How did mammals evolve from sea to land and back again? An article in The San Francisco Chronicle discusses the discoveries that have been made thus far with regard to intermediate sea-land links. One of the preeminent discoveries in this field was the discovery of Tiktaalik roseae:
“Tiktaalik was clearly a fish with scales, gills and fins, but at the ends of its fins were bones suggestive of developing fingers and toes. At the base of its fishlike head were bones found in land animals with flexible necks; its ribs were much more broad and robust than those of today's fish, and its flat head had eyes on top – much like those of crocodiles. Its gills were also modified to rely less on oxygen exchange in the water and showed indirect evidence of an ability to breathe air on land.”
Although Tiktaalik probably never walked, scientists conjecture that he was able to “prop himself up” in push-up style. Nevertheless, Tiktaalik constituted a clear intermediate step in the transition from sea to land. (See Tiktaalik for more discussion.)
After this remarkable finding, paleontologist Charles Carroll called the discovery “one of the most important evolutionary events in the history of vertebrate animals that made it onto the land – perhaps in the history of life itself.”
The article goes on to discuss the naturally selected transition back to sea again, as paleontologist Hans Thewissen recently discovered a “walking and swimming whale” in the waters by Pakistan. This species is thought to be a precursor to today’s whales and dolphins, explaining why these mammals have land-mammal characteristics like “fins, flukes and air-breathing lungs.”
Finally, an intermediate ungulate form – “Indohyus” – has freshly emerged that shares traits with horses, cows, pigs, giraffes, and the hippopotamus. As the Chronicle noted, “It looked like a fox-size deer that swam underwater and lived around the same time as Ambulocetus [the walking and swimming whale].” (See Indohyus for more discussion.)
What do these intermediate links mean in the grand scheme? Do they serve as evidence of steady evolution as opposed to punctuated equilibrium?