Symmetry in nature tells us a lot about the forces that shape an object: stars and planets are mostly spherical because of gravity, for example, but slightly flattened at the poles because of rotation. Crystal lattices in solids form repeating patterns due to the forces between atoms or molecules comprising them. Even biological symmetries—two arms, legs, eyes, lungs, and so forth in humans, five arms in starfish, etc.—are due to genetic expression; while these aren’t “forces” in the usual physics sense, there is a definite mathematical kinship.
Sometimes symmetry is important in the way it is broken: an ordinary lump of iron has no preferred direction for the electron spins inside it, so every direction has a roughly equal number of spins pointing that way. If the iron becomes magnetized, that symmetry is lost: one direction is more important than the others. An ammonia molecule is symmetrical, and that’s the source of its oscillations, but not all molecules work that way. Organic molecules (in standard scientific usage) are compounds containing one or more carbon atoms; they often come in two varieties containing the same atoms in the same quantities, but with subtle difference known as chirality.
Hands exhibit a simple example of chirality: both left and right possess five fingers and a palm, but they are not oriented the same way. You can’t turn a left hand into a right hand by any physical process; overlapping them can either line up the fingers or the palms, but not both simultaneously. In fact, the word “chiral” (the root of chirality) is derived from the Greek word for hand.
Not all organic compounds are used by life as we know it, but all life on Earth is based on organic molecules. (We have to qualify our language here, because if organic life does exist elsewhere in the universe, it may not be constrained to use exactly the same compounds.) Proteins (used to build cell structures) are built from amino acids with “left-handed” or L chirality; sugars (the fuel sources of cells and components of DNA) have “right-handed” structure or D chirality. A “right-handed” amino acid can’t be made into proteins by most organisms on Earth. Some archaea – tiny single-celled organisms – can in their cell walls, but they are very rare exceptions and even they still use left-handed amino acids for the rest of their proteins. A left-handed sugar, sucrolose, is marketed as an alternative sweetener because the body doesn’t recognize or metabolize it like its right-handed counterpart sucrose.
Why one chirality is more common than another is somewhat of a mystery; there doesn’t seem to be an inherent reason why life should prefer one over the other, but of course once organisms follow a particular chemical path, it’s probably impossible to change. Since life on Earth has a common origin, the biochemistry of our most ancient ancestor is inherited down the generations to today, so that as varied as life is, all species have more in common chemically than not.
Amino acids have been found in meteorites and comets. The Murchison meteorite, which fell in Australia, contains thousands of organic compounds including dozens of amino acids (including several not utilized by life on Earth). Of these, a higher fraction of left-handed amino acids are present than right-handed, so the bias favoring that chirality carries beyond life as we know it. Perhaps the chemical process producing primordial amino acids generates more of one chirality than the other.
Whatever the specific cause, it seems very much to be a broken symmetry: nothing inherent within the structure of the laws of physics favors one chirality in organic molecules over the other. In a magnet, below a certain temperature the electron spins will align: there is no reason for one or the other direction to be preferred, but staying unmagnetized is not an option. Perhaps the process producing the amino acids that led to life had to select one chirality over another; it didn’t matter which, but having a “preference” for neither wasn’t an option.
Acknowledgments: Thanks to Caleb Scharf, Summer Ash, Josh Rosenau, and Ben Lillie for helping this poor ignorant theoretical physicist understand basic biochemistry. Obviously any errors are mine, not theirs, because they rock.
And a Final Relevant Thought: I hope if we come across any ravening aliens, they are based on right-handed amino acids. Then they won’t be able to digest us.