Early Speculations
Charles Darwin postulated that a semi-aquatic land animal would, generation by generation, develop into fully aquatic life forms. He offered the bear, a predator of the water as well as land, as the type of animal that would have been a precursor to whales. At the time he was ridiculed, and wrote at the time that this suggestion caused him more grief than any particular point of his then-shocking theory.
Geologic History
It was not until a century later that the fossil record began to yield definite evidence of the origins of whales. The main source of relevant fossils has been Pakistan. 50 million years ago, India was still drifting toward mainland Asia. The shallow sea that existed in between is now called the Sea of Tethys by paleontologists. Here the early ancestors of marine mammals have been uncovered.
Ecology at Tethys
Uncannily, Darwin’s idea of an aquatic bear was actually not far off. However, the actual ancestor of whales was found to be a hoofed carnivore, rather like a wolf. This mesonychid, as it is called, also has evolutionary ties to the hippopotamus.
The Sea of Tethys, like the seas in general as this time, might well have seemed oddly empty; the large marine predators like pleisiosaurs and mososaurs had died along with the dinosaurs. With the exception of sharks, there were no large organisms in the seas. Shallow seas are rich in variety, because of the amount of sunlight they receive, so the pickings, even to a beachcomber, were rich.
Natural Selection
Evolutionary theory states that change in species is driven by subtle gains in advantage. In the case of the mesonychid, eating dead fish off the shore, and competing with other land predators for other food, mesonychids with an aptitude for wading into the water would have less competition and more fish, and thus breed more successfully. Over time, the advantage would be improved upon. Pakicetus, a land animal and successor to mesonychids, features ears adapted for hearing underwater and teeth primarily built for gripping fish, as well as feet built for swimming. Through this species, successive whale ancestors grow increasingly aquatic, until they lose their hind legs entirely.
Once in the sea, their diversity exploded, filling all the niches left open by the extinction of marine reptiles. They diverged by size and prey; within ten million years, the sixty foot Basilosaurus roamed the ocean along with the smaller Dorudon and others. Until forty million years ago, all whales were toothed. They eventually split into toothed predators and the gentle filter-feeders we recognize and try our best to conserve today.
Legacy
The origin of whales is an excellent illustration of the evolutionary process, and one that is very counter-intuitive. Humanity’s evolutionary history is often told in terms of mammal’s distant forebears climbing out of the sea and flourishing. Here is a case of a mammal going back into the sea and flourishing, and finding a unique place in Earth’s ecology.
Bibliography
Wallace, D. R. (2007). Neptune's Ark: From Ichthyosaurs to Orcas. Berkeley ; London: University of California Press.
BBC, Science and Nature, “Sea Monster Facts.” http://www.bbc.co.uk/sn/prehistoric_life/dinosaurs/seamonsters/
Ben Gazarra, Narrator. “Back to the Seas” Paleoworld, The Learning Channel, 1994.
Barry Cox, Colin Harrison, R.J.G. Savage, and Brian Gardiner. The Simon & Schuster Encyclopedia of Dinosaurs and Prehistoric Creatures: A Visual Who's Who of Prehistoric Life. Simon & Schuster. 1999
