Free Why Evolution Is True by Jerry A. Coyne

survey its surroundings. And, like the early amphibians, Tiktaalik has a neck. Fish don’t have necks—their skull joins directly to their shoulders.
    Most important, Tiktaalik has two novel traits that were to prove useful in helping its descendants invade the land. The first is a set of sturdy ribs that helped the animal pump air into its lungs and move oxygen from its gills (Tik taalik could breathe both ways). And instead of the many tiny bones in the fins of lobe-finned fish, Tiktaalik had fewer and sturdier bones in the limbs—bones similar in number and position to those of every land creature that came later, including ourselves. In fact, its limbs are best described as part fin, part leg.
    Clearly Tiktaalik was well adapted to live and crawl about in shallow waters, peek above the surface, and breathe air. Given its structure, we can envision the next, critical evolutionary step, which probably involved a novel behavior. A few of Tiktaalik’s descendants were bold enough to venture out of the water on their sturdy fin-limbs, perhaps to make their way to another stream (as the bizarre mudskipper fish of the tropics does today), to avoid predators, or perhaps to find food among the many giant insects that had already evolved. If there were advantages to venturing onto land, natural selection could mold those explorers from fish into amphibians. That first small step ashore proved a great leap for vertebrate-kind, ultimately leading to the evolution of every land-dwelling creature with a backbone.
    Tiktaalik itself was not ready for life ashore. For one thing, it had not yet evolved a limb that would allow it to walk. And it still had internal gills for breathing underwater. So we can make another prediction. Somewhere, in freshwater sediments about 380 million years old, we’ll find a very early land-dweller with reduced gills and limbs a bit sturdier than those of Tiktaalik.
    Tiktaalik shows that our ancestors were flat-headed predatory fish who lurked in the shallow waters of streams. It is a fossil that marvelously connects fish with amphibians. And equally marvelous is that its discovery was not only anticipated, but predicted to occur in rocks of a certain age and in a certain place.
    The best way to experience the drama of evolution is to see the fossils for yourself, or better yet, handle them. My students had this chance when Neil brought a cast of Tiktaalik to class, passed it around, and showed how it filled the bill of a true transitional form. This was, to them, the most tangible evidence that evolution was true. How often do you get to put your hands on a piece of evolutionary history, much less one that might have been your distant ancestor?
Into Thin Air: The Origin of Birds
    Of what use is half a wing? Ever since Darwin, that question has been raised to cast doubt on evolution and natural selection. Biologists tell us that birds evolved from early reptiles, but how could a land-dwelling animal evolve the ability to fly? Natural selection, creationists argue, could not explain this transition, because it would require intermediate stages in which animals have just the rudiments of a wing. This would seem more likely to encumber a creature than to give it a selective advantage.
    But if you think a bit, it’s not so hard to come up with intermediate stages in the evolution of flight, stages that might have been useful to their possessors. Gliding is the obvious first step. And gliding has evolved independently many times: in placental mammals, marsupials, and even lizards. Flying squirrels do quite well by gliding with flaps of skin that extend along their sides—a good way to get from tree to tree to escape predators or find nuts. And there is the even more remarkable “flying lemur,” or colugo, of South-east Asia, which has an impressive membrane stretching from head to tail. One colugo was seen gliding for a distance of 450 feet-nearly the length of six tennis courts—while losing only forty