The evolution of whalesThe first thing to notice on this evogram is that hippos are the closest living relatives of whales, but they are not the ancestors of whales. In fact, none of the individual animals on the evogram is the direct ancestor of any other, as far as we know. That's why each of them gets its own branch on the family tree.
These first whales, such as Pakicetus, were typical land animals. They had long skulls and large carnivorous teeth. From the outside, they don't look much like whales at all. However, their skulls — particularly in the ear region, which is surrounded by a bony wall — strongly resemble those of living whales and are unlike those of any other mammal. Often, seemingly minor features provide critical evidence to link animals that are highly specialized for their lifestyles (such as whales) with their less extreme-looking relatives.
Skeletons of two early whales.
Isotopic analyses help us figure out the likely habitats of extinct whales like Ambulocetus.
As whales evolved increasingly aquatic lifestyles, they also evolved nostrils located further and further back on their skulls.
As whales began to swim by undulating the whole body, other changes in the skeleton allowed their limbs to be used more for steering than for paddling. Because the sequence of these whales' tail vertebrae matches those of living dolphins and whales, it suggests that early whales, like Dorudon and Basilosaurus, did have tailfins. Such skeletal changes that accommodate an aquatic lifestyle are especially pronounced in basilosaurids, such as Dorudon. These ancient whales evolved over 40 million years ago. Their elbow joints were able to lock, allowing the forelimb to serve as a better control surface and resist the oncoming flow of water as the animal propelled itself forward. The hindlimbs of these animals were almost nonexistent. They were so tiny that many scientists think they served no effective function and may have even been internal to the body wall. Occasionally, we discover a living whale with the vestiges of tiny hindlimbs inside its body wall.
Skeleton of the early whale Dorudon. Notice the tiny hind limbs at left below the tail.
At left, the ankle bones of two middle Eocene protocetid archaeocetes, Rodhocetus balochistanensis (left) and Artiocetus clavis (right) from Pakistan, compared to those of the pronghorn Antilocapra americana (center). At right, the ankle region and foot of Basilosaurus. The pulley part of the astragalus (outlined) connects to the tibia and fibula.
Whale phylogeny from The Tangled Bank, used with permission of the author, Carl Zimmer, and publisher, Roberts & Company, Greenwood Village, Colorado; Photos of Dorudon, extinct whale and pronghorn ankle bones, and Basilosaurus foot © 1998, 1991 and 2001, respectively, Philip Gingerich.
Photo of Pakicetus and Ambulocetus courtesy of J.G.M. Thewissen, from:
J.G.M. Thewissen, L.N. Cooper, J.C. George, and S. Bajpai. 2009. From land to water: The origin of whales, dolphins, and porpoises. Evolution: Education & Outreach 2:272-288.
Isotopic analyses data from:
Bajpai, S., and P.D. Gingerich. 1998. A new Eocene archaeocete (Mammalia, Cetacea) from India and the time of origin of whales. Proceedings of the National Academy of Sciences 95:15464-15468.
Barrick, R.E., A.G. Fischer, Y. Kolodny, B. Luz, and D. Bohaska. 1992. Cetacean bone oxygen isotopes as proxies for Miocene ocean compostion and glaciation. Palaios 7(5):521-531.
Thewissen, J.G.M., L.J. Roe, J.R. O'Neil, S.T. Hussain, A. Sahni, and S. Bajpai. 1996. Evolution of cetacean osmoregulation. Nature 381:379-380.
Yoshida, N., and N. Miyazaki. 1991. Oxygen isotope correlation of cetacean bone phosphate with environmental water. Journal of Geophysical Research 96(C1):815-820.