A long-standing mystery about the origin of life has a new possible solution.
The puzzle is that amino acids, the constituents of proteins, occur in two chemically identical forms that have structures mirroring each other like two gloves. Most chemical processes yield left- and right-handed amino acids in equal amounts yet life forms contain left-handed amino acids almost exclusively. New findings show that a mineral common on the ancient Earth might have segregated the mirror-image versions.
Much speculation has focused on extraterrestrial influences behind early life. Some scientists suggest that extra left-handed amino acids arrived on meteorites as life was emerging on Earth (SN: 2/22/97, p. 118). In another theory, Earth formed from dust harboring a primordial excess of left-handed amino acids.
Researchers in Washington, D.C., now report that calcium carbonate, or calcite, can adsorb an excess of left-handed amino acids onto some of its crystal faces and of right-handed amino acids on other faces. Robert M. Hazen and Timothy R. Filley of the Carnegie Institution of Washington and Glenn A. Goodfriend of George Washington University report their results in the May 8 Proceedings of the National Academy of Sciences.
“It is very exciting,” says Max Bernstein of NASA’s Ames Research Center and the SETI (Search for Extraterrestrial Intelligence) Institute, both in Mountain View, Calif. “Such results provide a plausible scenario by which . . . amino acids could be separated from their mirror images, potentially solving a vexing problem of prebiotic chemistry.”
In their analysis, the scientists focused on calcite because it was widely present on the early Earth and is compatible with biological molecules. For instance, it’s the primary ingredient of sea shells.
The researchers placed each of four fist-size calcite crystals in a 50-50 solution of right- and left-handed aspartic acid, an amino acid. Two of the crystals had super-smooth surfaces, and two had microscopic terraces. The scientists found that the terraced crystals ended up with a 10 percent excess of right-handed amino acids on one type of face and a similar excess of left-handed amino acids on another. The smooth-faced crystals tended not to differentiate between the mirror-image forms of aspartic acid.
Hazen speculates that billions of years ago, amino acids might have lined up on crystals’ terraces. They could have then combined into the first all-lefty peptides, or short protein segments.
The calcite results are “of special significance for origin-of-life researchers,” says Noam Lahav of the Hebrew University in Jerusalem. But he cautions that the findings still do not point to a chemical mechanism by which southpaw amino acids overtook their right-handed siblings as life emerged.
John Cronin of Arizona State University in Tempe also remains puzzled. Since both the mirror forms become concentrated on crystal faces, “it would be necessary for life to have originated from a particular spatially isolated crystal surface, which seems rather improbable,” he says.
Hazen suspects that, in fact, that’s what happened. The left-handed amino acids in one place got the edge, he speculates, because by pure chance, these lefties bonded into peptides capable of self-replication.