Why does life use molecules of only one hand?

The question

Many molecules come in two forms that are mirror images of each other, like a left and a right hand: same atoms, same connections, but they can't be superimposed. Chemists call such molecules chiral. The two forms can behave very differently in the body: one form of the molecule carvone smells of spearmint, its mirror image of caraway.

In 1848 Louis Pasteur, sorting by hand the tiny crystals of the sodium ammonium salt of racemic tartaric acid, separated them into left- and right-handed forms and showed that their solutions turn polarised light in opposite directions. He later noticed something stranger: living things make only one of the two forms.

This turned out to be universal. The proteins of every known organism are built from left-handed amino acids; the sugars in DNA and RNA are right-handed. Yet ordinary chemistry, without help from life, produces both forms in equal amounts. So how did life end up with only one hand, and why this one?

What we know

Homochirality is necessary, not a curiosity. Proteins fold into precise shapes and DNA forms a regular double helix only if all their building blocks have the same handedness. A mixture would not work.
The laws of chemistry are almost perfectly symmetric between the two forms. The only known exception is the weak nuclear force, which slightly favours one hand, but the resulting energy difference between mirror-image molecules is calculated to be far too small to matter directly.
Small imbalances can be amplified. In 1953 Charles Frank showed in theory that a reaction in which a molecule speeds up its own production, while suppressing its mirror image, could turn a tiny excess into complete dominance. In 1995 Kenso Soai and colleagues found a real reaction that does this.
Crystals can do it too. In 2005 Cristobal Viedma showed that grinding a mixture of left- and right-handed crystals in solution can convert the whole mixture to one hand.
Some meteorites contain amino acids with a small excess of the left-handed form: in 1997 excesses of several percent were found in the Murchison meteorite in amino acids that are rare in living things, which makes contamination from Earth unlikely. Circularly polarised light, observed in star-forming regions, can destroy one form slightly faster than the other and may be one source of such excesses.
Samples of the asteroid Bennu, brought to Earth by NASA's OSIRIS-REx mission and never exposed to Earth's biosphere, gave a different picture in 2025: all their chiral amino acids that life does not use, including isovaline, which is left-enriched in Murchison, were equal mixtures of both forms within measurement error. Samples of the asteroid Ryugu showed no excesses either. The authors conclude that life's left-handedness may not come from a bias in molecules delivered from space.
In 2023 a group showed that a precursor of RNA crystallising on magnetite, a common magnetic mineral, gained an excess of about 60% of one form, because electrons of one spin interact differently with the two mirror forms; a second crystallisation then gave crystals of a single form.

Main hypotheses

A chance start, then amplification

The first excess was random — a statistical fluctuation, a single crystal — and autocatalysis or crystallisation amplified it. If so, life could equally well have been mirror-image, and on another planet it might be.

A physical bias

The excess came from a physical cause that favours one hand everywhere in a region: polarised light in space, magnetic minerals, or, ultimately, the weak force. If so, the same choice might be made wherever life appears, at least in our part of the galaxy.

Choice by biology, not chemistry

Perhaps early life or proto-life began with mixed chemistry, and homochirality emerged later, when self-copying systems that used one hand outcompeted mixed ones. The choice would then have been made by selection rather than chemistry.

Where understanding ends

We know several ways to create a small imbalance, and several ways to amplify one. What we don't know is which of them actually operated on the early Earth, and whether any of them could work under realistic, messy conditions rather than in a carefully designed experiment.

There is also a puzzle of coordination. Life needs left-handed amino acids and right-handed sugars together. A mechanism that explains one does not necessarily explain the other, or why the two choices are linked the way they are.

And with only one example of life, we can't tell whether the choice was forced or accidental. That question is tied to the deeper one about how life began: until we understand the path from chemistry to biology, the moment when one hand won will stay hidden.

What would settle it

A plausible prebiotic route that produces nearly pure building blocks of both proteins and RNA, starting from a mixture and using only conditions available on the early Earth.
An explanation of why some meteorites carry chiral excesses while samples of the asteroids Bennu and Ryugu do not — and whether such excesses could ever have reached the early Earth in meaningful amounts.
Finding life, or its remains, elsewhere — and seeing which hand it uses.