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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 humans
The ability to digest lactose into adulthood ('lactase persistence') was useful to humans after the invention of animal husbandry. This developed animals that gave 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.
- Malmstrom H. et al. 2010. High frequency of lactose intolerance in a prehistoric hunter-gatherer population in northern Europe. BMC Evolutionary Biology 10: 89.
- Coles Harriet (2007). "The lactase gene in Africa: do you take milk?". The Human Genome, Wellcome Trust. http://genome.wellcome.ac.uk/doc_WTX038968.html. Retrieved 2008-07-18.
- Coelho M. et al. 2002. Microsatellite variation and evolution of human lactase persistence. Human Genetics 117(4): 329–339.
- Bersaglieri T. et al. 2004. Genetic signatures of strong recent positive selection at the lactase gene. American Journal of Human Genetics 74(6): 1111–20.
- Wade N. 2006. Study detects recent instance of human evolution. The New York Times. December 10, 2006.
- Swaminathan, N. 2006. African adaptation to digesting milk is "strongest signal of selection ever". Scientific American.
- Aoki K. 2001. Theoretical and empirical aspects of gene–culture coevolution. Theoretical Population Biology 59(4): 253–261.