Eliezer argues that there can't possibly be a simple surgical procedure that dramatically increases human intelligence. Any physical effect it could have, he says, would necessarily have arisen before as a mutation. Since intelligence is highly beneficial in any environment, the mutation would spread throughout our population. Thus, evolution must have already plucked all the low-hanging fruit.
I'd be surprised if EY were actually arguing this.
The argument you summarize here isn't just an argument against the possibility of surgically improving intelligence, it's an argument against the possibility of technologically improving on any evolved capability (say, cars, or penicillin, or writing).
Come to that, it's an argument against any evolved species developing useful abilities that any other evolved species didn't develop.
I'm confused too. But I really didn't take any liberties with my paraphrasing. Here's an exact EY quote, albeit from a deprecated essay:
any simple intelligence enhancement will be a net evolutionary disadvantage - if enhancing intelligence were a matter of a simple surgical procedure, it would have long ago occurred as a natural mutation.
Edit: For an in-depth discussion of precisely this topic, see Nick Bostrom and Anders Sandberg's 2008 paper "The Wisdom of Nature: An Evolutionary Heuristic for Human Enhancement", available as a pdf here. This post was written before reading the paper.
There doesn't seem to be a thread discussing Eliezer's short-short story X17. While I enjoyed the story, and agreed with most of its points, I disagree with one assertion in it (and he's said it elsewhere, too, so I'm pretty sure he believes it). Edit: The story was written over a decade ago. Eliezer seems to have at least partially recanted since then.
Eliezer argues that there can't possibly be a simple surgical procedure that dramatically increases human intelligence. Any physical effect it could have, he says, would necessarily have arisen before as a mutation. Since intelligence is highly beneficial in any environment, the mutation would spread throughout our population. Thus, evolution must have already plucked all the low-hanging fruit.
But I can think of quite a few reasons why this would not be the case. Indeed, my belief is that such a surgery almost certainly exists (but it might take a superhuman intelligence to invent it). Here are the possibilities that come to mind.