According to Thomas, milk was a "superfood" to early man. It contains protein, calcium, fat and carbohydrates. The ability to digest milk without developing diarrhea would give it's drinker an evolutionary edge. The people who had the lactase mutation genes would survive the harsh winters, famines and even other illnesses that caused early death. Those genes would survive and be passed on to further generations and create more lactose tolerant milk drinkers. Other scientists have speculated that fresh milk provided a more pure fluid alternative as water was often contaminated or difficult to find in arid climates. Milk may have also given people a fertility advantage. We know that women need a certain amount of body fat to produce children so the lactose tolerant women may have produced more offspring.
I was surprised to learn that only about 35% of the adult population worldwide can drink milk without a problem. East Asians and Indigenous North Americans may be 80-100% lactose intolerant yet the Brits in the UK run a rate of only 5-15%.
People with lactose intolerance frequently get symptoms of nausea, cramping, bloating, diarrhea and gas about 30 minutes to 2 hours after drinking milk. But the symptoms can vary and some individuals can drink small quantities of milk or other dairy products without a problem. The intolerance can also change during pregnancy. Lactose intolerance may be the culprit in many instances of "Irritable Bowel Syndrome".
For ice-cream lovers, Breyers makes a lactose free ice-cream as does Turtle Mountain and Lactaid.