Things that tend to survive, tend to survive. That single sentence, a tautology, in fact, sums up the theory of evolution. The rest flows naturally from that simple observation.
If a chimp has two offspring, the one that is better equipped to survive in its environment will probably live longer. As a result of living longer, it will probably have more offspring. Children tend to resemble their parents. Therefore, the next generation of chimps will be more like that chimp than its sibling. Over a long period of time, the population of chimps will become more and more like that first chimp that was better equipped to survive in its environment.
No rational person can argue with the basic theory of evolution, as described above. Of course, there numerous ways in which the picture can be refined. For example, the phrase "better equipped to survive" can be replaced with "better at getting its genes into the next generation." The two are not always the same. If the chimp survives a long time by not expending energy fighting for the ability to mate, the story falls apart. A bee may gets its genes into the next generation by sacrificing itself for its genetically identical siblings. The details of evolution -- what kinds of animals in fact tend to evolve, and under what conditions -- are complex. The basic theory, however, is an obvious truth.
Many Americans refuse to accept that evolution applies to humans. But they have no basis to do so. There is no rational basis to believe that humans are somehow exempt from the laws of logic. Human populations are subject to the same forces of nature as any other population. The humans who tend to survive will tend to survive, and the next generation will be more like them.
That fact alone, however, does not prove that humans and chimps (and indeed all life) share a common ancestor. But a look at the real world makes it abundantly clear that we do. From our obvious physical similarities to other animals, to the distribution of plants and animals that exist, to the common DNA we share, to the fossil record, to the very ways in which are so well and so poorly designed at the same time, the evidence is beyond overwhelming. It would be easier to deny that the heart pumps blood (a fact which, like evolution, was unknown in Shakespeare's time) than it is to deny that we share a common ancestor with other primates.
But what does it matter whether people belief in evolution. I, for one, uses to think that creationist were foolish but harmless, like the Flat Earth Society or the people who think Elvis was abducted by space aliens. Whether we shared a common ancestor with chimps six million years ago makes no difference in our daily lives. Or so I thought. I was wrong. The process of evolution sculpted our bodies and minds of a period of more than a billion years. Evolutionary thinking is critical to understanding who we are, emotionally, intellectually and physically. Understanding evolution is important for medicine, but it is also important for understand ourselves, our loved ones and every other human with whom we share this earth.
I highly recommend the following books:
The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe Without Design, by Richards Dawkins http://www.amazon.com/Blind-Watchmaker-Evidence-Evolution-Universe/dp/0393315703/ref=sr_1_4?ie=UTF8&s=books&qid=1223754778&sr=1-4
The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution, by Richard Dawkins http://www.amazon.com/Ancestors-Tale-Pilgrimage-Dawn-Evolution/dp/061861916X/ref=pd_bbs_sr_1?ie=UTF8&s=books&qid=1223755602&sr=1-1
Darwin's Dangerous Idea: Evolution and the Meanings of Life, by Daniel C. Dennett http://www.amazon.com/Darwins-Dangerous-Idea-Evolution-Meanings/dp/068482471X/ref=sr_1_1?ie=UTF8&s=books&qid=1223754873&sr=1-1
Evolution for Everyone: How Darwin's Theory Can Change the Way We Think About Our Lives, by David Sloan Wilson http://www.amazon.com/Evolution-Everyone-Darwins-Theory-Change/dp/0385340923/ref=pd_bbs_sr_1?ie=UTF8&s=books&qid=1223754672&sr=1-1