A central puzzle in modern biochemistry is life’s chirality—that is, a tendency for life’s molecules to have a certain preferential orientation, or “handedness.” Much of life is said to be inherently left-handed, especially its amino acids. No one has ever been able to explain satisfactorily how life became so asymmetric. Yet broken symmetry seems as central to biology and life on Earth as it is to physics and matter in the early Universe. Asymmetry may well be an essential prerequisite for the origin and evolution of complexity throughout all of Nature.

Many molecules display two kinds of structures that are mirror images of each other. Their chemical formulas are the same in both cases, but the orientations of some of the molecules’ atoms are reversed left for right and right for left. For example, as shown in Figure 5.13, two forms of the alanine amino acid are possible; each is a mirror image of the other, much like our left and right hands are mirror images, as are the left- and right-handed wood screws also shown in the figure.

FIGURE 5.13 — Left-handed and right-handed screws (top) are in some ways analogous to the mirror-image arrangements of the atoms in simple molecules, such as those of the alanine amino acid shown here (bottom). In life as we know it, however, such molecules are constructed only in their left-handed configurations.

A molecule’s orientation can be determined by watching the behavior of polarized light passing through it. This is a type of light having an aligned plane of vibration, which can rotate right or left when encountering a material substance. Key molecules of life—especially the common 20 amino acids comprising the structure of all proteins—are almost exclusively of the left-handed variety, since light moving through them rotates left. By contrast, the nucleotide bases and sugars that comprise RNA and DNA tend to be right-handed. “Handedness” is one of the most striking properties of life on Earth. But we don’t understand why.

Life’s amino-acid preference for left-handedness is particularly puzzling because such molecules, when artificially produced in the laboratory, invariably show an equal mixture of left- and right-handed configurations. Furthermore, should a right-handed amino acid drift into a living organism, the catalysts that control protein production will quickly destroy it. Not only that, when a living organism dies and decays, thermal fluctuations change molecular shapes randomly, so that eventually an even left-right mixture results. Why terrestrial life employs only left-handed amino acids or right-handed nucleic acids is one of the great unsolved mysteries of chemical evolution.

One possibility is that the first organism's proteins just happened, simply by chance, to be left-handed. If life arose only once on Earth, all its descendants would then also be left-handed; the continuity of life is a mere copying process. An alternative, less chancy possibility is that both left- and right-handed organisms originated, perhaps on different occasions billions of years ago, but that left-handed life proved advantageous in eliminating all competitors. The ability to make an extra amino acid or a healthy vitamin, for instance, might have provided such an advantage. If minerals acted as catalytic templates for life’s origin, as appraised later in this CHEMICAL EPOCH, some crystals might have attracted left- and right-handed amino acids differently. For example, the rock calcite (a common mineral, CaCO₃, that forms limestone and marble) does display this kind of asymmetric property, possibly acting as a determined selector and not a purely chance event, that could explain why much of life is preferentially left-handed.

Yet another intriguing idea interfaces physics with biology. In brief, life’s handedness might result from one of Nature’s basic forces, once again invoking determinism more than chance. The weak nuclear force operates on size scales smaller than nuclear dimensions and is thus often dismissed as unimportant to atomic physics, let alone molecular biology. However, as noted in the earlier PARTICLE EPOCH, the weak force has now been merged with the electromagnetic force, which biologists often refer to as the “life force.” And since some weak-interaction events studied in nuclear laboratories do show a preference for one handedness over the other (more elementary particles spin clockwise than counterclockwise during weak-force radioactive events), there might well be a very small (thus far undetected) difference in total energy between left- and right-handed molecules. If true, the left-handed amino acids could have been advantageously selected, as this is the lower-energy state preferred by Nature.

Polarized radiation, whereby waves of energy have specific orientations, is another possibility since energy is needed to drive the production of organic molecules. Circularly polarized light has in fact been detected from distant supernovae, their radiative beams perhaps having been emitted by collapsed neutron stars, although such specialized radiation hasn’t been noticed coming from the Sun. These light waves move in corkscrew fashion, spinning either clockwise or counterclockwise while traveling through space. Researchers have shown that such light can skew chemical reactions toward producing one type of chiral molecule at the expense of its twin—a preference that could have affected the origin of life’s biomolecules on early Earth. Closer to home, star-forming regions, such as the Orion Nebula, emit polarized infrared radiation that also might have favored left-handed interstellar organic molecules capable of reaching Earth while embedded in comets, meteors, and interplanetary dust.

Though these ideas amount to mostly speculation, they well exemplify frontier science at the intersections of physics, chemistry, and biology. Such interdisciplinary efforts will almost surely be increasingly needed, as specialists cross over into each others’ disciplines in order to crack the puzzling case of Nature’s uneven handedness.