Earth's beginning: the origins of "left-handed" forms

Evidence that the building blocks of terrestrial life were delivered from outer space is published today.

Molecules can come in left and right handed forms and one puzzle is that most life on Earth contains left handed amino acids, the building blocks of proteins that build and operate the bodies of living things.

Now an American team has found evidence that meteorites delivered the 'seeds' of Earth's left-hand life.

This sinister start to life came around four billion years ago when Earth was a hot, dry and lifeless place.

In a report at a meeting of the American Chemical Society, New Orleans, Prof Ronald Breslow, Columbia University, described how our amino acid signature came from outer space, when an object from outer space planted the chemical seeds of life on Earth.

The term that chemists use to describe left and right handed molecules is chirality. "If you mix up chirality, a protein's properties change enormously. Life couldn't operate with just random mixtures of stuff," he says.

With the exception of a few right-handed amino acid-based bacteria, left-handed "L-amino acids" dominate on earth and Prof Breslow believes that this lefty dominance originated at the dawn of life.

"These meteorites were bringing in what I call the 'seeds of chirality,'" says Prof Breslow.

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 some classes of stars, their light rays trigger the selective destruction of one form of amino acid.

The reason is that these stars, called neutron stars, emit light of a certain handedness - light can be in left and right handed form too, which is how Polaroid glasses work.

Experiments have shown that this destroys the right handed amino acids. Evidence of this left-handed excess has been found on the surfaces of meteorites, which have crashed into Earth even within the last hundred years, landing in Australia and Tennessee.

Working with Mindy Levine, he simulated what happened on Earth, after the dust settled following a meteor bombardment, when the amino acids on the meteor mixed with the primordial soup.

In desert-like temperatures, and with a little bit of water, these cosmic amino acids caused a kind of domino effect, causing an existing amino acids in living things on Earth to convert to the left handed form.

Thus far, Prof Breslow's team is the first to demonstrate that this kind of handedness transfer is possible under these conditions.

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 the left handed of amino acid dissolved in water and found that, if the amino acid formed crystals, they would be a 50-50 blend of the two, enriching the water in the left handed form.

Eventually, only the left handed form was left dissolved and it is in this dissolved form that it would have contributed to life.

"The evidence that these materials are being formed out there and brought to us on meteorites is overwhelming," says Prof Breslow.

However, the steps afterward that led towards the genesis of life are shrouded in mystery.

"This work is related to the probability that there is life somewhere else," said Breslow. "Everything that is going 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."

But elsewhere in the universe, it could be possible that right handed life dominated. "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," he says.

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