How to Build a Dinosaur by Jack Horner Read Free Book Online Page A

ancient skull structure that had been around before anyone had thought of fur. Changes in size are easy to understand occurring quickly. A different regulation of growth, a few genes turned on and off at different times, or producing more or less of the regulatory proteins, and the tiny mammal would roar, or bellow. Shape inevitably changed with size. Teeth changed with diet. Depending on food availability, stomachs and metabolisms changed. But over the course of sixty-five million years the fundamental mammal has stayed visible.
    It is too long a time to track every change that occurred in the continental interior, or worldwide, where one branch of mammals, the primates, were developing bigger and bigger brains and new behaviors. Hominins first appeared about six million years ago, according to current thinking, shortly after the last common ancestor of humans and chimpanzees, probably six to eight million years ago. The succession of hominins that led to humans is long and not entirely clear. But we do know that not until about fifty thousand years ago did physically and behaviorally modern humans leave Africa. They quickly spread around the world.
    When humans first came to the Americas is a matter of some dispute. The first undisputed evidence of their arrival puts the colonization of the open continent at a little over thirteen thousand years ago. These were the so-called Clovis people, who used distinctive stone points to hunt mammoth and other large animals. Stone points of this type were first found at Clovis, New Mexico. Humans may have arrived earlier, however, as some sites, like one in Monte Verde, Chile, have materials that date to more than fourteen thousand years ago and show evidence of settled rather than nomadic life. That would seem to suggest that these people came to the New World from Asia earlier than the Clovis hunters.
    The first culture to be widely represented in North America, however, is that of the Clovis hunters, who quickly spread across the continent. They came at a time when the last glaciers were receding, and a passable overland route from Asia existed where the Bering Strait is now. If other humans had come to North and South America earlier, they did not leave a mark on the environment that we have found.
    The Clovis people encountered animals that we have never seen, the last of the great mammals of the ice ages. Mammoths and mastodons were common. Huge short-faced bears, bigger than grizzlies, may have been present in smaller numbers. Long-horned bison were plentiful. There were at least some ground sloths surviving, as well as tapirs, a giant beaver, horses, and other animals.
    All of these—the Ice Age megafauna—disappeared rapidly right at the time when the Clovis people hunted their way east and south. For some time these hunters have been viewed as the human equivalent of a Chicxulub meteor crash, a destructive wave of humanity sweeping east in a way that strangely prefigures the European sweep westward thousands of years later. A more recent view is that a change in climate played a role in the extinctions. Tim Flannery, in his excellent book The Eternal Frontier , still argues for the rapid extinction of large mammals by the Clovis people, who, he points out, left no art that has been found and seemed to live in more rudimentary shelters than did the earlier Asian cultures from which they came. Flannery suggests that they put all their effort into the beautiful and deadly Clovis points. He estimates that they accomplished the extinction of North America’s ice age animals in three hundred years.
    Whether or not he is right, it is clear that thirteen thousand years ago, the time when the Clovis people arrived, is the time when the animal ecology of North America changed. Brown bears, moose, and elk migrated from Asia. Gray wolves, a global Arctic species, replaced the larger dire wolf. The High Plains began to look something like they do today. Climate may have changed the