Also called “chirality,” handedness is a directional property exhibited by the physical systems.
A system is “right handed” if it twists in the direction in which the fingers of the right hand curl when the thumb points along the natural axis. Most naturally occurring sugar molecules are, for example, right handed. The fundamental sub-atomic particles also exhibit chirality. In this case, the twist is defined by the particle’s spin, and the natural axis by the direction in which the particle is moving. If the spin and motion of the particle point in the same direction, the particle is right handed and if these point in the opposite direction, then the particle is left handed. While as the molecules can be right handed or left handed, “Nature” seems exclusively left handed.
Ever since scientists first looked at the structure of amino acids that make up life on Earth, they had a mystery on their hands. Virtually all life on Earth is “left-handed.” Further testing on meteorites show left-handed amino acids contained within their rocky treasure chests. That gives credence to the theory that life originated in space and was deposited on Earth in meteor and comet bombardment. But again, we come to the important question, why are left-handed amino acids more common and why is there a strong bias against the right-handed?
More than 4 billion years ago, when our home planet was in its fiery and temperamental youth, both the biological building blocks and their mirror reflections were present. In fact, if we cook up a batch of amino acids or their precursor molecules in a laboratory, we will always get a 50-50 mixture of left and right. But somehow, as life emerged in the countless millennia that followed the Earth’s formation, only the left-handed amino acids were selected. Thus our proteins are made up of lefty molecules. Proteins are the workhorse molecules of life, used in everything from structures like hair to enzymes, the catalysts that speed up or regulate chemical reactions. Just as the 26 letters of the alphabet are arranged in limitless combinations to make words, life uses 20 different amino acids in a huge variety of arrangements to build millions of different proteins.
The study of how atoms radioactively decay has played a critical role in developing the standard model, our modern understanding of our universe’s evolution since the Big Bang. Experiments investigating one form of how a nucleus decays, where a radioactive nucleus emits a beta particle to become more stable, have led to revolutionary ideas that are part of the standard model. The most surprising result from beta decay is that Nature is not ambidextrous, but is “left-handed.” No right-handed beta particles have ever been observed and there is no compelling reason why they should not exist.
Life prefers the left-handed version, which is puzzling since both mirrored types form equally in the laboratory. But a new study suggests that this may be because the star-forming cloud that created the first-ever biological molecule, before our sun was even born, made it left-handed.
In 2004, NASA’s Stardust spacecraft swept through the nebulous “halo” surrounding a comet. What it found was the simplest of life’s building blocks: the amino acid glycine. Since comets are frozen remnants from the earliest days in our solar system, their material is therefore not made in planets, but likely originates in the natal gas cloud that formed our sun. A research team recreated the freezing conditions inside such a star-forming cloud. In apparatus sealed completely from the already crisp air in the laboratory, the temperature was brought down to minus 263 degrees Celsius, just ten degrees above absolute zero where even molecules stop vibrating. They believed that on the surface of dust grains suspended in this chilly gas, glycine may have undergone a change that made it left-handed.
Chiral glycine is very similar to original glycine, but with an important extra property. Laboratory experiments have shown that chiral glycine is a catalyst for other chiral molecules. That is, it promotes the production of other species with the same handedness as itself. The result is that if glycine became a left-handed molecule, then future biological molecules would also be predominantly left-handed. When life developed on Earth, it would therefore build from a pool of left-handed molecules, giving it the bias we observe today. Exclusive, left handed amino acids, existed because only one choice was possible for “communal” life forms. It was a random process with a winner and a loser.
The big questions still remains that how and why did life choose only one of two mirror reflections to construct every single creature in her menagerie? Does life require homochirality to get its start, or could life forms exist that use both the earthly building blocks and their alter egos? Did the seeds of homochirality originate in the depths of interstellar space, or did they evolve here on Earth?
For now, all scientists can do is keep asking questions about molecules here on Earth and on the celestial bodies that surround us. In the hopes of unlocking one more piece of this puzzle, researchers are now developing new technologies to determine if there are excesses of one hand in interstellar space over the other. In the meantime, life on Earth continues to be mysterious and asymmetric as ever.