[mikeash]

Realizing that self-replicating entities with heritable traits subject to selection and variation will evolve over time just takes some thought. Applying it to the natural world just requires that you notice that living things are subject to selection, have variation, and have heritable traits.

DNA evidence requires some pretty advanced science to obtain. The basics of evolution have been known for millennia (that's how you breed plants/animals for specific traits, after all) but DNA evidence for evolution as the source of present-day biodiversity is quite recent.

[pdonis]

Of course DNA evidence is more recent; that's why I said it was obvious now (but wasn't before we knew about DNA). "Obvious" always requires background knowledge; the basics of evolution, as applied to plant and animal breeding for example, were known for millennia to plant and animal breeders, but not to people with no background in those disciplines.

Also, humans have only been breeding plants and animals for ten thousand years or so; would the consequences of self-replicating entities having heritable traits have been "obvious" to a Cro-Magnon 30,000 years ago? They were just as intelligent as we are (at least that's what the data on brain size indicates), but they didn't have our background knowledge.

Finally, saying it "just takes some thought" is vague: how much thought? How much compared to the knowledge and cognitive ability of the average lay person? If you randomly selected lay people and asked them to (1) briefly explain the consequences of self-replicating entities, and (2) briefly explain what having similar DNA means, giving them time to think about each question, which question would they, on average, do better on?