- The milk sugar lactose is broken down by the enzyme lactase .
- A competing hypothesis argues that ancient europeans domesticated milk-bearing livestock because lactase persistence was already quite common in certain populations .
- In most cases mammals stop producing lactase after weaning but a nucleotide switch in their dna can keep lactase flowing into adulthood a trait called lactase persistence .
- Switching from milk formula to soy formula is helpful for severe diarrhea ( from acquired lactase deficiency ) and for vegetarianism .
- Lactase persistence is far from a universal condition ( half of modern humans cannot digest lactose ) and there are at least two competing theories as to how it became the norm for european populations .