All sugars, amino acids, DNA and RNA exist in one of two possible orientations, left-handed or right-handed. This handedness is known as "chirality." The theory is that for life to be possible, proteins must contain only one chiral form of amino acids, left or right, for example. Apart from a few bacteria, the chirality of amino acids of all life on Earth is left-handed.
One theory of how life was spawned on Earth is through a mechanism known as "panspermia" - basically, life has the ability to "hop" from one planet to the next encased in the protective shell of meteoroids. If life on Earth was indeed started via a cosmic "seed," then perhaps life evolved elsewhere in a similar manner as it did on our planet. Perhaps life even evolved with a different chirality than Earth.
"Such life forms could well be advanced versions of dinosaurs, if mammals did not have the good fortune to have the dinosaurs wiped out by an asteroidal collision, as on Earth," Breslow speculates. "We would be better off not meeting them."