mammals normally cannot drink milk as adults, but humans are an exception. milk is digested by the enzyme lactase, which switches off as mammals stop taking milk from their mothers. the human ability to drink milk during adult life is supported by a lactase mutation which prevents this switch-off. human populations have a high proportion of this mutation wherever milk is important in the diet. the spread of this 'milk tolerance' is promoted by natural selection, because it helps people survive where milk is available. genetic studies suggest that the oldest mutations causing lactase persistence only reached high levels in human populations in the last ten thousand years. therefore, lactase persistence is often cited as an example of recent human evolution. as lactase persistence is genetic, but animal husbandry a cultural trait, this is gene–culture coevolution.