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Meteorites left “seeds” of Earth’s left-handed life

April 6, 2008
Courtesy American Chemical Society
and World Science staff

Flash back three or four bil­lion years to a hot, dry, life­less Earth. All is still. Sud­den­ly, a me­te­or plunges in­to the des­ert sev­er­al times faster than a speed­ing bul­let. With it, sci­en­tists be­lieve, this crash may have planted the chem­i­cal seeds of life on Earth.

A "chi­ral" mol­e­cule is one that is not su­per­im­pos­able with its mir­ror im­age. Like left and right hands, which are mir­ror im­ages but not the same, chi­ral mol­e­cules have the same el­e­ments at­tached in the same or­der, but are dis­tinct. Most ami­no acids can ex­ist in both left and right hand­ed forms, but life on Earth is made al­most en­tire­ly of left hand­ed ami­no acids. (Im­age cour­te­sy NA­SA Ames Re­search Cen­ter)

Re­search­ers are now pre­sent­ing ev­i­dence that des­ert heat, a lit­tle wa­ter, and me­te­orite im­pacts may have cooked up an early pre­req­ui­site for life: the dom­i­nance of “left-hand­ed” ami­no ac­ids, the build­ing blocks of Earth­ly life.

Chains of ami­no ac­id mo­le­cules make up the pro­tein found in peo­ple, plants, and all oth­er known life. 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, ac­cord­ing to chem­ist Ronald Bres­low of Co­lum­bia Uni­ver­s­ity.

“If you mix up chi­ral­ity, a pro­tein’s prop­er­ties change enor­mous­ly. Life could­n’t op­er­ate with just ran­dom mix­tures of stuff,” he said.

In a re­port Sun­day at the an­nu­al meet­ing of the Amer­i­can Chem­i­cal So­ci­e­ty in New Or­leans, Bres­low de­scribed new re­search sug­gest­ing how our ami­no ac­id sig­na­ture may have came from out­er space. 

With the ex­cep­tion of a few types of bac­te­ria, left-hand­ed “L-a­mi­no ac­ids” pre­vail on Earth. Bres­low said ami­no ac­ids de­liv­ered to Earth by me­te­orite bom­bard­ments left us with those left-hand­ed pro­tein un­its. 

These rocks brought “the ‘seeds of chi­ral­ity,’” said 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. That’s 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 “seeds” formed in in­ter­stel­lar space, pos­sibly on as­ter­oids as they ca­reened through space, Bres­low said. At the out­set, they have equal amounts of left and right-hand­ed ami­no ac­ids. But as these rocks soar past a type of super-dense star known as a neu­tron star, the light rays trig­ger the se­lec­tive de­struc­tion of one form of ami­no ac­id. The stars emit cir­cu­larly po­lar­ized light—a type in which light waves are aligned to­geth­er and twist like a cork­screw.

Bres­low said ex­pe­ri­ments have con­firmed that cir­cu­larly po­lar­ized light se­lec­tively de­stroys one chi­ral form of ami­no ac­ids over the oth­er. The re­sult is a five- to ten-per­cent ex­cess of one form, in our 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 added.

Bres­low 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” con­di­tions sim­u­lat­ing early Earth—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 found that these cos­mic ami­no ac­ids could trans­fer their chi­ral­ity to sim­ple ami­no ac­ids in liv­ing things.

Bres­low next sim­u­lat­ed the chem­istry that he said 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 found 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. 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 early Earth as he turns his at­ten­tion to nu­cleic ac­ids, the chem­i­cal un­its of DNA and its more prim­i­tive cous­in RNA. 

“This work is re­lat­ed 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.”

* * *

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Flash back three or four billion years and Earth hot, dry and lifeless. All is still. Suddenly, a meteor plunges into the desert several times faster than a speeding bullet. With it, scientists believe, this crash may have planted the chemical seeds of life on Earth. Researchers are now presenting evidence that desert heat, a little water, and meteorite impacts may have cooked up an early prerequisite for life: the dominance of “left-handed” amino acids, the building blocks of life on this planet. 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, according to chemist Ronald Breslow of Columbia University. “If you mix up chirality, a protein’s properties change enormously. Life couldn’t operate with just random mixtures of stuff,” he said. In a report Sunday at the annual meeting of the American Chemical Society in New Orleans, Breslow described new research suggesting how our amino acid signature may have came from outer space. With the exception of a few types of bacteria, left-handed “L-amino acids” dominate on earth. Breslow said amino acids delivered to Earth by meteorite bombardments left us with those left-handed protein units. These rocks brought “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. That’s 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 “seeds” formed in interstellar space, possibly on asteroids as they careened through space, Breslow said. At the outset, they have equal amounts of left and right-handed amino acids. But as these rocks soar past a type of super-dense star known as a neutron star, the light rays trigger the selective destruction of one form of amino acid. The stars emit circularly polarized light—a type in which light waves are aligned together and twist like a corkscrew. Breslow said that previous experiments confirmed that circularly polarized light selectively destroys one chiral form of amino acids over the other. The result is a five- to ten-percent excess of one form, in our 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 added. Breslow 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” conditions simulating early Earth—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 found that these cosmic amino acids could transfer their chirality to simple amino acids found in living things. Breslow’s next experiment simulated the chemistry that he said 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 found 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. 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 early Earth as he turns his attention to nucleic acids, the chemical units of DNA and its more primitive cousin RNA. “This work is related 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.”