I seem to recall that about 3 million years ago was when the two hemispheres became connected by the isthmus of Panama, which caused major changes in ocean circulation and temperature distribution.
I was there. Horrible, horrible thing ~ just a peaceful middle-class school of sharks and next thing you know we were separated into two species ~ and it took forever to swim around Tierra del Fuego to meet up with old friends (and kissing cousins of course).