All of us come from a long and unbroken line of ancestors who accomplished two critical tasks: they survived to reproductive age, and they reproduced. If any one of your ancestors had failed at reproduction, you would not be here today to contemplate their existence. In this sense, every living human is an evolutionary success story. As descendants of these successful ancestors, we carry with us the genes for the adaptive mechanisms that led to their success. From this perspective, our human nature—the collection of mechanisms that defines us as human—is the product of th evolutionary process. Nonetheless, humans are rarely aware of these mechanisms.
Long before Charles Darwin, the originator of evolutionary theory, it was known that change takes place over time in or ganic structures. The fossil record showed the bones of long extinct dinosaurs, suggesting that not all species in the past are with us today. The paleontological record showed changes in animals' body forms, suggesting that nothing remains static. Moreover , the structures of species seemed extraordinarily well adapted to their environments. The long necks of giraf fes enabled them to eat leaves from tall trees. The turtle's shell seemed designed for protection. The beaks of birds seemed suited for cracking nuts to get at their nutritious meat. What could account for the dual observations of change over time and appar-
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