Thought Leadership

Non-human handedness

By Colin Walls

On a previous occasion, I wrote about my interest in handedness in humans. This phenomenon continues to intrigue me and that feeling is increased when I think beyond the human sphere. There is, I believe, little or no evidence of handedness in other animals – although I would be very unsurprised if it were discovered in other primates. The topic becomes more interesting still if one looks beyond living beings’ bodies and considers chemistry and physics …

In chemistry, the primary mechanism that is studied is the combination of various elements to form compounds, which is more commonly thought of as combining atoms to form molecules. If we ignore isotopes, all the atoms of a given element are identical and, when combined in a given ratio, yield identical molecules. For example, if you combine x atoms of sodium [Na] with x atoms of chlorine [Cl], you end up with x identical molecules of salt [sodium chloride, NaCl]. [Yes, I do know about ions and ionic lattices, but I am just trying to keep this discussion simple.] As compounds [molecules] become more complex, this rule continues to apply.

However, in organic [carbon based] chemistry, there is a small wrinkle. There is no problem with simple compounds. For example, methane is a combination of one carbon atom and four hydrogen atoms and every methane molecule is identical. If you could see a methane molecule, you would observe that it has a tetrahedral shape – a triangular pyramid. It is this shape, produced by the arrangement of hydrogen atoms around a central carbon atom, that leads to an interesting effect. If you insert an atom of oxygen between one of the hydrogen atoms and the carbon, you have a new compound: methanol [or methyl alcohol]. What is interesting is that the methanol molecule is asymmetric – you can envisage two different arrangements, which are mirror images of one another. These are thought of as left- and right-handed forms.

From a chemist’s perspective both forms of methanol appear quite identical. As you might expect, if you perform a chemical process in a test tube that yields this compound, you get a 50:50 mix of left- and right-handed molecules. However, if methanol is made by a biological process [like fermentation – an old name for methanol is “wood alcohol”], only molecules of one handedness are produced. This asymmetry in biologically produced molecules is very common [perhaps universal] and, as far as I can determine, not clearly explained. There are some odd side-effects. For example, if you use sugar made artificially so that all the molecules are of the “wrong” handedness, it still tastes sweet, but is not ingested by the body [i.e. it has no calories]. I find it odd that the taste receptors are not geared to a single handedness. Another implication is that an accumulation of handed molecules found in an extra-terrestrial context may be considered an indirect indication of the presence of life.

So much for chemistry. The world of physics is mostly a lot more ordered. Everything seems to be quite symmetrical. For example, all subatomic particles seem to come in nice neat pairs. The odd this is that their distribution is not evenly distributed. In fact, this asymmetry accommodates the very existence of the material universe. If all the particles were in existence in symmetric quantities, the universe would contain equal quantities of matter and anti-matter. Since, if matter and anti-matter come into contact with one another, they are both annihilated and converted to energy, the universe would have long since self-destructed. Again, no clear explanation of this other form of handedness is forthcoming.

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This article first appeared on the Siemens Digital Industries Software blog at