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.

1. The surgery might introduce some material a human body can't synthesize.¹
2. The surgery might require intelligent analysis of the unique shape of a subject's brain, after it has developed naturally to adulthood.
3. The necessary mutation might simply not exist.  The configuration space for physically possible organisms must surely be larger than the configuration space for human-like DNA (I get the sense I'm taking sides in a longstanding feud in evolutionary theory with this one).
4. The surgery might have some minor side effect that would drastically reduce fitness in the ancestral environment, but isn't noticeable in the present day.  Perhaps it harnesses the computing power of the subject's lymphocytes, weakening the immune system.