Why Life on Earth is Left-Handed
A mystery has long surrounded the "left-handed" bias in the building blocks of life. Now scientists have confirmed the same left-handed bias in meteorites, which may suggest that life on Earth originated from space rocks.
That bias exists in amino acids, the basic components of proteins, which can come in a left-handed or right-handed configuration. Left-handed or right-handed life can only break down and use their respective amino acids, which means that left-handed life could have gained an advantage in an Earth environment with more left-handed amino acids.
Researchers examined meteorites dating back more than 4.5 billion years, or older than Earth's existence as a planet, and found that meteorites with the longest exposures to water within had a much stronger left-handed bias.
"We don't have records on Earth, so we look to meteorites," said Daniel Glavin, an astrobiologist at NASA's Goddard Space Flight Center in Maryland. "They tell us a very interesting story that there was a left-handed bias prior to the emergence of life."
Researchers have known about a left-handed bias on Earth for years, but it first came to light for space rocks in a 1997 study of the Murchison meteorite found in Australia. Since then, Glavin and another NASA Goddard astrobiologist examined six meteorites that fit into three different classifications. Half of the meteorites showed the left-handed bias.
"The two meteorites where we saw the highest left-handed enrichment had the longest exposure to water," Glavin told SPACE.com. The evidence suggested that the meteorites had been exposed to liquid water over time periods ranging from 1,000 to 10,000 years.
Glavin added that the most pristine meteorites with little water exposure showed no evidence for the left-handed bias, with water exposure being estimated based on the presence of clays and minerals.
Previous lab experiments have shown that liquid water can amplify any inequality in amino acids, whether a small bias exists toward left-handed or right-handed types but a neutral experiment should turn up a 50:50 ratio for left-handed and right-handed.
Now the new research provides the first evidence of water's effect on amino acids in the natural world, Glavin said. Such liquid water could have arisen within asteroids when radioactive decay heated and melted the ice, long before the asteroids fell to Earth as meteorites.
Other effects may have played a role in the left-handed bias within space rocks. For instance, polarized light from neutron stars could have selectively destroyed more right-handed molecules as opposed to left-handed molecules, when the right-handed molecules absorbed more light.
The polarized light may account for a percent or two of the imbalance, Glavin noted. But he and Dworkin found around 15 percent more left-handed amino acids within some meteorites.
"You would have to destroy too much of the compound [with polarized light]," Glavin said. "That's why we really like the idea of water exposure."
This still leaves the question of what created the small left-handed bias in the first place. But the confirmation of water's amplification of the imbalance within meteorites may lend more weight to the notion that life on Earth came from outer space or at least some space rock in the main asteroid belt between Mars and Jupiter.
And that still doesn't mean right-handed life never existed on primitive Earth, or doesn't exist elsewhere in the universe. However, all the current evidence suggests that both Earth and the solar system lean left.
Glavin and Dworkin plan on studying even more meteorites, including samples from hundreds which have turned up in Antarctica. But their skepticism about the left-handed bias which exists beyond Earth has vanished, after they spent several years ruling out factors such as faulty analyses or possible contamination of the meteorites.
"We just recently ran out of explanations," Glavin said. "It's a really rock-solid case."
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