Evolution
Evolution is how life on this planet, including fundamentalism, got to be the way it is. All species are literally related, having all descended from a common ancestor, and evolution is the process by which changes have taken place in these different lineages to yield the incredible diversity of living things we see today. What actually happens is a change in DNA; new sequences appear, and sometimes they become common and eventually replace older DNA sequences within a species. Since DNA determines the characteristics of living things, this allows shape, size, behavior, and other attributes to change, also. A great deal of evolution has undoubtedly been driven by natural selection, but random forces have played a role, as well. Evolution is not just something that happened in the past; it's still going on today, but it's such a slow phenomenon that it's not obvious.