I never said adults could not digest dairy nor did I say we could not utilize it. I said it was not neccessary after infancy. For all I know cows produce lactase all their lives too, but I have never heard of an adult cow still nursing its mother. While it is true that most humans nursed their offspring much longer in the past than they do today (some up to 5 years of age), I doubt that adult humans still nursed the lactating women in their tribes (but ya never know! ). Seriously, milk from non-human sources was probably not used by humans until they domesticated animals during Neolithic times. That means that the earliest humans did not drink milk after they were weaned in childhood.

ps: Just think of cats for an example. Domesticated cats would never drink milk after they were weaned unless humans gave it to them or they were starving and just accidentally happened upon a bowl of milk. The same is true of all mammals. I believe humans are the only mammals that choose to drink milk (use milk products) after they are weaned.