A nineteenth-century Englishman, Charles Darwin shown at left, was the first to present a coherent scientific theory of human origins. His famous book, On the Origins of Species by Means of Natural Selection, published in 1859, advanced the theory that species came into existence by evolution, or gradual changes over a long period of time. Darwin suggested two basic "mechanisms" at work in this process: first, "variation" continually occurred within species. That is, individuals of the same species vary slightly from one another. He noted that members of the same litter of pups or the thousands of offspring of a pair of salmon are not identical. Darwin was not certain why variation occurred; he only observed that it, in fact, happened that way. Individual offspring, he noted, because of their different characteristics, were not equally successful in the struggle to survive and reproduce. He called this weeding out process with the consequent differences in reproductive success "natural selection"--which is the second of his two "mechanisms."Darwin concluded from these observations that those individuals whose characteristics fitted them to the circumstances in which they found themselves would have more offspring. Over time, the characteristics which gave them a reproductive advantage would be fully expressed in their descendants. Given more time, these small accumulating changes would result in a new species. The theory is, in essence, very simple: new species have emerged on earth because of variation and natural selection.
| Previous Page | Next Page | Long Foreground Home |