All mammals (including humans) are supposed to be born with the ability to produce an enzyme called lactase to breakdown lactose (a sugar found in milk), which allows them to take in the nutrients and energy from their mother’s milk. However, after weaning, lactase production decreases as the organism is switch over to other foods. One blatant exception to this norm is demonstrated within our species Homo sapiens, but we have only recently become the exception. In fact, the ability to digest produce lactase/digest lactose in one’s adult life, called lactase persistence, is only present in 35% of humans beyond the age of about seven or eight years old (Gerbault et al., 2011).
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Like many new traits in an organism, lactase persistence was a result of a mutation. There is now evidence that several mutations led to lactase persistence in humans in Europe, the Middle East, Africa, and Tibet (Gerbault et al., 2011; Leonardi, 2013; Jeong & di Rienzo, 2014; Bayoumi et al., 2016; Liebert et al., 2017). However, the first mutation that was discovered to be linked to lactase persistence originated in central Europe between 12,300 and 7,450 years ago, and one of the lactose persistence mutations that originated in Africa is dated to a slightly more recent time than the European variant (Leonardi, 2013). While this mutation allowed for humans to digest milk in their adult lives, it did not qualify as an adaptation on its own. In order for this trait to be beneficial, humans had to have access to milk.
Around the same time that the mutation for lactase persistence came about in central Europe (and eventually Africa), humans in the Fertile Crescent had already begun to lead agricultural lives, which included domesticated animals, around 12,000 years ago. As these agriculturalists spread into the neighboring lands of Western Asia, Africa, and Europe, they brought with them cattle, goats, and sheep (Leonardi, 2013). Along with those stock animals, they also brought dairying practices with them.
It is suspected that dairying began not long after the animals began to be domesticated. Pottery that was found in the Middle East and dated back to the time when domestication of animals occurred about 9,000 to 7,000 years ago was analyzed for fat residues, and there were traces of fat residues found in the milk of cattle and other dairy animals (Curry, 2013; Leonardi, 2013; Spiteri et al., 2016). There was also evidence of cheese and yogurt-making amongst dairy farmers in the Middle East and southern Europe. Even though lactase persistence was not present in these areas, the lactose levels in cheese and yogurt is low enough that even lactase non-persisters (those who cannot digest lactose as adults) are able to digest cheese and yogurts when taken in small amounts (Curry, 2013). Thus, despite not being lactase persistent, consuming dairy products was part of dairy culture that was being swept into Europe and parts of Africa and Western Asia.
As agriculturalists and pastoralists began to settle in Europe and admix with the indigenous populations of hunter gatherers, dairying became interwoven with the fabric of the European lifestyle at the time. By about 6,000 years ago, Western, Central, and Northern Europe had a well-established dairy economy (Lukito, Malik, Surono & Wahlqvist, 2015). Using the same idea of looking for milk fat residues on pottery, it was found that Europeans were dairy farming and collecting milk as the practice spread gradually from southeast Europe to central Europe and then to northwest Europe (Gerbault et al., 2011; Cramp et al., 2014). It was through the overlapping timeline of the spread of dairying into Europe and the emergence of the lactase persistence mutation that lactase persistence was able to spread as it did.
Before dairying reached central Europe, the allele frequency for lactase persistence was low (Gerbault et al., 2011), but after the introduction of dairy farming, lactase persistence began to spread with the wave of pastoralism and dairy farming. With the transition to a lifestyle where milk and other dairy products were consumed more frequently, those who could consume more (due to their lactase persistence) were better suited to survive as they could obtain more nutrients from milk. This also meant that milk and other dairy products, especially cheese and yogurts that have a slightly longer shelf-life, became a reliable fallback food in case of famine or a bad harvest. It could even be used as a source of hydration in case of drought (Curry, 2013). The advantage that lactase persistence provides in a dairy community makes it an adaptation, so much so that “researchers estimated that people with the mutation would have produced up to 19% more fertile offspring than those who lacked it,” (Curry, 2013). The effects of this environmentally dependent selective advantage are seen in the geographic distribution of lactase persistence seen today.
