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Chemists 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 form and a right hand form, but nearly all living organisms only use the left hand form. Now, there's a chemical explanation for how that could have happened.

NPR's Joe Palca explains.

JOE PALCA reporting:

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 structure is important; think about it. Your left hand is made of the same muscles, bones, and tendons as your right hand, but try to put a left hand glove on your right hand. Doesn't work.

Everything in your body is designed to work with left-handed amino acids. If you were plopped down in a right-handed world, a world where all your body could get was right handed amino acids, you'd die.

Now, it's assumed that the soup of chemicals sloshing around on earth before life evolved had equal amounts of the left and the right; so how did we wind up on the left? Chris Welsh(ph) is a chemist at the pharmaceutical company Merck. He's been thinking about this right hand, left hand business for years.

Mr. CHRIS WELSH (Chemist, Merck Pharmaceuticals): There's a lot of debate and speculation of could it - did it necessarily go that way, or could it have gone to the right handed amino acid world. And, of course, it's a question that you can't really answer experimentally, but people have debated that for quite some time.

PALCA: 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 is a chemistry professor at Imperial College, London. She says that paper had a huge impact on her research.

Professor DONNA BLACKMOND (Professor of Chemistry, Imperial College, London): It was a purely theoretical paper; in the last sentence, the guy said an experimental demonstration may not be impossible. And so that's a challenge to any red-blooded chemist. You got to go in the lab and find it if the theory says it should be there.

PALCA: Blackmond says a team of Japanese chemists came up with a solution, but Blackmond didn't feel it really explained what happened on earth billions of years ago, so she and her colleagues kept working.

They found that so long as there was the tiniest imbalance between left and right forms in a mixture of amino acids, there were times when the soup would become richer in one form or the other. But, Blackmond says, they didn't really understand why.

Prof. BLACKMOND: Just last summer my one (unintelligible) doc walked in my office and said you know I think I know what's going on.

PALCA: Blackmond and her colleagues 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 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 would explain why today we live in a left handed world.

Blackmond's results appear in the journal Nature. Chris Welsh is convinced there's value in doing experiments trying to mimic the conditions as our planet was getting its act together.

Mr. WELSH: We're not going to unambiguously know the answer through this, but we'll know which mechanism is more probable than another.

PALCA: And besides people seem to have an innate curiosity about how life on earth began.

Joe Palca, NPR News, Washington.

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