Shifts in ocean currents and the development of Antarctic bottom waters drop ocean temperatures by as much as five degrees centigrade. Life thrives and expands in these nippy, nutrient-rich ocean waters.
Perhaps some combination of increased ocean productivity, shifting sea margins, the complexities of megafaunal movement on land, and a nostalgia and logic we've yet to understand — inspires a change. The whales’ land-roving mammalian ancestors, after thoughtful ambling on the edge, return to the sea. They do go home again.