Also called “chirality,” handedness is a directionalproperty exhibited by the physical systems.
A system is “right handed” if it twists in thedirection in which the fingers of the right hand curl when the thumb pointsalong the natural axis. Most naturally occurring sugar molecules are, forexample, right handed. The fundamental sub-atomic particles also exhibitchirality. In this case, the twist is defined by the particle’s spin, and thenatural axis by the direction in which the particle is moving. If the spin andmotion of the particle point in the same direction, the particle is righthanded and if these point in the opposite direction, then the particle is lefthanded. 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 aminoacids that make up life on Earth, they had a mystery on their hands. Virtuallyall life on Earth is “left-handed.” Further testing on meteorites showleft-handed amino acids contained within their rocky treasure chests. Thatgives credence to the theory that life originated in space and was deposited onEarth in meteor and comet bombardment. But again, we come to the importantquestion, why are left-handed amino acids more common and why is there a strongbias against the right-handed?
More than 4 billion years ago, when our home planet was inits fiery and temperamental youth, both the biological building blocks andtheir mirror reflections were present. In fact, if we cook up a batch of aminoacids or their precursor molecules in a laboratory, we will always get a 50-50mixture of left and right. But somehow, as life emerged in the countlessmillennia that followed the Earth’s formation, only the left-handed amino acidswere selected. Thus our proteins aremade up of lefty molecules. Proteins are the workhorse molecules of life, usedin everything from structures like hair to enzymes, the catalysts that speed upor regulate chemical reactions. Just as the 26 letters of the alphabet arearranged in limitless combinations to make words, life uses 20 different aminoacids in a huge variety of arrangements to build millions of differentproteins.
The study of how atoms radioactively decay has played acritical role in developing the standard model, our modern understanding of ouruniverse’s evolution since the Big Bang. Experiments investigating one form ofhow a nucleus decays, where a radioactive nucleus emits a beta particle tobecome more stable, have led to revolutionary ideas that are part of thestandard model. The most surprising result from beta decay is that Nature isnot ambidextrous, but is “left-handed.” No right-handed betaparticles have ever been observed and there is no compelling reason why theyshould not exist.
Life prefers the left-handed version, which is puzzlingsince both mirrored types form equally in the laboratory. But a new studysuggests that this may be because the star-forming cloud that created thefirst-ever biological molecule, before our sun was even born, made itleft-handed.
In 2004, NASA’s Stardust spacecraft swept through thenebulous “halo” surrounding a comet. What it found was the simplest of life’sbuilding blocks: the amino acid glycine. Since comets are frozen remnants fromthe earliest days in our solar system, their material is therefore not made inplanets, but likely originates in the natal gas cloud that formed our sun. Aresearch team recreated the freezingconditions inside such a star-forming cloud. In apparatus sealed completelyfrom the already crisp air in the laboratory, the temperature was brought downto minus 263 degrees Celsius, just ten degrees above absolute zero where evenmolecules stop vibrating. They believed that on the surface of dust grainssuspended in this chilly gas, glycine may have undergone a change that made itleft-handed.
Chiral glycine is very similar to original glycine, but withan important extra property. Laboratory experiments have shown that chiralglycine is a catalyst for other chiral molecules. That is, it promotes theproduction of other species with the same handedness as itself. The result isthat if glycine became a left-handed molecule, then future biological moleculeswould also be predominantly left-handed. When life developed on Earth, it wouldtherefore build from a pool of left-handed molecules, giving it the bias weobserve today. Exclusive, left handed amino acids, existed because only onechoice was possible for “communal” life forms. It was a randomprocess with a winner and a loser.
The big questions still remains that how and why did lifechoose only one of two mirror reflections to construct every single creature inher menagerie? Does life require homochirality to get its start, or could lifeforms exist that use both the earthly building blocks and their alter egos? Didthe seeds of homochirality originate in the depths of interstellar space, ordid they evolve here on Earth?
For now, all scientists can do is keep asking questionsabout molecules here on Earth and on the celestial bodies that surround us. Inthe hopes of unlocking one more piece of this puzzle, researchers are nowdeveloping new technologies to determine if there are excesses of one hand ininterstellar space over the other. In the meantime, life on Earth continues tobe mysterious and asymmetric as ever.