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SALT LAKE CITY — Hold your hands together like you're praying. You'll notice what probably everyone has noticed since they were children - they are mirror images of each other. What most people don't know is that the molecules of life also often come in two versions that are mirror images of each other. Just like our hands. And just like we all show a preference for one hand over the other from very early on, life has shown a preference for the left hand over the right as long as life has existed. Life is left-handed.
Left-handedness is ingrained deeply into life, right down to some of the the smallest and most basic molecules in our bodies - amino acids. Life shows an amazingly consistent preference for molecules that are left-handed, but the question that has never fully been answered is why. Life ought to work out just as well if it was right handed, but it just isn't the case. We use certain right- handed molecules, like the sugars that make the backbone of our DNA, but other than that, Earth is basically the equivalent of Ned Flanders' Leftorium.
A few NASA scientists think they have the beginnings of an answer, and it starts with an explosion over a distant lake in Canada's Yukon Territory. In January of 2000, a meteorite flew over Tagish Lake and blew itself to pieces. Those pieces were scattered all over, and some of them were frozen quickly in ice, protecting them from biological contamination.
Dr. Daniel Glavin of NASA's Goddard Space Flight Center in Greenbelt got his hands on some, and with a team of scientists he analyzed those fragments, paying special attention to how much right-handed verses left-handed aspartic acid was on them. Aspartic acid is one of the amino acids found in every protein in all forms of life. It's essential for everything, and there turned out to be four times as much of the left-handed version on the asteroid as one would expect.
"At first, this made no sense, because if these amino acids came from contamination by terrestrial life, both amino acids should have large left-handed excesses, because both are common in biology," Glavin said. "However, a large left-hand excess in one and not the other tells us that they were not created by life but instead were made inside the Tagish Lake asteroid." The team confirmed that the amino acids were probably created in space using isotope analysis.
They're sure the aspartic acid was made on the asteroid not by life on Earth, which leaves the question open: How did it all get there" Their answer is in crystallization. Aspartic acid - and many other amino acids - will crystallize only with a single handed version of itself. There are no crystals of aspartic acid that contain both versions of the molecule. So, if an asteroid is depositing these molecules on Earth, and has an abundance of one hand over the other, then crystallization can concentrate the left-handed version to the exclusion of the right-handed one.
Left-handedness could have originated in space while asteroids were flying around and getting exposed to polarized light, giving preference to the left, and then deposited on Earth and amplified by crystallization. By the time life began to evolve, only left-handed molecules were around to build up proteins, for the most part.
This is, of course not the end of the story, but it does provide some evidence toward an explanation of life's origins in outer space. If not for the excess created by polarized light during the formation of the solar system, it's possible none of us would be here to ask these kinds of questions.
