One peculiarity of our world is that certain biological molecules are handedmeaning that only one of two possible mirror-image configurations occur in living things. Sugars, for instance, tend to be right-handed, whereas the amino acids are lefties. Now Robert M. Hazen and his colleagues are presenting strong evidence to support an intriguing explanation in today's Proceedings of the National Academy of Science. According to their experiments, handedness may have resulted from the way that primordial biochemicals sloshed over simple rock crystals billions of years ago.

The scientists worked with calcite (CaCO3), which has the same composition as chalk and marble and was one of the most abundant minerals in our planet's earliest days. Calcite crystals are also formed such that different facets have mirror-image atomic-scale structures. When the researchers dunked mineral samples in solutions containing both left- and right-handed versions of the amino acid aspartic acid, they discovered that the different versions preferentially stuck to different facets of the calcite crystals.

Hazen and his colleagues note that other minerals have the potential for sorting handed molecules. And in the April 2001 issue of Scientific American, Hazen described other ways in which minerals could have helped jump-start life on Earth. "Minerals could have served at least five significant functions," he wrote, "from passive props to active players, in life-inducing chemical reactions." Apart from selecting for handedness, Hazen suggests that minerals may have provided support and scaffolding for growing chains of molecules, and catalyzed or fueled biochemical reaction