All life is left-handed, and no one knows why

Hold up your hands and try to lay one exactly over the other, both palms down. You can’t. They are mirror images, the same in every part and impossible to match up. A great many of the molecules that life is built from have that same problem, and here is the strange thing: every living thing on Earth has chosen the same hand.

Take the amino acids, the beads that all proteins are strung from. Almost every one of them comes in two forms, a left and a right, identical in every chemical respect, mirror twins. Stir them up in a tube with nothing alive involved and you get a clean fifty-fifty mix. But life is not fifty-fifty. Of the twenty amino acids your body runs on, nineteen are left-handed and one has no hand at all. Not just in you. In the bacteria in your gut, in the redwood, in the mold on old bread, in everything that has ever lived, the proteins are built from left-handed parts. The sugar in the backbone of your DNA runs the other way, all of it right-handed, in every genome on the planet. Life is single-handed, top to bottom, and has been for close to four billion years.

It matters more than it sounds. A creature built from the mirror molecules could not eat your food. Its enzymes would reach for a left-handed world and find ours all backwards; a mirror steak would slide through it like sand. You and your mirror twin could shake hands and never share a meal.

Here is the part I keep turning over. Nobody knows why life picked this hand. The other one would have worked exactly as well. The best guess is that some tiny accident tipped the very first scales, a faint twist in starlight, or an asymmetry buried in the weak nuclear force so slight no one has ever managed to measure it in a molecule, and that life seized on that whisper and amplified it forever. We even went looking in space for the answer. Some meteorites carry a faint left-lean in their amino acids; the freshest asteroid samples we have carry none at all. The sky has not settled it either.

So the deepest thing everything alive has in common is not a molecule or a gene. It is a choice, made once, before there was anyone to make it, between two options that were exactly equal, and then kept without a single exception for four billion years. We are all standing on the same side of a coin, and no one saw it land.