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