I can link EPHX2 biologically to damage caused by smoking, but I found no evidence linking it to smoking behavior.
I'm pretty confident in CHRNA2 here.
I agree that CHRNA2 is the best candidate to explain smoking behavior, but would bet that there is some co-evolution between the regulation of the two smoking related genes, assuming that apes did not smoke.
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I'm pretty confident in CHRNA2 here.