Mirror life: a biological branch that could coexist

In the nineteenth century, Louis Pasteur found that many tartrate crystals come in two mirror forms, like left and right hands. This property is called chirality, or handedness. The forms have the same chemical formula, but enzymes can interact with them differently. Nearly all amino acids in proteins made by life on Earth have the L form. Many sugars used in RNA and metabolism have the D form. Bacteria, plants, animals, and people share this molecular pattern.

Reactions without life often produce both molecular forms. Without a selection process, left-handed and right-handed amino acids can appear in nearly equal amounts. Why, then, does life on Earth use one direction? An early living system may have selected one form. Its enzymes and genetic material could then have copied that choice for billions of years.