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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