A team of scientists has identified a new physical mechanism that could help explain one of the most persistent mysteries in science: why life consistently uses one "handed" version of its molecules and not the other. 

The researchers show that electron spin, a fundamental quantum property, can cause mirror-image molecules to behave differently during dynamic processes, even though they are otherwise identical. The work appears in Science Advances.

Many molecules essential to life come in two mirror-image forms, known as enantiomers. Chemically, these forms are nearly indistinguishable. Yet in living systems, only one version is typically used: amino acids are almost exclusively one type, while sugars follow the opposite pattern.

This phenomenon, known as homochirality, has puzzled scientists for more than a century.

Image source: Wikipedia

The new study suggests that the answer may lie not in the molecules themselves, but in how they behave when electrons move through them. The researchers found that when electrons pass through chiral molecules, their spin interacts with the molecular structure in a way that is not perfectly symmetric between mirror images.
As a result:
The two forms can produce different levels of spin polarization
These differences can influence how efficiently each form participates in physical and chemical processes
This breaks a long-standing assumption that mirror-image molecules should behave identically in magnitude, differing only in sign.

The study combines theoretical analysis, experiments, and advanced calculations to show that this asymmetry arises from how electron spin aligns within each molecular structure.

Although the two enantiomers have the same energy, their spin-related properties during motion are not exact mirror images, leading to measurable differences in behaviour. Importantly, these differences appear in dynamic processes, such as electron transport and interactions with magnetic environments, rather than in static properties.

These findings offer a possible route toward understanding how one molecular "hand" came to dominate in biology. If one enantiomer consistently interacts more efficiently with its environment under spin-dependent conditions, even small differences could accumulate over time, leading to a global preference. This provides a new perspective on how physical processes, rather than purely chemical ones, may have influenced the earliest stages of biological development.

The work opens new directions for research at the intersection of physics, chemistry, and biology:
Exploring how spin-dependent effects influence chemical reactions
Designing materials that exploit chirality and electron spin
Investigating how quantum properties shape biological systems
More broadly, the study suggests that symmetry in chemistry may be more subtle—and more easily broken—than previously thought.

Yossi Paltiel et al, Dynamic Breaking of Mirror Symmetry in Spin-Dependent Electron Transport through Chiral Media Causes Enantiomeric Excesses, Science Advances (2026). DOI: 10.1126/sciadv.aec9325. www.science.org/doi/10.1126/sciadv.aec9325