Mammals in their infancy usually make lactase: they need it for digesting the lactose found in their mother's milk. Mammalian adults usually lose the capacity to produce lactase. Thus, if you give milk to an adult cat it causes discomfort and diarrhea.
Lactase evolution in humansEdit
The ability to digest lactose into adulthood ('lactase persistence') was useful to humans after the development of animal husbandry. This created a system providing a consistent supply of milk.
A mutation on chromosome #2 stops the shutdown in lactase production, making it possible for those with the mutation to continue drinking fresh milk (and eating other dairy products) throughout their lives.
This appears to be a recent adaptation to dairy consumption. It has occurred independently in both northern Europe and East Africa in populations with a pastoral lifestyle. Lactase persistence, allowing lactose digestion to continue into adulthood, is a dominant allele, making lactose intolerance a recessive trait.
Genetic studies suggest that the oldest mutations associated with lactase persistence only reached appreciable 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.
- Itan, Yuval et al. 2010. A worldwide correlation of lactase persistence phenotype and genotypes. BMC Evolutionary Biology. 10(1). p36.
- Swaminathan, N. 2007. Not milk? Neolithic Europeans couldn't stomach the stuff. Scientific American.
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- Coles Harriet (2007). "The lactase gene in Africa: do you take milk?". The Human Genome, Wellcome Trust. Retrieved 2008-07-18.
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- Swaminathan, N. 2006. African adaptation to digesting milk is "strongest signal of selection ever". Scientific American.
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