"the 10,000 years since the advent of agriculture is a very small evolutionary time step"

True, although those 10,000 (I've actually heard 20-40K) years are the most recent ones in our evolution and therefore have more direct bearing on our current state. In addition, humans experienced a population bottleneck around 60,000 that brought us down to 10-20K individuals. Since then, there have been repeated agricultural famines. These stresses should have favored people able to eat the grains that formed the basis of our diet.