Brandon Keim writes: "The chemical seeds of Earthly life were planted by meteorites, says Columbia University chemist Ronald Breslow — and if it happened here, it could happen elsewhere.
"Flash back three or four billion years Earth is a hot, dry and lifeless place. All is still. Without warning, a meteor slams into the desert plains at over ten thousand miles per hour. With it, this violent collision may have planted the chemical seeds of life on Earth, it was explained.
"Researchers presented evidence today that desert heat, a little water, and meteorite impacts may have been enough to cook up one of the first prerequisites for life, it was explained.:"The dominance of left-handed amino acids, the building blocks of life on this planet.
"Ronald Breslow, Ph.D., University Professor, Columbia University, and former ACS President, described how our amino acid signature came from outer space.
"Chains of amino acids make up the protein found in people, plants, and all other forms of life on Earth. There are two orientations of amino acids, left and right, which mirror each other in the same way your hands do. This is known as chirality. In order for life to arise, proteins must contain only one chiral form of amino acids, left or right, Breslow noted.
"If you mix up chirality, a proteins properties change enormously. Life couldn't operate with just random mixtures of stuff, he said.
"With the exception of a few right-handed amino acid-based bacteria, left-handed L-amino acids dominate on earth. The Columbia University chemistry professor said that amino acids delivered to Earth by meteorite bombardments left us with those left-handed protein units.
"These meteorites were bringing in what I call the seeds of chirality, stated Breslow. If you have a universe that was just the mirror image of the one we know about, then in fact, presumably it would have right-handed amino acids. Thats why I'm only half kidding when I say there is a guy on the other side of the universe with his heart on the right hand side.
"These amino acids seeds formed in interstellar space," he said, "possibly on asteroids as they careened through space. At the outset, they have equal amounts of left and right-handed amino acids. But as these rocks soar past neutron stars, their light rays trigger the selective destruction of one form of amino acid. The stars emit circularly polarized lighting one direction, its rays are polarized to the right.180 degrees in the other direction, the star emits left-polarized light.
"All earthbound meteors catch an excess of one of the two polarized rays. Breslow said that prior experiments confirmed that circularly polarized light selectively destroys one chiral form of amino acids over the other. The end result is a five to ten percent excess of one form, in this case, L-amino acids. Evidence of this left-handed excess was found on the surfaces of these meteorites, which have crashed into Earth even within the last hundred years, landing in Australia and Tennessee.
"Breslow explained that he simulated what occurred after the dust settled following a meteor bombardment, when the amino acids on the meteor mixed with the primordial soup. Under credible prebiotic conditions desert-like temperatures and a little bit of water he exposed amino acid chemical precursors to those amino acids found on meteorites.
"Breslow and Columbia chemistry grad student Mindy Levine observed that these cosmic amino acids could directly transfer their chirality to simple amino acids found in living things. Thus far, Breslow's team is the first to demonstrate that this kind of handedness transfer is possible under these conditions.
"On the prebiotic Earth, this transfer left a slight excess of left-handed amino acids, Breslow said. His next experiment replicated the chemistry that led to the amplification and eventual dominance of left-handed amino acids. He started with a five percent excess of one form of amino acid in water and dissolved it.
"Breslow observed that the left and right-handed amino acids would bind together as they crystallized from water. The left-right bound amino acids left the solution as water evaporated, leaving behind increasing amounts of the left-amino acid in solution. Eventually, the amino acid in excess became ubiquitous as it was used selectively by living organisms.
"Other theories have been put forth to explain the dominance of L-amino acids. One, for instance, suggests polarized light from neutron stars traveled all the way to earth to zap right-handed amino acids directly. But the evidence that these materials are being formed out there and brought to us on meteorites is overwhelming, said Breslow.
"The steps afterward that led towards the genesis of life are shrouded in mystery. Breslow hopes to shine more light on prebiotic Earth as he turns his attention to nucleic acids, the chemical units of DNA and its more primitive cousin RNA.
"This work is correlation to the probability that there is life somewhere else, said Breslow. Everything that is going on on Earth occurred because the meteorites happened to land here. But they are obviously landing in other places. If there is another planet that has the water and all of the things that are needed for life, you should be able to get the same process rolling.
"In a report delivered to the American Chemical Society's annual meeting, Breslow simulated the behavior of "left-handed" amino acids in an Earth-like environment. The acids' appendage appellation derives from their molecular orientation, also known as chirality; except for a few oddball bacteria, all living creatures are composed of proteins made from left-handed amino acids. This phenomena has long perplexed scientists. Why left instead of right-handed? Why not both?
"One possible explanation: because the first amino acids came from space, riding to Earth on meteorites. When the meteorites passed neutron stars, they were blasted with circular polarized light, in which photons corkscrew to either the left or right. Breslow's earlier experiments showed that CPL damages amino acids of corresponding handedness: if, on a fateful journey some four billion years ago, an Earthbound meteorite passed a star emitting right-spiraling CPL, it would have been stripped of right-handed acids, leaving the present orientation of life's building blocks as a biochemical echo of our extraterrestrial origin.
"In his ACS report, Breslow added left-handed amino acids to the amino acid precursors found on pre-biotic Earth. The resulting acids were also left-handed and eventually dominated Breslow's mixture, suggesting that a similar process could have happened four billion years ago after a rock from space hit our hot and lifeless world.
"Will we ever know for sure? Of course not. But it's plausible, and excesses of left-handed amino acids have indeed been found on meteorites landing in Australia and Tennessee during the last century.Breslow next plans to study the origin of nucleic acids — the chemical units of DNA.
"This work is related to the probability that there is life somewhere else," said Breslow in a press release. "Everything that is going on on.
"Earth occurred because the meteorites happened to land here, Breslow explained. But they are obviously landing in other places. If there is another planet that has the water and all of the things that are needed for life, you should be able to get the same process rolling."
Meteorites delivered the 'seeds' of Earth's left-hand life [press release]
