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We take for granted today that dairy consumption is something that is typical or even natural to human diet. However, that is not the case, as it contains lactose, a complex carbohydrate that is not digestible in most adult mammals.<ref> For lactose ingestion in mammals, see: Sahi, T. 1994. “Hypolactasia and Lactase Persistence Historical Review and the Terminology.” ''Scandinavian Journal of Gastroenterology'' 29 (s202): 1–6. dos:10.3109/00365529409091739. </ref> In other words, the vast majority of human beings, at one point, were physically unable to consume dairy products after their childhood; in fact, we see today a large percentage of people still unable to consume milk or dairy products in adulthood. This leads to the question on how did human societies become or evolve to the point where dairy consumption became prevalent?

==Evolution and Agriculture==

Natural selection, the process first uncovered by Charles Darwin, tells us that specific traits, over time, could become selected for, leading to a transformed species as given traits become advantageous for reproduction.<ref>For information on the process of natural selection, see: Endler, John A. 1986. ''Natural Selection in the Wild''. Monographs in Population Biology 21. Princeton, N.J: Princeton University Press.</ref> Dairy consumption, in fact, is an example of how relatively recent natural selection pressures have transformed our human DNA. Over 11,000 years ago, that is before the invention of agriculture, almost all societies depended on hunting and gathering. Their diets were much more diverse than ours and consumed a large variety of meats, fruits, and vegetables.<ref> For information on the human diet in pre-agricultural societies, see: Hockett, Bryan, and Jonathan Haws. 2003. “Nutritional Ecology and Diachronic Trends in Paleolithic Diet and Health.” ''Evolutionary Anthropology: Issues, News, and Reviews'' 12 (5): 211–16. dos:10.1002/evan.10116. </ref> However, dairy was not a caloric resource they would have typically encountered during adulthood.

This is made possible by the production of the lactase enzyme need to breakdown lactose into digestible carbohydrates. With greater availability of milk in early settled societies, the advantages of having a high caloric product that has calcium and fats that are beneficial became a greater selective pressure for human populations. The genes that then produce the lactase enzyme became selected for or gave advantages to given human populations that then allowed the consumption of lactose found in diary. The fact that milk was utilized very early in the Near East has meant that populations there do show a relatively high ratio of lactose persistence in adulthood (Figure 1).

==Coevolution of Dairy==

The story of how dairy developed as a product of consumption is, however, more complex. Once agriculture was invented, it did begin to spread to Europe, initially in southeast Europe before spreading to northern and western Europe.<ref>For tracing the spread of agriculture, see: Pinhasi, Ron, Joaquim Fort, and Albert J Ammerman. 2005. “Tracing the Origin and Spread of Agriculture in Europe.” Edited by Chris Tyler-Smith. ''PLoS Biology'' 3 (12): e410. dos:10.1371/journal.pbio.0030410.</ref> Looking at a modern map of populations that are lactose persistent (Figure 1), we see pockets where particular populations have a much greater portion of the population able to consume dairy. This indicates that the spread of dairy consumption was not even and that different populations have adapted differently to dairy consumption. In Europe, we see eastern Europe, around Poland and the Czech Republic, there is a greater percentage of people who are lactose persistent. A closer look at the haplotypes, or genetic groups, that include the genes for dairy consumption indicate that not all human populations that have evolved to consume dairy show the same genes involved in the consumption of lactase.<ref>For further on the genetic makeup of Europeans in relation to the consumption of diary, see: Leonardi, Michela, Pascale Gerbault, Mark G. Thomas, and Joachim Burger. 2012. “The Evolution of Lactase Persistence in Europe. A Synthesis of Archaeological and Genetic Evidence.” International Dairy Journal 22 (2): 88–97. doi:10.1016/j.idairyj.2011.10.010.</ref>