How the Primordial Soup Took a Left Turn

Scientists in England may have cracked a puzzle that lies at the very core of all life on Earth. The amino acids that form the building blocks of life come in two forms: a left-hand and a right-hand version. But nearly all living organisms only use the left-handed form. Now there's a chemical explanation for how that could have happened.

Left-hand amino acids and right-hand amino acids have all the same ingredients. But they differ in one important quality, their three-dimensional structure. They are mirror images of one another, and their structure is vital to how they interact.

A suitable comparison can be found in the way hands and gloves fit. Your left hand is made of the same muscles, bones and tendons as your right. But try to put a left-hand glove on your right hand, and it doesn't fit.

The same holds true with entire beings, scientists say. Everything in our bodies is designed to work with left-hand amino acids. If we were suddenly thrust into a right-hand-based world, with only right-handed amino acids, humans would die.

About 50 years ago, a British physicist named Charles Frank wrote a paper suggesting it just might be possible to imagine the primordial circumstances where one form would win out over the other. Donna Blackmond, a chemistry professor at Imperial College, London, says Frank's paper inspired her research.

Blackmond and her colleague realized there were two key factors. One was which amino acid they used in their experiments. The other had to do with the relationship between the amount of amino acid that was dissolved in the primordial soup, versus the amount that had formed crystals as the soup evaporated.

She says you can imagine an amino-acid laden puddle sitting on Earth's surface. As the puddle evaporated, and left more crystals, it would reach a point where the conditions were just right for one form to win out over the other. And if the earliest life evolved from the amino acids in a left-handed puddle, that could explain why today we live in a left handed world.

"My model doesn't tell you where the initial imbalance came from, it doesn't tell you where the amino acids themselves came from," Blackmond says. "But given that there's a way to get even a tiny imbalance of one over the other, my model tells you we're off and running."

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