Biology letters has the scoop, from scientists who studied scars on snail shells:

Using the Plio–Pleistocene fossil record of crab predation on morphologically similar pairs of right- and left-handed snail species, we show here for the first time, contrary to traditional wisdom, that rare left-handed coiling promotes survival from attacks by right-handed crabs. This frequency-dependent result influences the balance of selection processes that maintain left-handedness at the species level and parallels some social interactions in human cultures, such as sports that involve dual contests between opponents of opposite handedness.

Perhaps that is supposed to say “duel contests”? This gives some perspective to the argument that using Mac OS X or Linux will increase system survivability on the Internet.