"Other researchers have suggested that fresh milk provided a more pure fluid alternative to contaminated water sources in arid environments"

http://www.npr.org/blogs/thesalt/2012/12/27/168144785/an-evo...

"Milk is uncontaminated by parasites, unlike stream water, making it a safer drink."

http://darwinstudents.blogspot.com/2009/02/evolution-of-lact...