Lactase

Lactase is the enzyme which breaks down lactose. That's a special kind of sugar, in milk, which mammals (ideally) should grow out of drinking. Failure to do that - lactase persistence - results in Kate Upton obsessives and nobody wants those around.

Over the Neolithic, men who'd already learnt to domesticate dogs and, maybe, rabbits learnt to domesticate the big dumb mammals, starting I think with sheep. It occurred to some of these men to milk the beasts. That allowed a sort of interim-weaning for human infants and, further, led to lactose-light cheeses and several other dairy products. The one exception, in adults, was milk itself - but who needed that anyway, in an age before Louis Pasteur.

David Reich is here to constrain lactase persistence. Milk-guzzling did not come with the Neolithic farmers and herders. It came with the Bronze Age - with western Europeans, like the Sabines.

Why they chose to keep drinking milk as adults, I know not - but I'll take a guess at it. It is often the case that a sugary liquid crop is fermented for drink, before it is consumed "raw". Milk, in a pre-Pasteur age, historically was fermented as kumiss. Indo-European gatherers were famed connoisseurs of mead honey-wine; berry boozes are all over the Balkans to this day. A pastoral society, with direct descent from gatherers, would experiment with milk.