The difference is that the evolution for lactose digestion involves turning off an already existing gene. Digesting phytic acid involves evolving a new gene -

The difference is that the evolution for lactose digestion involves turning off an already existing gene. Digesting phytic acid involves evolving a new gene - a new enzyme maker. So you are talking about one random mutation vs. XXXX mutations. A genetic study has found that humans did evolve rapidly in the last 10,000 years, but most of the changes were one-off mutations such as these.