On the other hand biologists have shown that a complex eye can evolve and has evolved—actually on several independent occasions—from simpler forms of light- sensitive cells. Once you see how much change can be produced within a species, it’s not hard to see how evolution can transform one species into another. Is it such a stretch to believe that the lion and the tiger evolved from a common ancestor, even if there is no way to see this process occur?

I am not a biologist, but what impresses me is that virtually every biologist in the world accepts the theory of evolution. While the debate goes on, it seems improbable that the small group of intelligent design advocates is right and the entire community of biologists is wrong. Consider what two leading Christian biologists say about evolution. Kenneth Miller writes, “Evolution is as much a fact as anything we know in science,” and Theodosius Dobzhansky famously said, “Nothing in biology makes sense except in the light of evolution.”

The great strength of evolution as a scientific theory is that it makes sense of two huge facts about life. On the one hand, all living things from trees to cats to humans are formed from the same genetic material. Beyond this, it is evident that many groups of organisms show similar characteristics. So there is a unity to life.

At the same time, living creatures exhibit incredible diversity There are literally millions of living species with widely varying characteristics. Evolution accounts both for the similarities and the differences. It accounts for common characteristics by positing that the creatures possessing them descended from the same ancestor. It explains differences by suggesting that creatures evolved new traits over a long period of time under the pressures of survival.

One of the strongest proofs for evolution is that the geological record, for all its imperfections, shows a single invariant trajectory. The oldest rocks contain only single- celled creatures. Later strata show the appearance of invertebrates. Then we see the first fishes, then amphibians, then reptiles, and finally mammals. Man appears latest on the scene. The fossils are found in exactly the places and at exactly the times that we would expect if Darwin’s theory is correct.

Not a single fossil has ever been found in a place where it is not supposed to show up. If we ever discover the fossil of a single reptile in a rock so old that fishes had not yet arrived, or if we find human skeletons at the time when dinosaurs also lived, then Darwin’s theory will be proven false and biologists will have to come up with a new one.

Until this happens—and I don’t think it will—evolution remains the best and most persuasive account of our origins. It is impossible to deny the theory’s explanatory power. Evolution by natural selection helps us to explain why pesticides and antibiotics frequently result in the pests and bacteria developing new strains more resistant to human efforts to wipe them out. In a word, they evolve.