there's a lot of misinformation in this thread. A major one was RunningArt's commet that humans are meant to eat at milk past infancy. Someone else posted that we're not meant to drink cow's milk at all.
How is drinking cow's milk any different than say, eating a carrot? It's not. Indeed, plants do a much better job of manufacturing toxic chemicals than a cow. Why the world is green is a major theme in ecology and one of the answers is because plants do such a good job resisting being eaten (by producing all the toxic chemicals).
We have an enzyme, lactase, that digests lactose sugar in milk. We obviously need this because we are a mammal and nurse, like all other mammals, but the enzyme isn't so different from the other disachharideases that almost all organisms on earth have. We didn't evolve lactase recently - like I said, all mammals have the gene. Primitively, the gene was "turned off" after weaning, meaning the enzyme wasn't made. So if you are lactose intolerant, its not because you don't have lactase its because you aren't making it any more. Several pops of humans independently evolved the mutation that kept this gene from being turned off - that is, lactase continues to be made after weaning. It is very well documented that this evolved in response to raising dairy animals (and drinking milk past weaning). So many of us of northern european descent are actually "made" to drink cow's milk.