Evolution Read Online Free PDF

the long day her legs scraped and scratched at the gritty young sand.

    • • •
    Cretaceous Earth was a world of ocean, of shallow seas and shore.
    A giant ocean called the Tethys— like an extension of the Mediterranean— cut off Asia from Africa. Europe was little more than a scattered collection of islands. In Africa, even the mid-Sahara was an ocean floor. The world was warm, so warm there were no polar ice caps. And for eighty million years the sea levels had been rising. The post-Pangaea spreading of the continents, and the formation of huge reefs and shelves of chalk around their coasts, had pushed huge volumes of solid matter into the oceans: It had been like putting bricks into an already full bucket of water, and the brimming oceans had flooded the continents. But the vast shallow oceans were almost tideless, and their waves were gentle.
    Life in the sea was richer and more varied than at any other time in Earth’s long history. Tremendous blooms of plankton filled the waters, drinking the sunlight. The plankton were the base of the ocean’s vast pyramid of eaters. And in the plankton were microscopic algae called haptophytes. After a brief free-swimming phase, the haptophytes constructed for themselves tiny, intricate suits of armor from calcium carbonate. As they died, billions of tiny corpses sank to the warm seabeds, where they settled and hardened into a complex white rock, chalk.
    Eventually tremendous chalk beds, kilometers thick, would smother Kansas and the gulf coast of North America, and stretch along the southern half of England and into northern Germany and Denmark. Human scientists would call this era the Cretaceous— after creta, meaning “chalk”— for its most enduring monuments, constructed by the toiling plankton.
    When the light began to seep out of the sky, Purga emerged from her shelter.
    She scampered with difficulty through dry sand that yielded with every step, sometimes billowing around her belly. She was rested. But she was hungry, and confused, and pulsed with loneliness.
    She came to the top of the rise she had crossed yesterday. She found herself facing a broad, gently rolling plain extending to the rising smoke-wreathed mountains to the west. Once the great American inland sea had flooded this place. But now the sea had receded, leaving a plain littered with broad, placid lakes and marshes. Everywhere there was life. Giant crocodiles cruised like gnarled submarines through the shallow waters, some of them with birds riding on their backs. There were flocks of birds, and birdlike, furry pterosaurs, some of whom built huge rafts to support their nests at the center of the lakes, far from the land-based predators.
    And everywhere she looked there were dinosaurs.
    Herds of duckbills, ankylosaurs, and a few gatherings of slow, clumsy triceratops clustered around the open water, jostling and fighting. Around their feet ran and hopped frogs and salamanders, lizards like iguanas and geckos, and many small, snapping dinosaurs. In the air pterosaurs and birds flapped and called. On the fringe of the forest, raptors could be seen stalking, evaluating the jostling herds.
    The hadrosaurs, the duck-billed dinosaurs, were this era’s most common herbivores. Though they were larger than later mammalian equivalents like wildebeest or antelope, they walked on two legs like outsized ostriches, their strides long, their heads bobbing. Males led the way, elaborately ornamented by huge crests over their noses and foreheads. The crests acted as natural trumpets, capable of producing notes as low as a piano’s bottom register. Thus the voices of the duckbills hooted mournfully across the misty plain.
    In the foreground a herd of vast anatotitans was crossing the floodplain. It was a convoy of flesh. These immense creatures looked oddly unbalanced, with powerful hind legs— each of them taller than an adult human— but comparatively spindly forelegs, and they trailed long, fat conical tails. The