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Wednesday, April 9, 2008

NATURAL SELECTIONS

Life and left-handed meteorites


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

News photo
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)."


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