New research published today bolsters theory that life exists elsewhere in the universe.
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.