I know it is dishonest to ask LW-ers to spend time on studying exactly human anatomy, but even a thorough look at some skeleton should give you a vibe of how defined human bodies are.
I see three lines of addressing this concern:
1) Anatomy was over a long time under strong evolutionary pressure. Human intelligence is a fairly recent phenomena of the last 100,000 years. It's a mess that's not as well ordered as anatomy.
2) Individual humans deviate more from the textbook anatomy than you would guess by reading the textbook.
3) The brain seems to be build out of basic modules that easily allow it to add an additional color if you edit the DNA in the eye via gene therapy. People with implented magnets can feel magnetic fields. It's modules allow us to learn complex mental tasks like reading texts which is very far from what we evolved to do.
Also, human intelligence has been evolving exactly as long as human anatomy, it simply leaped forward recently in ways we can notice. That doesn't mean it hasn't been under strong evolutionary pressure before. I would say that until humans learned to use tools, the pressure on an individual human had had to be stronger.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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