Many of us are lactose intolerant.  It turns out that most animals are lactose intolerant in their adult stage.  This stands to reason given that most animals are weaned off of milk early in life.  Thus the gene coding for the production of lactase becomes dormant. 

Dr Katherine Pollard gives an insightful precis of her work in this area:

Original Link:
http://fora.tv/2009/10/03/Dr_Katherine_Pollard_What_Makes_Us_Human#Lactose_Intolerance_and_the_Evolution_of_Human_Digestion