Jerry Adler in Smithsonian:
Water gave birth to life, and guarded it jealously. For billions of years, the land was barren, while life proliferated in the buoyant, nurturing bath of the seas, ignorant of such terrestrial concerns as falling down. The first invaders were plants, which began creeping upland from the streams and swamps some 450 million years ago, followed by arthropods and a few brave mollusks, which became the land snails. But waiting in the shallows was a nine-foot-long, crocodile-headed fish with both gills and, on the top of its head, air-breathing nostrils called spiracles. With a fateful lunge landward, it changed the course of life on earth forever. In 2004, when the fossil bones of Tiktaalik roseae were dug from the ground of Ellesmere Island, in the Canadian Arctic, the discovery was hailed as a breakthrough not just for paleontology, but for beleaguered science teachers trying to keep creationism out of their classrooms. A fish (with scales and gills) clearly resembling a tetrapod (with a flat head, a neck and prototypes of terrestrial limb bones in its lobelike fins), it precisely filled one of the gaps in the fossil record that creationists cited as evidence against Darwinian evolution. Scientists can’t say whether Tiktaalik itself is the ancestor of any species alive today; there were likely several related genera making the same transition around the same time. But the marvelously preserved fossil sheds new light on how the vertebrate invasion of land took place, some 375 million years ago.
Until this year, Tiktaalik was known only from its front half, but in January, evolutionary biologist Neil Shubin of the University of Chicago and his colleagues reported excavating the posterior skeleton of their original specimen. The hip and pelvis were surprisingly robust, suggesting more powerful rear limbs than previously believed. Although almost certainly still encased in fleshy lobes, appendages could have helped support or even propel the animal in shallow water or mud flats. If so, it changes our view of the evolution of tetrapods, whose ancestors were believed to drag themselves by their forefins, only developing useful hind legs once ensconced on land. As for what drove this epochal migration, “it’s extremely bloody obvious: There were resources on land, plants and insects, and sooner or later something would evolve to exploit them,” says vertebrate paleontologist Mike Benton of the University of Bristol.