Darwin's Kidneys: Fossil Friday: Thar She Blows; Whale evolution.

Thar She Blows: Evolution of whales!

by Rich Feldenberg

Welcome back to Fossil Friday. The evolution of the cetacean group (marine mammals like whales and dolphins) is one of the coolest and most beautiful demonstrations of a clear link of fossil evidence from primitive forms to modern forms with many transitional fossils present.

Whales, are of course, mammals and descended from air breathing land vertebrates. All tetrapods descended from lobe finned fish (see my Tiktaalik post). From there they diversified into amphibians, reptiles, mammals, and birds. Some of the groups of reptiles (like the great marine reptiles during the age of the dinosaurs), and mammals (like the whales) returned to the sea many millions of years later. Based on molecular genetics studies, the closest living land mammals to the whales is the hippopotamus.

Cute little Pakicetus was one of the earliest known proto-whales. These hoofed footed mammals were alive about 50 million years ago. Based on bone structure of the skull around the auditory region, they fit into the cetacean group, but were not thought to be good swimmers. Good swimmers in the family would come later!

It is thought that changes in the regulation of genes such as Sonic Hedge Hog (Shh) and Tbx4 may have been important in the loss of the hind limbs in the cetaceans. By affecting when and how genes are expressed, major changes in structure can be made due to relatively small genetic changes. It is also pretty amazing to see the embryology of modern whales also betrays their ancestry. For example, in the whale fetus the nostrils start out in the usual position for a mammal, but as the maxillary bones grow to huge proportions this forces the nasal bones to the top of the skull. This type of evolutionary effect is called allometry and refers to a change in body parts due to changing the growth rate of different parts in relation to one another.

Over time the cetaceans evolved their characteristic echolocation apparatus, as well as, the development the blow hole from nostrils that were originally forward on the face. Today, cetaceans are beautifully adapted for life in the oceans.