For the first few months after birth mammals generally tend to live off milk until they become weaned and very rarely, if ever, drink milk again (Simoons 1978). The lactose or milk sugar is broken down into galactose, glucose and monosaccharides (Feldman 2005) by the lactase gene, allowing it to be used as a nutritional source. In general, the production of lactase declines rapidly at a very young age. Therefore, after weaning most people cannot catabolise or break down lactose in large amounts. This condition is referred to as Primary Lactose Intolerance (Feldman 2005). However, in some human populations, particularly those of central and northern Europe (Burger et al. 2006), and some parts of Africa, individuals have developed lactase persistence (Itan et al. 2009), therefore continuing to produce lactase. By observing our evolutionary history, as well as the distribution of lactase persistence in adults, it is evident that the variation in the lactase gene is a recent evolutionary adaptation, and is the result of mutation (a variation in the genetic code) (Relethford 2010) and natural selection.

Lactase persistence is unseen among mammals and very rare among most adults in the world. However, frequencies of lactase persistence are noticeably high among central and northern Europeans, as well as in some African pastoralist groups (Itan et al. 2009). A mutation on the lactase gene located on chromosome two is believed to be related to lactase production (Relethford 2010). Research indicates that the genetic variant of the lactase gene closely related to lactase persistence in Europe is -13,910*T (Itan et al 2009). Figures in Table 1 of the appendix show a close relation between the -13,910*T allele found in European populations and lactase persistence. The eastern and southern European populations have the lowest range of -13,910*T allele frequency and lowest occurrence of lactase persistence. The frequency of the -13,910*T allele and the occurrence of lactase persistence among the western and central Europeans was higher than the eastern and southern populations. However, the Northern European population indicated a much higher -13,910*T allele frequency and consequently a higher occurrence of lactase persistence (89 – 96%). These results suggest that there is a strong relation between the -13,910*T allele and lactase persistence. Furthermore, recent research of ancient DNA suggested that early farmers of eastern and central Europe did not express the -13,910*T allele (Itan et al 2009), suggesting a mutation of the lactase gene must have occurred and resulted in lactase persistence. However, this relationship between the -13,910*T allele is not associated with all Lactase persistent frequencies, such as some African populations (Myles 2005). This suggests several mutations must have occurred and is evidence of convergent evolution.

Africans unlike Europeans do not possess the -13,910*T allele and generally have a lower frequency of lactase persistence. For example, African populations such as the Ibos and Yoruba have a high frequency of lactose intolerance (100%), (Relethford 2010). However, some African populations such as the Fulani people have a much lower frequency of lactose intolerance (22%), (Relethford 2010) even though they do not express the same allele as the lactase persistent European populations. The differences between lactose intolerance and lactose tolerance among these African populations is characterised by their pastoral history. Dairy farming was not a tradition or practice of the Ibos and Yoruba populations (Relethford 2010). However, as nomadic cattle herders, the Fulani people consumed a large amount of milk as part of their diet. (Relethford 2010) This suggests that populations with a dairy farming history show a lower frequency of lactose intolerance (Mace et al. 2003). It also suggests that the mutation on the lactase gene was most likely caused after the introduction of dairying. This theory is further supported with evidence of European populations. Northern Europeans have a long dairy farming history and as mentioned earlier, a higher frequency of lactase persistence. This is further supported with the following observations of lactose persistence among different populations: hunters and gatherers (12.6%), non-dairying pastoralists (15.5%) and northern Europeans (91.5%), (Simoons 1978).

The relationship between dairy farming and a higher frequency of lactase persistence supports the Co-evolutionary hypothesis as suggested by Simoons (Burger et al. 2006). The theory suggests that human behaviour results from both genetic and cultural evolution (Mace 2003), therefore meaning that these cultures influenced their biological evolution by consuming milk as their main nutritional source. This is in turn caused a selection pressure resulting in lactase persistence (McCracken; Simmons, cited in Myles 2005). Furthermore, this provides evidence of natural selection – a mechanism by which certain traits that increase survival and reproductive fitness in organisms are favoured over others (Relethford 2010). Lactase persistence is one such example of natural selection as individuals who are able to include milk in their diet have a survival or fitness advantage in times of food shortage, as the milk provides an extra nutritional source (Feldman 2005).

Adult lactose tolerance in humans has clearly evolved over time. Firstly, humans are the only mammals to show signs of lactose tolerance. Furthermore, by looking at the distribution of the lactase persistent allele (such as its high frequency in northern Europe) as well as the trends between lactose tolerance and dairying populations, it is evident that lactose tolerance is the result of both a mutation on the lactase gene and natural selection. Its advantage in increasing human survival and fitness has resulted in its ever increasing frequency, as it provides an additional nutritional source and a selective pressure.

References
Burger, J, Kirchner, M, Bramanti, B, Haak, W, Thomas, M.G. 2006, ‘Absence of the lactase-persistence-associated allele in early Neolithic Europeans,’ Proceedings of the National Academy of Sciences of the United States of America Journal, vol. 104, no.10, pp. 3736 - 3741. Available from: PubMed Cenral [28 March 2007]

Itan, Y, Powell, A, Beaumont, MA, Burger, J, Thomas, M.G. 2009, ‘The Origins of Lactase Persistence in Europe,’ PLOS Biology Journal, vol.10, pp. 1-13 Available from PLOS Computational Biology [28 August 2009]

Feldman, MW, Sforza L 2005, On the Theory of Evolution under Genetic and Cultural Transmission with application to the Lactose Absorption problem. Available from: http://stanford.edu/group/morrinst/pdf/3.pdf [8 April 2010]

Mace, R, Jordan F, Holden, C 2003, ‘Testing evolutionary hypotheses about human biological adaptation using cross-cultural comparison,’ Comparative Biochemistry and Physiology – Part A: Molecular & Integrative Physiology Journal, vol. 136, no. 1, pp. 85-94. Available from: Science Direct [20 June 2003]

Myles, S, Bouzekri, N, Haverfield, E, Cherkaoui M, Dugoujon, J 2005, ‘Genetic Evidence in support of a shared Eurasian-North African dairying origin,’ Human Genetics Journal, vol. 117, no.1, pp 34-42. Available from: SpringerLink [2 April 2005]

Relethford, J 2010, The Human Species, McGraw-Hill, New York

Simoons, F 1978, ‘The Geographic Hypothesis and Lactose Malabsorption – A Weighing of the Evidence,’ Digestive Diseases, vol. 23, no.11, pp. 963. Available from: SpringerLink [31 March 2005]