terran: the planet that humans reside on.
contra-terran (CT): anti-matter. People made out of anti matter could have reversed handed DNA. The term comes from two neat books by Jack Williamson, Seetee Ship and Seetee Shock
Of course direct contact with antimatter would cause a horrific explosion, so we will have to assume that the left handed DNA is made out of regular matter.
A friend of mine points out that, if you were not sure, normal individuals with a preference for using their left hand still have right handed DNA. But, of course, since most people are right handed we might expect that CT people tend to be left handed ... ;-)