Banner
    Why Life Is Left-Handed: Amino Acids From Space, Says Astrobiologist
    By News Staff | March 17th 2009 01:00 AM | 7 comments | Print | E-mail | Track Comments
    NASA scientists analyzing the dust of meteorites say they have discovered new clues to a long-standing mystery about how life works on its most basic, molecular level.

    Over the last four years, the team carefully analyzed samples of meteorites with an abundance of carbon, called carbonaceous chondrites. The researchers looked for the amino acid isovaline and discovered that three types of carbonaceous meteorites had more of the left-handed version than the right-handed variety – as much as a record 18 percent more in the often-studied Murchison meteorite. 

    "We found more support for the idea that biological molecules, like amino acids, created in space and brought to Earth by meteorite impacts help explain why life is left-handed," said Dr. Daniel Glavin of NASA's Goddard Space Flight Center in Greenbelt, Md. "By that I mean why all known life uses only left-handed versions of amino acids to build proteins."


    Artist's conception of the left and right-handed versions of the amino acid isovaline. Credit: NASA/Mary Pat Hrybyk-Keith

    Proteins are the workhorse molecules of life, used in everything from structures like hair to enzymes, the catalysts that speed up or regulate chemical reactions. Just as the 26 letters of the alphabet are arranged in limitless combinations to make words, life uses 20 different amino acids in a huge variety of arrangements to build millions of different proteins. Amino acid molecules can be built in two ways that are mirror images of each other, like your hands. Although life based on right-handed amino acids would presumably work fine, "you can't mix them," says Dr. Jason Dworkin of NASA Goddard, co-author of the study. "If you do, life turns to something resembling scrambled eggs -- it's a mess. Since life doesn't work with a mixture of left-handed and right-handed amino acids, the mystery is: how did life decide -- what made life choose left-handed amino acids over right-handed ones?"

    "Finding more left-handed isovaline in a variety of meteorites supports the theory that amino acids brought to the early Earth by asteroids and comets contributed to the origin of only left-handed based protein life on Earth," said Glavin.

    All amino acids can switch from left-handed to right, or the reverse, by chemical reactions energized with radiation or temperature, according to the team. The scientists looked for isovaline because it has the ability to preserve its handedness for billions of years, and it is extremely rarely used by life, so its presence in meteorites is unlikely to be from contamination by terrestrial life. "The meteorites we studied are from before Earth formed, over 4.5 billion years ago," said Glavin. "We believe the same process that created extra left-handed isovaline would have created more left-handed versions of the other amino acids found in these meteorites, but the bias toward left-handed versions has been mostly erased after all this time."

    The team's discovery validates and extends the research first reported a decade ago by Drs. John Cronin and Sandra Pizzarello of Arizona State University, who were first to discover excess isovaline in the Murchison meteorite, believed to be a piece of an asteroid. "We used a different technique to find the excess, and discovered it for the first time in the Orgueil meteorite, which belongs to another meteorite group believed to be from an extinct comet," said Glavin.

    The team also found a pattern to the excess. Different types of meteorites had different amounts of water, as determined by the clays and water-bearing minerals found in the meteorites. The team discovered meteorites with more water also had greater amounts of left-handed isovaline. "This gives us a hint that the creation of extra left-handed amino acids had something to do with alteration by water," said Dworkin. "Since there are many ways to make extra left-handed amino acids, this discovery considerably narrows down the search." 

    If the bias toward left-handedness originated in space, it makes the search for extraterrestrial life in our solar system more difficult, while also making its origin a bit more likely, according to the team. "If we find life anywhere else in our solar system, it will probably be microscopic, since microbes can survive in extreme environments," said Dworkin. "One of the biggest problems in determining if microscopic life is truly extra-terrestrial is making sure the sample wasn't contaminated by microbes brought from Earth. If we find the life is based on right-handed amino acids, then we know for sure it isn't from Earth. However, if the bias toward left-handed amino acids began in space, it likely extends across the solar system, so any life we may find on Mars, for example, will also be left-handed. On the other hand, if there is a mechanism to choose handedness before life emerges, it is one less problem prebiotic chemistry has to solve before making life. If it was solved for Earth, it probably has been solved for the other places in our solar system where the recipe for life might exist, such as beneath the surface of Mars, or in potential oceans under the icy crust of Europa and Enceladus, or on Titan."

    The research was funded by the NASA Astrobiology Institute, the NASA Cosmochemistry program, and the NASA Astrobiology: Exobiology, and Evolutionary Biology program

    Glavin is lead author of a paper on this research appearing in the Proceedings of the National Academy of Sciences March 16.

    Comments

    logicman
    If a theory gets developed about the cosmic distribution of left and right-handed amino acids, it could be a 'rosetta stone' for theories of C.E.T.I. - Communication with Extra-Terrestrial Intelligence.  Many ideas in language can be explained from basic principles, but handedness, being a cultural idea,  must be physically demonstrated.  That might be done with a chemical formula for an amino acid, together with its L-R ratio.

    In this context, the term 'handedness' includes all related ideas: east / west, clockwise / anticlockwise, etc. together with all scientific ideas which make use of these terms.
    It's not C.E.T.I. It's S.E.T.I. The Search for Extra-Terrestrial Intelligence. Not Communication with extra-terrestrial intelligence.

    logicman
    It's not C.E.T.I. It's S.E.T.I. The Search for Extra-Terrestrial
    Intelligence. Not Communication with extra-terrestrial intelligence.

    Have you never heard of C.E.T.I., Carl Sagan or Dr John Elliott?

    Communication implies two communicators with a common communication code, or language.   It's no good Searching for ExtraTerrestrial Intelligence without some notion of how to Communicate with an ExtraTerrestrial Intelligence, or of how to determine that an  E.T.I. may be trying to communicate with homo semisapiens.

    These days, CETI is usually seen as being incorporated in SETI.
    Hank
    It sounds like a lot of work.  This is why I support W.E.T.I.
    Success rate so far; 100%.  No one else can say that.
    WETI

    P.S.  I sort of like when someone gets all smug about correcting things and turns out to be wrong.   ha ha   You are quite patient, Patrick.
    logicman
    WETI

    Wunderbar!
    You are quite patient, Patrick.
    When debugging spell-checkers, patience is a virute.
    has anyone ever toyed with the idea that our earths magnetic field is the natural selector for left handed amino acids? the amount of water in a sample tells us how much of the left handed amino acids there will be in that given sample... doesn't it stand to reason that the amount of water relates to the amount of magnetite in the sample?

    If the L aminos are created by integration with water could the amount of h2o on earth play a part in our handedness