We drank milk 6000 years ago

https://link.springer.com/article/10.1007/s12520-019-00911-7

Intolerance to lactose due to the lack of lactase could have been overpassed by fermentation. As fermentation is a spontaneous process it is possible that humans discovered very early the advantage of fermentation. Fermented milk and cheeses didn’t lead to symptoms of lactose intolerance.

https://www.pnas.org/content/113/48/13594.abstract