Rock Offers Mirror-Image Clues to Life's Origins

Daniel Glavin, an astrobiologist at the Goddard Space Flight Center in Greenbelt, said he and his colleagues recently discovered that a particular amino acid in the Murchison meteorite, isovaline, was disproportionately left-handed at a level of almost 18 percent. Other left-over-right imbalances had mostly been in the single digits. The logical conclusion, Glavin said, is that the imbalance arose on a water-containing asteroid or comet well before it broke up.

"There are signs that this compound existed in a watery environment and that the change in handedness happened because of that," he said.

Chirality is a simple concept that is hard to fully grasp. A "chiral" molecule is one that cannot be superimposed on its mirror image. Like left and right hands that have a thumb and fingers in the same order but are mirror images, chiral molecules have the same things attached in the same order but are mirror images and not the same. To make things a bit more complex, while almost all proteins are left-handed, almost all sugars are right-handed.

When researchers initially reported the higher proportion of left-handed amino acids in the Murchison meteorite -- which now resides at the Smithsonian's National Museum of Natural History -- the news was met with skepticism. The main criticism was that the meteorite was no doubt contaminated by earthly chirality when it struck, accounting for the overabundance of left-handedness.

Further study, however, has shown that some of the disproportionately left-handed Murchison amino acids are rare on Earth and contain much more of the heavy isotope of carbon (with an extra neutron) than is found in organic carbon on Earth.

Some researchers conclude that chirality and its complex origins make it less likely that life exists on other planets. But Breslow and Glavin hold the opposite view.

"I think this kind of chemistry could exist on many other planets and asteroids," Breslow said. "Meteorites crash into celestial bodies all the time. Given at all similar conditions that early Earth had, I don't see why some of those meteorites couldn't have the same effect when they hit other planets, too."