As a previous poster has said, the absurdity heuristic works very well indeed—if something seems absurd to me, I need a lot of evidence before I’ll believe it. As Hume said:
“no testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous than the fact which it endeavors to establish.”
If someone claims that a talking snake is the reason for every bad thing that anyone has ever done, they’re going to have to provide some evidence that this is the case. If they claim that people are related to monkeys (which seems inherently less absurd to me, but I’m probably biased by the fact that it’s true), then they’re also going to have to provide some evidence, and whatever the extraordinary claim, the evidence is going to have to be enough to shift my belief from “that’s absurd” to “oh, I guess that’s true then”.
On the other hand, having written that I guess it’s more likely that the Absurdity Heuristic is more specific, and is the tendency to stick our fingers in our ears, say “that’s absurd, la la la”, and refuse to listen to any evidence to the contrary. I suppose this is a heuristic that people might use, and might be useful (in ruling out hypotheses which aren’t worth spending time falsifying), but as you say, does have its dangers. It’s not clear how to avoid the danger of ruling out an hypothesis which is absurd but true while getting the benefits of ruling out hypotheses which are simply absurd, but the heuristic still has its uses (perhaps some threshold of absurdity, some “suspension of absurdity” for certain types of proposition?)
I’d second that. In fact, I think that knowing about evolution is probably a necessary prerequisite to being a rational atheist. Even Dawkins admits that it would have been pretty difficult not to believe in God before there was a plausible naturalist explanation for the complexity of life.
Of course, it’s possible to know that there is a plausible naturalist explanation without really understanding the nuts and bolts of how it works (I’d probably put myself in that category), but maybe it really does help to hammer home the point if you really understand that The Creator wasn’t necessary to make life, what exactly is He for?