When we choose a partner with whom we want to propagate our genes, we choose one that in our judgment is beautiful and strong. One of our criteria for beauty is symmetry. To us, a beautiful face is one that is symmetrical. Lack of symmetry is often taken as a defect, caused by disease, or defective genes, or a past accident that could be a debilitating handicap. And, indeed, Nature has made us symmetrical - we have bilateral symmetry, that is, our left side is the mirror image of our right side.
But we are not quite perfectly symmetrical. Very few of us are ambidextrous (with equal facility in the use of our left and right hands), so that most of us have one arm slightly bigger and stronger than the other. Moreover, our bilateral symmetry is no longer faithfully maintained beyond our external surface. We do have two lungs and two kidneys and other organs that are positioned symmetrically inside our body. But we have but one heart, which is off to one side, and its insides do not have symmetrical structures or functions. And our digestive system is a mess in terms of symmetry.
Departure from symmetry is particularly evident in our molecules. When atoms are put together to form molecules, the resulting structures, especially the big ones like proteins and DNA, could be quite asymmetric. Molecules, like other structures, could have mirror images. Thus, amino acids, the components from which proteins are built, could be left-handed, or right-handed, and those have similar properties and reactivities. Surprisingly, our proteins are made up of only left-handed amino acids. In fact, left-handed amino acids are the only ones our body makes. (Some simpler organisms do make some right-handed amino acids in addition to the usual left-handed ones - and they incorporate those in their toxins.) Further, when some parts of our proteins are folded in the form of a helix, the helix is almost always right-handed. And our DNA is made up of nucleotides that assemble into right-handed spirals.
Why? Why is there such asymmetry in our molecules? Why is one molecular form favored over its mirror image? Actually, handedness, or chirality, figures prominently in many scientific fields. (Some scientists prefer to use the term chirality, which means exactly the same thing, but is quite a bit more fancy because it is descended from χειρ (kheir), the Greek word for hand.) Is there a fundamental basis for chirality? Let's consider one possibility.
During the first half of the 20th century, physicists believed that the fundamental interactions of matter are not chiral – that is, that they do not distinguish between left and right. In the terminology of the physicists, an interaction that does not distinguish between left and right “conserves parity.” In the 1950s, however, the decay patterns of some elementary particles made some wonder whether one of the fundamental interactions, the weak interaction in particular, may not in fact conserve parity.
Two young Chinese physicists, Tsung-Dao Lee and Chen-Ning Yang, who had emigrated to the United States during the Second World War, investigated the experimental evidence and concluded that there was really no experimental basis for the assumption that the weak interaction conserves parity. They suggested a number of experiments to check whether the weak interaction really discriminated between left and right, and challenged experimentalists to do the checking. The challenge was accepted by Chien-Shiung Wu, then at Columbia University, respectfully referred to as Madame Wu by most physicists of her generation. Her research team showed that the decay pattern of spinning Cobalt-60 nuclei differed from that of their mirror images. This and results of subsequent experiments by other groups confirmed that the weak interaction indeed does not conserve parity. (Lee and Yang were awarded the Nobel Prize for Physics in 1957.)
This could very well be the physical basis for the handedness of our molecules - or at least one possible basis. But whatever the reason for handedness is, it is fortunate that the bias exists. If we were capable of producing molecules of either hand, we would have a very difficult time finding a partner whose molecules are all compatible with ours. The situation would be chaotic. And we would have to have x-ray vision to discern the handedness of each of our potential partner's molecules. So, Nature has made it simpler for us mere mortals to choose our partners. She has limited the handedness of our molecules and caused this asymmetry to be invisible to our eyes, thereby making the propagation of ourselves and of our species easier and more likely. So now we judge our potential partners, at least initially, by simply looking at them.
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Eduardo A. Padlan was a Research Physicist in the US National Institutes of Health until his retirement in 2000. He currently serves as an adjunct professor in the Marine Science Institute, College of Science, University of the Philippines Diliman. He may be contacted at eduardo.padlan@gmail.com. Alfonso M. Albano is Marion Reilly Professor Emeritus in the Department of Physics, Bryn Mawr College, Bryn Mawr, Pennsylvania, USA. He may be contacted at aalbano2010@gmail.com.
Both authors are corresponding members of the NAST.