Extraterrestrial influences may have helped shape the origin of life on Earth billions of years ago.
That's the potentially far-reaching conclusion of two scientists studying chemical compounds that reached Earth aboard a meteorite - the detritus from a passing comet.
Their work, published today in the journal Nature, adds to a growing body of evidence that interstellar chemistry, not just terrestrial evolution, gave life's basic building blocks the consistent structure they need to develop.
If true, "it's a good sign for the prospect of life elsewhere" in the universe, says Christopher Chyba, a planetary scientist at the University of Arizona in Tucson.
The focus of the researchers' study was a set of amino acids borne by a meteorite that struck Australia in 1969. Amino acids form proteins, the biological workhorses of living organisms.
They were part of the chemical stew that enveloped the early Earth. Like mittens, amino acids come in left-handed and right-handed varieties - designations they received after scientists saw how polarized light twisted as it passed by sample compounds. Other amino acids show no "handedness" at all, but are symmetrical.
The amino acids found in organisms today are all left-handed, implying that for life to take hold, all the amino acids used for life must have a common "handedness." Yet over the years, as researchers tried to reproduce conditions of the early Earth in the lab, the amino acids they got split 50-50 between right-handed and left-handed, suggesting Earth's prebiological goo held the same mix.
Which hand are you?
These observations have generated a debate among scientists trying to explain the path to left-handedness. One camp holds that the change came through evolution; the other holds that the chemical deck was stacked before life began.
Australia's Murchison meteorite, with its amino-acid passengers, has given researchers the opportunity to test those theories.
In their work, reported in the current issue of Nature, geochemists Michael Engel and Stephen Macko from the Universities of Oklahoma and Virginia took samples of the meteorite, chemically separated amino acids, then examined their "handedness." They also looked at the nitrogen content of the acids for indications that would signal either a terrestrial or extraterrestrial origin.
They found that of the amino acids that exhibited a handedness, the majority were left-handed. And high levels of a nitrogen isotope in the acids led them to conclude that the chemicals making up the amino acids, if not the acids themselves, had an extraterrestrial origin.
This is the duo's second attempt at demonstrating the outer-space origin of amino acids on the meteorite. Seven years ago, they made similar measurements but used isotopes of carbon instead of nitrogen. Although the results pointed to an extraterrestrial source for the left-handed amino acids, the work was dogged by concerns that the duo's samples had become contaminated by amino acids and carbon sources on Earth.
"It's hard to see how contamination explains their results" this time, says Dr. Chyba.
The latest findings come on the heels of research reported last February by a team at Arizona State University in Tempe. In their experiment, chemists John Cronin and Sandra Pizzarello isolated amino acids from the Murchison meteorite that either had no known terrestrial counterparts or were extremely rare on Earth. They discovered a slightly higher number of left-handed amino acids than right-handed ones, becoming the first researchers to demonstrate that processes in some parts of the cosmos may tip the amino-acids scale.
"The work by Cronin and Pizzarello was immaculate," says Jeffery Bada, a marine chemist at the Scripps Institution of Oceanography in San Diego and one of several scientists who remain skeptical of the latest results, based on concerns about contamination.
Just how cosmic processes might shape the "handedness" of amino acids remains a matter of speculation. One explanation holds that radiation from a spinning neutron star could alter the structure of acids in clouds of gas and dust that pass nearby. Or, under the right conditions, ultraviolet light striking the molecules could do the same.
Whatever the mechanism, the latest findings add weight to the view that the cloud of gas and dust that formed the solar system could well have contained amino-acid molecules already primed for the eventual emergence of life. That adds weight to the theory that life exists elsewhere.