Some human populations have developed the ability to digest nutrients in milk throughout their lives. As newborns, mammals rely solely on milk for their nutritional needs and, therefore, possess the enzymes necessary to digest milk and its nutrients. However, all mammals, except select populations of humans, lose the ability to digest the main carbohydrate in milk, lactose, after weaning (Ingram et. al, 2008). Lactase is the small intestinal enzyme responsible for breaking down lacrosse into the absorbable monosaccharides glucose and galactose. The presence or absence of lactase is a genetic polymorphism; individual is lactase persistent or lactase non-persistent. Individuals who do not have persistent lactase do not absorb lactase in the intestine, so it travels to the colon, where it is metabolized by bacteria. This causes fermentation and excess gas, which causes abdominal symptoms such as bloating, cramps, nausea, diarrhea and vomiting. On the other hand, those who show lactase persistence are able to break it down throughout their lives. Lactase persistence began with the domestication of animals, which led to dairy farming. Milk production and consumption of adult human milk have provided selective pressure that has led some human populations to develop lactase persistence, allowing them to digest the beneficial nutrients of dairy products throughout their lives. Frequencies and origin of lactase persistence in human populations It has long been thought that most humans had elevated lactase activities as adults, most likely due to the fact that early research was conducted in countries where lactase persistence lactase was the norm (Swallow, 2003). However, lactase non-persistence is actually more… the focus of the article… neurons have high frequencies of lactase persistence and low frequencies of the T-13910 allele. Two SNPs have been associated with lactase persistence in the Saudi population, T/C-3712 and T/G-13915 (Enattah et. al, 2008). Both have been shown to increase lactase transcription in vitro (Enattah et. al, 2008). Conclusion The persistence of lactase in humans has been one of the most prominent examples of natural selection in humans. It is an example of convergent evolution, as the persistence of the lactase phenotype has developed independently in different human populations. Although the single nucleotide polymorphism, -T 13910, is highly associated with lactase persistence in European populations, there are multiple variants in SNPs that control lactase persistence in Middle Eastern and African populations. Further research is needed to fully understand the evolution of lactase persistence worldwide.