The Next Species: The Future of Evolution in the Aftermath of Man Read Online Free

dinosaurs until, in the late 1970s, a team of scientists at the University of California, Berkeley, came up with a theory. Luis Walter Alvarez, a bespectacled Noble Prize–winning nuclear physicist and leader of the team, found unusually high levels of iridium—a heavy substance rarely found on the surface of the planet, but quite common in meteorites—in layered deposits of earth that represented the Cretaceous extinction in both Italy and Demark.
    Alvarez, his son the geologist Walter Alvarez, and colleagues shook the scientific community with their announcement that the mystery of the Cretaceous extinction had been solved: an asteroid got the dinosaurs.
    Scientists were at first skeptical. Older hypotheses cited volcanism or glaciation as the primary cause of this mass extinction. But eventually high levels of iridium were found at more than one hundred sites, all marking the Cretaceous extinction, and the evidence couldn’t be ignored. But where was the crater?
    The Alvarez team went looking for a depression somewhere on the planet big enough to have fit the job. The team calculated that the asteroid must have been about seven miles in diameter. In June 1990, a decade after the original Alvarez proclamation, geologists discovered a huge crater underlying the northern tip of the Yucatán Peninsula near the town of Chicxulub (“Chick-sha-loob”), Mexico, from which the crater eventually took its name.
    The crater revealed that the asteroid must have been about7.5 miles (12 kilometers) wide and was traveling about 44,640 miles per hour (20 kilometers per second) on impact, roughly twenty times the speed of a bullet. The collision would have releaseda million times more energy than the largest nuclear bomb ever tested.
    The impact blasted thousands of tons of rock as well as the mass of the asteroid back into the atmosphere, with some elements going into orbit, while others returned to the ground in a barrage of flaming meteors. These fireballs ignited the verdant late Cretaceous landscape, burning half the earth’s vegetation in the weeks following the impact. Dust along with the smoke from the fires obscured the light of the sun, dealing a deadly blow to plant life.
    In the ocean,huge tidal waves spread out to the continental shores, leaving a line of beached and bloated dinosaurs skewered on shoreline trees. Scavengers had a field day on the plentiful carcasses. After the initial fires burned out, the earth descended into a period of perpetual night caused by a blanket of smoke and dust in the air. Trees and shrubs began to die, as did the animals that ate them and the carnivores that ate the plant eaters. The Cretaceous extinction killed off the dinosaurs and many but not all of the mammals.

    At the top of the Capitan Reef, we looked out over the fossils, rocks, precipices, and the valley below us, and imagined life over 250 million years ago at the pinnacle of the Permian period. Dry land, which was then about fifteen miles northwest of the reef, was growing drier. The lush swamp forests that had existed before the Permian had been replaced by conifers, seed ferns, and other types of vegetation that were drought-tolerant. Giant cattail-like trees grew up to eighty feet. Ten-foot relatives of the centipede splashed through inshore water.
    The first vertebrates had crawled out onto the land only about 100 million years earlier. Giant amphibians, which roamed the marshlands, were up to six feet in length and two hundred pounds in weight. They sucked down dinner with enormous mouths filled with sharp teeth, tossing their captives little by little back into their deep throats,like a crocodile or alligator would. There were flying lizards and large armored herbivores the size of oxen. There were a number of sharks in the Permian oceans, the most bizarre being Helicoprion , which had a spiral jaw fitted with backward-leaning teeth that looked like a buzz saw. Primitive pelycosaurs about ten feet (three meters) long with