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In most of the world's
population the ability to digest lactose declines sharply after infancy.
High lactose digestion capacity in adults is common only in populations of
European and circum-Mediterranean origin and is thought to be an
evolutionary adaptation to millennia of drinking milk from domestic
livestock. Milk can also be consumed in a processed form, such as cheese or
soured milk, which has a reduced lactose content. Two other selective
pressures for drinking fresh milk with a high lactose content have been
proposed: promotion of calcium uptake in high-latitude populations prone to
vitamin-D deficiency and maintainance of water and electrolytes in the body
in highly arid environments. These three hypotheses are all supported by the
geographic distribution of high lactose digestion capacity in adults.
However, the relationships between environmental variables and adult lactose
digestion capacity are highly confounded by the shared ancestry of many
populations whose lactose digestion capacity has been tested. The three
hypotheses for the evolution of high adult lactose digestion capacity are
tested here using a comparative method of analysis that takes the problem of
phylogenetic confounding into account. This analysis supports the hypothesis
that high adult lactose digestion capacity is an adaptation to dairying but
does not support the hypotheses that lactose digestion capacity is
additionally selected for either at high latitudes or in highly arid
environments. Furthermore, methods using maximum likelihood are used to show
that the evolution of milking preceded the evolution of high lactose
digestion. |