My basic answer is: I’ve no idea. I’ve been doing this for twenty years, and I’m not sure I’ve ever persuaded someone that creationism is inconsistent with what we know. That’s borne of the fact that these are two entirely different ways of knowing the universe. We talk about science as being a way of knowing – a way of knowing which is reliant on evidence, experimentation, observation and all those things which pool together into a grand theory, which is the theory of evolution by natural selection. And that’s not a theory in the vernacular sense – it’s a theory in the scientific sense, which is the absolute top of the hierarchical pile of the strength of our ideas.
But there are other ways of knowing, and one of them is via faith, and creationism is a form of faith. There is literally and singularly no physical evidence we are aware of in the universe which supports the notion that, if you’re a young-Earth creationist, that the Earth is 6,000 years old – or, if you are certain forms of Islamic creationist, that it’s a million years old; which is a bit more generous but still massively, massively wrong.
These are classic examples of the non-overlapping magisteria argument, as first formalised by Stephen J. Gould. The problem arises when one encroaches on the other – when creationism encroaches on evolution, which it has to do, in the sense that it is an attempt to explain the diversity and radiation of life on Earth, which evolution by natural selection does perfectly satisfactorily with a million forms of evidence to suggest that it’s correct.
“If there was a supreme, perfect designer who is responsible for the human eye, he’s not a very good one.”
There’s the argument that some things in biology are too complex to rely on an explanation based on evolution. The classic example is the eye – an extremely complex organ, and they don’t really work if you don’t have all the components working exactly as they do; so the argument goes that the eye has to have been designed as such. But what we’ve shown, over the last 150 years or so, is that every single possible step in the evolution of the human eye is present in extant nature. And there’s another argument against the idea of the irreducible complexity of the eye, which is that human eyes aren’t that great. If there was a supreme, perfect designer, who is responsible for the human eye, he’s not a very good one.
I don’t know whether there’s one killer fact: there are so many different strands. Back in the 19th century, it was more to do with morphology being clearly shared: the bones of a human hand are very similar to the ones in a whale’s flipper, which are very similar to the bones of a horse’s hoof. Only a very unimaginative designer would use the same system to create those different limbs. From the 1950s onwards, it’s the emergence of genetics, which has said exactly the same thing: at a molecular level, there is clearly common ancestry, and the closer species are related in time, the more DNA they have in common.
I think the one I like best is the fact that we have 46 chromosomes, and gorillas and chimpanzees have 48. When you look at Chromosome 2 in humans, it’s pretty much identical to the fusion of two chromosomes from chimpanzees and gorillas. We can see the joints. It very clearly shows that the fusion of two chromosomes in a common ancestor of all the great apes sent us down one path, and chimpanzees and gorillas down another. I mean, even the Pope acknowledged this.
Just to add to the point about the eye, Richard Dawkins gave an excellent demonstration about the evolution of the eye in his Christmas Lectures in 1991.
The entire series of lectures is a must watch though and answers several basic questions about Evolution in a clear, understandable manner.