“Symmetry is a fundamental aspect of nature. For example, certain molecules exist in two forms which are symmetrical mirror images of each other. They are called chiral molecules. Common chemical synthesis generally produces equal amounts of the two forms of chiral molecules. In living systems, however, this symmetry is broken. Naturally occurring proteins are composed of L-amino acids but not D-forms, whereas DNA and RNA contain only D-sugars. How did this homochirality originally form in the context of the origin of life is a giant conundrum and has received much attention.”
The recognition of chiral (i.e. 'handed' ) molecules goes back to the time of Louis Pasteur. Since then, exhaustive investigations in a wide range of life-forms has shown 100% consistence in the homochirality of biological chemicals.
There are many theories on how this might have evolved (chemical, mathematical, quantum-based etc etc) but none is yet accepted as a definitive answer.
Note: There are also questions about whether a 'mirror image' of life, with the complex chemicals reversed, could exist.
Further info at Wikipedia