Homochirality: a prerequisite or consequence of life?

TL;DR

This paper explores whether homochirality was a necessary condition or a result of life, examining physical, chemical, and evolutionary pathways, and discusses implications for detecting life on Mars.

Contribution

It reviews theories on the emergence of homochirality as a consequence of early polymerization events and proposes experimental approaches for detecting chirality on Mars.

Findings

Homochirality may have arisen from polymerization processes in early life.

Physical and chemical mechanisms could explain the transition from racemic to homochiral molecules.

Future Mars missions could detect chirality in primitive life forms.

Abstract

Many of the building blocks of life such as amino acids and nucleotides are chiral, i.e., different from their mirror image. Contemporary life selects and synthesizes only one of two possible handednesses. In an abiotic environment, however, there are usually equally many left- and right-handed molecules. If homochirality was a prerequisite of life, there must have been physical or chemical circumstances that led to the selection of a certain preference. Conversely, if it was a consequence of life, we must identify possible pathways for accomplishing a transition from a racemic to a homochiral chemistry. After a discussion of the observational evidence, I will review ideas where homochirality of any handedness could emerge as a consequence of the first polymerization events of nucleotides in an emerging RNA world. These mechanisms are not limited to nucleotides, but can also occur for peptides, as a precursor to the RNA world. The question of homochirality is, in this sense, intimately tied to the origin of life. Future Mars missions may be able to detect biomolecules of extant or extinct life. We will therefore also discuss possible experimental setups for determining the chirality of primitive life forms in situ on Mars.