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Why would optical activity in organic molecules be useful evidence in a life-detection experiment?

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Planet TOI 700e — extraterrestrial life An illustration of the Earth-size planet TOI 700e orbiting within the habitable zone of its star. The habitable zone is the area around a star where liquid water, and thus possibly life, can survive on a planet's surface. Its Earth-size sibling, TOI 700d, can be seen in the distance.

In the context of astrobiology, detecting optical activity in organic molecules serves as a compelling biosignature because of the principle of homochirality—the tendency of living systems to utilize only one specific mirror-image form (enantiomer) of a molecule.1

Most biological building blocks, such as amino acids, are chiral, meaning they exist in two optically active forms that are mirror images of each other. In terrestrial life, proteins are almost exclusively composed of l-enantiomers. This preference is not a result of random chemical processes but is driven by the specific evolution of enzymes that synthesize these molecules.23

Because non-biological, abiotic chemical reactions typically produce a racemic mixture (an equal amount of both d- and l-enantiomers), the discovery of a sample dominated by a single enantiomer would strongly suggest the presence of a selective, biological process. Consequently, measuring the rotation of polarized light by organic samples provides a powerful method for identifying the organized, asymmetric molecular structures characteristic of life.143