In cultures where dairy farming and consuming fresh milk is common, lactase persistence offers a large advantage. However, in cultures where dairying did not have as large a foothold or where consuming processed dairy products rather than milk is more common, lactase persistence is not as imperative and offers less of an advantage. For instance, in northern and western Europe most of the population is lactase persistent (up to 96% of the population in the UK) (Itan, Powell, Beaumont, Burger & Thomas, 2009), which makes sense because dairy farming spread to northern and western Europe after lactase persistence emerged, so they coevolved. On the other hand, many areas in southern Europe practiced dairy farming long before lactase persistence spread to the region, and they had already integrated processed dairy (with less lactose) into their diets to compensate for the lack of lactase, so lactase persistence did not provide as significant an advantage. As a result, less than half of southern and eastern Europe are lactase persistent (Itan et al., 2009; Gerbault et al., 2011). There is a similar trend in India where the frequency of lactase persistence decreases as you get further south or east, which is farther away from the point of origin of lactase persistence in the Middle East and of dairy farming (Gerbault et al., 2011).
This trend is less obvious but still relevant when considering the spread of lactase persistence into Africa, which is still being researched. From what is known now, cases of lactase persistence from a mu outnumber those of lactase non-persistence in African pastoralist communities who herd dairy animals, but there are much fewer cases of lactase persistence in non-pastoral African communities (Ranciaro et al., 2014). The populations in Africa that do exhibit lactase persistence are thought to have interacted and admixed with foreign populations from the Middle East, Europe, and other pastoral communities in Africa. As a result, there is evidence of the European lactase persistence gene variant in parts of northern and central Africa, a Middle Eastern variant in eastern Africa, some variants that originated in eastern Africa and spread within eastern Africa and to southern Africa through gene flow or admixture with pastoralists (Ranciaro et al., 2014).
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Simulations have been done to recreate the circumstances under which lactase persistence spread within Europe. The parameters have been set to simulate the spread of dairy farming, competition with hunter gatherers in shifting the community niche (dairy or fishing and hunting), as well as the selective advantage of lactase persistence and the effect of lactase persisters who do not drink milk or lactase non-persisters who do drink milk (may be a result of the individual’s microbiome). The results of the simulations best matched the lactase persistence distribution seen in Europe today when lactase persistence originated in central Europe as dairy farming was just spreading into the region on an overlapping timeline (Itan et al., 2009; Gerbault et al., 2011). The simulation parameters align with what is suspected to have happened based off of the theory of gene-culture coevolution.
The research up to now has pointed to lactase persistence emerging as an adaptation that coevolved with the cultural practice of dairy farming, particularly in communities where fresh milk was consumed. This is seen in the populations of the world today where cultures that were known for dairy farming or that still dairy farm today have a higher prevalence of lactase persistence. However, further research needs to be done to pinpoint the degree to which lactase persistence is thought to be selectively advantageous is yet to be decided upon and to determine all the possible variants that can result in lactase persistence.
There are other factors that could be contributing to the increase in lactase persistence frequency that we see in today’s populations. As lactase persistence is based on a genetic mutation, we must take into account how those genes are passed on and what may be affecting how those genes are passed on. One potential confounding variable for the drastic increase in lactase persistence is transmission ratio distortion (Liebert et al., 2017). Transmission ratio distortion is when there is a disruption of the gametic or embryonic development stages of life that results in preferential transmission of parental alleles so that the distribution of parental alleles in an offspring is not 50/50 as predicted by Mendelian genetics (Huang, Labbe & Infante-Rivard, 2013). The death of an embryo, competition between sperm, which of 4 oocytes will develop into an egg, and many more aspects of reproduction that determine the success of the delivery of fertile offspring with the parent’s genes are not controlled by the lactase gene but still have an effect on whether or not a particular variant of the lactase gene is passed on. Further research into transmission ratio distortion; how, when, and why it occurs; and its relevance to lactase persistence are needed to determine just how selectively advantageous lactase persistence is.
As the production of lactase is coded for by one gene, it is more likely to be affected by a single mutation, which is what is observed as researchers continue to uncover more and more variants of the lactase persistence genes (Jeong & di Rienzo, 2014; Ranciaro et al., 2014). Many of the studies into lactase persistence were done by looking for one particular variant in a population, but with the development of new genetic technology, researching lactase persistence can now be done by looking in populations for observed lactase persistence and then finding the mutation in the gene responsible for it (Jeong & di Rienzo, 2014). That research could result in data that could provide us with a deeper insight to the development of lactase persistence in history and how it happens now as it keeps evolving.
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