In ancient times, humans became lactose intolerant, or unable to fully digest the sugar (lactose) in milk, after childhood. However, according to several reports, this changed circa 10,000 B.C., when a human mutation occurred, likely in Europe, which allowed adults to consume dairy.
Why lactose intolerance later became prominent once again baffles scientists. Lactose intolerance was first discovered by Hippocrates, the ancient Greek physician 2,500 years ago. In 1906, R. H. Plimmer was the first scientist to discover lactase enzymes in the intestines of animals. In 1978, a breath hydrogen test was used by Michael D. Levitt to diagnose lactose intolerance.