Here’s what you’ll learn when you read this story:

• Chirality is a property that makes it impossible to superimpose an object perfectly on its mirror image, and chiral molecules are known as enantiomers.
• Enantiomers that favor one “hand” or the other tend to dominate in the ingredients of life, but why that’s so has largely remained a mystery.
• Now, scientists think that the spin of electrons inside a molecule makes the right- or left-handed version more efficient, which may have impacted the formation of RNA billions of years ago.

Hold your hands out in front of you, both palms facing the same way. For most people, it would be safe to say that your hands are pretty good mirror images of each other—identical, but flipped over a central vertical axis. Now, move your hands together, without flipping them, so that one rests on top of the other. While your middle three fingers might line up pretty well, your pinkies and thumbs are not going to have anything to perfectly map to. Even though your hands are mirror images of each other, when you superimpose them one over the other, they don’t perfectly line up. This property is called chirality, and in addition to making it possible for you to make weird-looking shadow puppets, it’s often observed in quantum physics.

Specifically, chirality pops up a lot in the molecules in your body, which tend to be right-handed or left-handed. In fact, most biological molecules are chiral, and the two chemically identical forms of a molecule that look like reflections are known as enantiomers. For over a century and a half, the reason why organisms have an unspoken preference for one or the other enantiomer—a phenomenon known as homochirality—has been confounding scientists. Seeking an answer, physicists Yossi Paltiel (from Hebrew University in Israel) and Ron Namaan (from the Weissmann Institute in Israel) led a research team that investigated why life consistently chooses one mirror molecule over the other.

“Over the years, several schemes have been proposed regarding the emergence of chirality in life, but none of them have explained a specific handedness,” the team wrote in a study recently published in the journal Science Advances. “On the basis of the identical energetics of the two enantiomers and symmetry-imposed mapping of their properties, it has been generally expected that the absolute value of any physical effect will be the same for the two enantiomers.”

To try and solve this conundrum, the team focused on an effect discovered decades ago—the chirality-induced spin selectivity (CISS) effect—which partially explains handedness in biomolecules. The CISS effect makes it seem as if electron motion through both enantiomers of a molecule would be the same, because it’s technically the same in every other way. As it turns out, however, the ways electrons spin through the enantiomers are often similar, but they’re not exactly symmetrical. And because the spin of an electron in one enantiomer may make it work more efficiently, it keeps appearing in living things over and over again.

The direction in which electrons spin within a molecule depends on their angular momentum (momentum of rotation, as opposed to moving forwards or backwards). The research team figured out that projecting the path of these electrons involves spin-orbit coupling—an interaction inside an atom that links an electron’s spin with its rotational momentum as it orbits the nucleus—and this spin is what causes enantiomers to act differently in certain biological processes. Despite sharing the same energy, enantiomers that are in motion or functioning as transporters behave slightly differently because of opposite electron spins. Usually, it turns out that one spin direction (and thus one enantiomer) performs more efficiently than another, which is why an organism uses that version. Amino acids, for instance, are almost exclusively left-handed, while sugars tend to be right-handed.

The team believes that this research could open a portal into the distant past, when the ancestor of everything alive on Earth today came into being in the crevices of an ancient rock or a blob of primordial ooze. Magnetic rocks such as iron and magnetite were abundant on early Earth. It’s thought that ribose aminooxazoline (RAO) could have been the precursor to RNA, and that interactions with a magnetic surface could have shaped molecules in the first life-form’s genome to favor one handedness over another.

A magnetic rock has two poles (north and south), and depending on which pole faces upward, it will attract and hold one version of a chiral molecule on its surface while leaving the other version free. This happens because when a chiral molecule is drawn toward a magnetic surface, both its electrical charge and its electron spin become polarized. Which pole attracts which molecule depends on whether it's the left- or right-handed version. In other words, a magnetic rock could act like a filter, collecting one enantiomer while ignoring its mirror image. The researchers tested this idea using ribose aminooxazoline (RAO), a molecule thought to be a precursor to RNA. Their analysis suggests that interactions with magnetic surfaces on early Earth could explain why biology universally favors D-sugars (used in RNA) and L-amino acids (used in proteins).

“This work provides a plausible and universal route for better understanding the origins of biomolecular homochirality and the specific handedness of chiral molecules in nature,” they wrote. “Moreover, it provides new insights on spin-dependent enantiospecific processes and devices.”