[meteorite-list] Japan Times article: Life and left-handed meteorites 9APR08

From: drtanuki <drtanuki_at_meteoritecentral.com>
Date: Sun, 13 Apr 2008 06:14:00 -0700 (PDT)
Message-ID: <712685.46819.qm_at_web53202.mail.re2.yahoo.com>

Wednesday, April 9, 2008
http://search.japantimes.co.jp/cgi-bin/fe20080409rh.html

NATURAL SELECTIONS
Life and left-handed meteorites


By ROWAN HOOPER
 wonder if Empress Gensho, who ruled Japan for nine
years and died in 748, had something against
left-handed people.

 
The Willamette Meteorite on display at the Rose Center
for Earth and Space on Feb. 19, 2000, at New York's
American Museum of Natural History. AP PHOTO
 

It was Gensho who decreed that kimono should be worn
migi-mae (right side over the left at the front)
rather than hidari-mae (left over right), as was the
style until then. Now only the dead have hidari-mae
kimono, so it's not done to wear one like that.

(Once when I wore a yukata the wrong way, a Japanese
colleague said to me, "Are you dead?" It took me a
while before I understood what he was on about.)

The kimono custom comes to mind because I was reading
this week about how the building blocks of life ?
amino acids ? are predominantly "left-handed." In the
case of kimono-wearing, it was Empress Gensho who,
long ago, caused the dominant form to be one
particular side over the other. This was just chance;
she might have decreed it the other way around. With
amino acids, it seems to have been meteorites which
fell to Earth billions of years ago that decreed it.

Chains of amino acids make up the proteins found in
all forms of life on Earth, from plants to people. The
funny thing about amino acids is that they come in two
versions, a left and right form, just as hands come in
a left and right form. Anything that is not identical
to its mirror image in this way is called "chiral."

Almost all living things have left-handed amino acids,
known as L amino acids (some bacteria have
right-handed, or D, amino acids). Change L-type for
D-type, and life would grind to a halt.

So why do most amino acids come in the left-handed
form?

The immediate answer is: because they arrived from
outer space like that.

This week, Ronald Breslow, of Columbia University, New
York, described his evidence for this idea. Amino
acids can form spontaneously wherever you get the
right chemicals. When this happens in space ? say on
asteroids ? equal amounts of left- and right-handed
forms are made.

But asteroids roam across great distances of
interstellar space. As the rocks pass neutron stars,
the light rays from the stars reach the amino acids,
and that triggers the selective destruction of one
form of amino acid. The stars emit circularly
polarized light. This means that in one direction, the
light is polarized to the right. Conversely, 180
degrees in the other direction, light rays are
left-polarized.

As a meteor tumbles toward Earth, it is bathed in an
excess of one of the two polarized rays. Breslow has
confirmed in experiments that circularly polarized
light selectively destroys one chiral form of amino
acids over the other. The end result is that meteors
landing on Earth have a 5 to 10 percent excess of
L-type amino acids.

Evidence of the left-handed bias has been found on the
surfaces of meteorites that have crashed into Earth
even within the last 100 years.

But a mere 5 to 10 percent bias doesn't explain why
the vast majority of living things on Earth use L-type
amino acids. So Breslow has gone on to simulate what
happens after the dust settles following a meteor
bombardment.

Imagine Earth 4 billion years ago. It's a rock covered
in a warm soup of basic chemicals, including equal
amounts of both types of amino acids ? but without any
life on it yet. Then in comes a meteor carrying a
payload of extraterrestrial acids. What Breslow has
found is that when the amino acids on the meteor mixed
with those in the primordial soup, the cosmic amino
acids directly transferred their chirality to the
native Earthling amino acids.

This experiment is the first to demonstrate that
"handedness transfer" ? like a cosmic version of
Empress Gensho's decree ? occurs under conditions
found on prelife Earth.

This means that there was a slight excess of
left-handed amino acids.

Breslow has also shown how left-handers came to
dominate.

In warm conditions, such as those found in deserts,
water will evaporate and leave the amino acids
crystallized. As this happens, the left- and
right-handed forms bind together, leaving behind
increasing amounts of L-type amino acid in the
remaining water.

Eventually, life got going, and the amino acid in
excess became ubiquitous as it was used selectively by
living organisms.

"Everything that is going on on Earth occurred because
the meteorites happened to land here. But they are
obviously landing in other places," said Breslow. "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."

Some scientists have suggested that an intricately
linked process leads from the predominance of L-type
amino acids to the fact that we process language and
motor control in the left hemisphere of the brain.
This could explain why most of us are right-handed,
why our heart is on the left side of the body ? and
even why the accelerator of a car is always on the
right and the brake always on the left.

Asymmetry, it seems, is something we find everywhere
we look.

But what if, on that other planet, there was an excess
of R-type amino acids?

If life took off there, it would lead to aliens built
from R-type amino acids ? mirror images of us, but
unable to process our L-type amino acids.

The second volume of Natural Selections columns
translated into Japanese is published by Shinchosha at
?1,500. The title is "Hito wa Ima mo Shinka Shiteru
(The Evolving Human: How New Biology Explains Your
Journey Through Life)."
Received on Sun 13 Apr 2008 09:14:00 AM PDT