Life is built almost entirely from left-handed molecules, and nobody knows why

The problem

Many molecules come in two forms that are mirror images of each other, like a left and right hand — and ordinary chemical reactions produce both in equal amounts. Yet life is startlingly one-handed: it builds proteins almost exclusively from left-handed amino acids and uses right-handed sugars, a property called homochirality that is essential for biology to work. How the early Earth broke the symmetry and locked in a single handedness — whether by chance, mineral surfaces, circularly polarized starlight, or some amplifying chemistry — remains unresolved. Pasteur discovered chirality in 1848, and the origin of life's specific choice has been debated ever since.

Why it matters

Homochirality is a prerequisite for the origin of life, so explaining it is part of explaining how life began. It also matters intensely to medicine: the two hands of a drug molecule can have completely different effects in the body, and understanding chiral selection guides how we make them.

Progress so far

• 1848Louis Pasteur separates mirror-image tartaric acid crystals, discovering molecular chirality
• 1953the Miller–Urey experiment produces amino acids, but as equal mixtures of both hands
• 1995–2001the Soai reaction demonstrates chemical autocatalysis that amplifies a tiny chiral imbalance into near-total one-handedness
• 2020sanalyses of meteorite amino acids and circularly polarized light keep the origin of the initial bias an open question

References

1. Wikipedia — the puzzle, proposed mechanisms, and why it remains unresolved
2. Wikipedia — Pasteur's 1848 discovery and the chemistry of mirror-image molecules