We tend to think of evidence for evolution in terms of complex facts of geology and biology. But in fact there is pretty good evidence for the theory of evolution which is available to almost everyone, or least everyone who has some familiarity with various kinds of plants and animals, without any complicated study.

What happens when you take a thing, then make a copy, then make copies of the copies, and so on? If your copies are perfect, you will just get a bunch of identical copies of the original. But if your copies are not perfect, something else happens. Suppose you perform this process with photocopies of a sheet of paper with text on it. Over time, various discrepancies will creep in. For example, during one of your copies there may be a hair on the surface of the copy machine, and this hair will show up as an extra line on the copy.

Then, when you make copies of the sheet with the extra line, all the copies you make of it, and all the copies of the copies of that sheet, and so on, will all have an extra line.

At the end you will be able to divide your copies into at least two families: ones with the extra line, and ones without it. In practice you will not get just two families, but families within families within families, and so on.

There are two facts about living things, neither of which is all that hard to notice.

First, living things make copies of themselves. They are not perfect copies but imperfect ones, with differences from the original.

Second, living things are organized in the way discussed above, as families within families. Thus there are various kinds of dog such as the chihuahua and the golden retriever, which are both kinds of dog. And then there are dogs and wolves, which are pretty similar themselves. And wolves have a similar relationship with coyotes and jackals. And all of these canine species have a similar relationship with cat families, and so on.

These two facts are evidence for common descent, that is, evidence that all of these living things are remote descendants of a lengthy process of the imperfect copying of one original ancestor.

Nonetheless, the theory was rarely proposed, if not non-existent, before the eighteenth century. Empedocles anticipated the theory of natural selection, as in this statement by Aristotle:

Wherever then all the parts came about just what they would have been if they had come be for an end, such things survived, being organized spontaneously in a fitting way; whereas those which grew otherwise perished and continue to perish, as Empedocles says his ‘man-faced ox-progeny’ did.

Empedocles, however, implied (for example by speaking of “man-faced ox-progeny”) that things came to be by chance, and does not seem to have suggested common descent in particular.

Given the presence of evidence for common descent, why was the theory not proposed much earlier? I have two guesses regarding the reason for this. First, the existence of an apparently settled account in the book of Genesis. Second, the fact that it is difficult for people to conceive of long periods of time and of their effects. People have a hard time even with much shorter periods of time, let alone the idea of considering the effects of the passage of millions of years.