Not surprisingly, the right side evolved first, paving the way for the evolution of the parasitic left... Yes, eventually followed by the all-left sided creature, exemplified by Tom Daschel, Hillary, Teddy, etc.
Fortunately, as having two lefts is an example of devolution, we can expect to see such creatures return to their roots in the primordial slime before too long, much like Teddy oozes now...