No other species has adult members capable of digesting milk, including cats. It is not advisable to feed an adult cat milk, popular images to the contrary notwithstanding. Some may tolerate the milk and merely suffer from gas, while others will get diarrhea and/or throw up. That’s one kind of hot mess you do not want running around your house.
The 40% or so of the human adult population carrying a genetic variant which continues to produce lactase into adulthood is not evenly distributed.
The numbers are often given as close to 0% of Native Americans, 5% of Asians, 25% of African and Caribbean peoples, 50% of Mediterranean peoples and 90% of northern Europeans. Sweden has one of the world’s highest percentages of lactase tolerant people.
The old theory about the origin of this unusual ability was that humans living in the colder, less sunny parts of Northern Europe had difficulty getting sufficient vitamin D and individuals who could tolerate milk into adulthood were at a great advantage.
Actually, vitamin D isn’t really a vitamin. It’s a hormone.
Here’s the new theory, published in PLoS Computational Biology:
Using data on −13,910*T allele frequency and farming arrival dates across Europe, and approximate Bayesian computation to estimate parameters of interest, we infer that the −13,910*T allele first underwent selection among dairying farmers around 7,500 years ago in a region between the central Balkans and central Europe, possibly in association with the dissemination of the Neolithic Linearbandkeramik culture over Central Europe. Furthermore, our results suggest that natural selection favouring a lactase persistence allele was not higher in northern latitudes through an increased requirement for dietary vitamin D. Our results provide a coherent and spatially explicit picture of the coevolution of lactase persistence and dairying in Europe.
In other words, the lactose tolerance began with dairy farmers in Central Europe and gradually spread outwards, rather than spontaneously appearing in Northern Europe.
The remarkable part is that the ancient Europeans were not the only ones to develop that type of mutation.
The European mutation is different from several lactase persistence genes associated with small populations of African peoples who historically have been cattle herders.
Researchers at the University of Maryland identified one such mutation among Nilo-Saharan-speaking peoples in Kenya and Tanzania. That mutation seems to have arisen between 2,700 to 6,800 years ago. Two other mutations have been found among the Beja people of northeastern Sudan and tribes of the same language family in northern Kenya.