Something that should be considered when discussing speed of evolution is
that some of our digestion is done by micro-organisms, not necessarily
resident.
Bacteria are said to evolve in much shorter time than we, so digestive
changes could take place in days, rather than tens of thousands of years.
The big problem here is the internal environment, which is polluted in
modern man. Microbes will have a hard time living in a gut environment
full of chlorine, fluorides, PCBs, etc.
William