Meteorites delivered the 'seeds' of Earth's left-hand life (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Wed, 16 Apr 2008 22:37:31 GMT
American Chemical Society
Washington, D.C.
Contact:
Charmayne Marsh
504-670-5115 (New Orleans, April 6-10)
202-872-4400 (Washington, DC)
Michael Bernstein
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202-872-4400 (Washington, DC)
April 6, 2008
Meteorites delivered the 'seeds' of Earth's left-hand life
NEW ORLEANS -- 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.
Scientists 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: The dominance of "left-handed" amino acids, the
building blocks of life on this planet.
In a report at the 235th national meeting of the American Chemical
Society, 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 protein's 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. 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 amino acids "seeds" formed in interstellar space, 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 light -- in 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 previous 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 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 found 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 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 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 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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