when viewed externally, many of the internal organs are arranged asymmetrically. So for example the heart is biased to the left, and the liver to the right, while the right kidney is positioned lower than the left kidney. Occasionally, someone is born in whom all the asymmetries are the other way round, a mirror-image arrangement which is called situs inversus. This occurs in about 1 in 10,000 people. Complete reversal like this does not necessarily create any additional problems for the person. It is interesting that in conjoined twins who are joined at the thorax or abdomen, one of the twins will show situs inversus. This probably results from cross-signalling between the two primitive streaks in the embryonic disc. However, if there is only a partial reversal of position, as for example when the heart is biased to the right (dextrocardia) and all the other organs are in their usual positions, then clinical problems can arise because of the changed inter- relationships within the body. In mouse embryos, a mutation in the gene called inv causes situs inversus, and is associated with a reversal of the normal distribution of nodal and lefty. It is becoming clear that the asymmetries of the body are tightly controlled by genes that have been conserved during evolution, and are implemented by cascades of signalling molecules.

The preference for a particular hand appears to be linked with an asymmetry in the brain that controls it. It has been found that the regions of the cerebral cortex which control each upper limb do indeed differ in volume by about 7%. Thus, in right-handed people the left somatomotor cortex controlling the right upper limb is about 7 % larger than the corresponding cortex of the right side of the brain (motor pathways to the limbs cross the midline), although it is difficult to decide whether this difference is causing the preference or a result of the preferential use of one hand. There is also evidence for a link between hand preference and speech: in most people, the speech centre is located in the left cerebral hemisphere, the same side as the cortex controlling the preferred hand. Thirty percent of left-handed people have their speech centre located in the right hemisphere (only 5% in right-handers).

So, internal asymmetries arise early in embryonic development, perhaps triggered by preferred molecular configurations and/or genetic events, and in some way lead to behavioural bias in the use of the upper limbs several months before birth.