This is incorrect, and there are many examples to prove it. For example, alleles conferring lactose tolerance increased to high frequencies in Europe just a few thousand years after animal husbandry was invented. Similarly, recent increases in the number of copies of the gene for salivary amylase, which digests starch, are related to agriculture.
Basically, the response to the change of the environment of a species depends on three factors:
1. Heritability
2. Intensity of the selection
3. Number of generations that selection acts.
What this means is that 10,000 can be more than sufficient to fully evolve the ability to eat and digest dairy and grains.
Whilst I'm sure that is true, it certainly won't be true for the entire human population.
I strongly suspect that my excellent reaction to low carb diets is because I haven't evolved the ability to consume them as well as other humans. Clues include oral allergy to birch pollen and my 'output' when I eat dairy and grains is less than optimal.
I don't believe low carb works for everyone. It doesn't work for my wife. But it is a useful tool, that I'm very glad I found.
Basically, the response to the change of the environment of a species depends on three factors:
1. Heritability 2. Intensity of the selection 3. Number of generations that selection acts.
What this means is that 10,000 can be more than sufficient to fully evolve the ability to eat and digest dairy and grains.