What’s in your tea, worth £4.6 billion per year in the UK, and comes from a different species?
Milk consumption is a strange phenomenon. We are quite happy to drink milk from a cow, a completely different species, intended for its own babies. Perhaps more strange is how our preparation of food has allowed us to develop the huge brains we possess. These two, pertinent, but everyday, phenomenon are examples of how culture can influence your genes at a rapid rate in a process called gene-culture coevolution. We uniquely possess two lines of inheritance, cultural and genetic, which both interact with each other.
A quick glance on the label of a bottle of milk tells you why drinking it would be a good idea. High in fats, proteins (6.7 grams in a serving), sugars and calcium (30% of your recommended intake for the day in one serving); it’s a nutritional goldmine, so being able to drink it seems to be very advantageous. Yet, our ability to drink milk is surprisingly unique and recent. The ability to digest milk comes from the enzyme lactase, which breaks down lactose in milk. In our evolutionary history, only babies could digest milk (Malmström et al., 2010). From birth, babies are equipped to break down the contents of milk from their mother but, as they grow older, their ability to digest milk declines as production of lactase decreases (Swallow, 2003). This is seen not only in humans but all mammals after weaning (Gerbault et al., 2011). However, some humans can continue to produce lactase enzymes into adulthood and consequently retain the ability to digest milk, this is known as lactase persistence. This is not particularly common for much of the world today as seen in the graph below, where blue areas indicate low frequencies of lactase persistence.
How did lactase persistence come about in these red areas? This can be explained by gene-culture coevolution. In order for milk to have a selective advantage, there has to be milk available in the first place. This arose with the development of domesticating animals. While it may not have been the original intention, having animals like cows and sheep around meant that there was milk to drink when an adult and thus the selection for genes to digest it was started (Henrich, 2016). Those that had the mutated gene for lactase persistence were at a strong selective advantage (i.e. they are more likely to survive and reproduce) since a source of high nutrition was readily available. Hence, this gene would have spread quickly in these areas with domesticated animals, leading to the dark red areas on the map where lactase persistence is high. The strength is shown by a near 100% lactase persistence in northern Europe despite the allele for lactase persistence (called ‘-13,910*T’), arising only 7,500 years ago (Itan et al., 2009), highlighting how powerful culture can be on genes. In brief, areas with lactase persistence correlates to areas which have culturally developed dairying. Blue areas, like China, did not develop domesticated animals and as such there was no selective pressure for the ability to digest milk as an adult (Gernet, 1962). However, some areas with lower frequencies of lactase persistence had developed milking, but did so along with cheese and yoghurt production, culturally evolved practices which removes nearly all the lactose. Therefore, the nutritional bounty milk provides can be accessed without the need to process lactose and hence the selective pressure for persistence is reduced, giving lower frequencies of lactase persistence in those areas (Henrich, 2016). The red areas developed milking before developing yoghurt and cheese production which meant that to get the benefits from milk, you had to be able to digest lactose. The different areas of red on the map had different reasons for lactase persistence. In northern Europe, the strong selection pressure may have arisen from the need for the vitamin D in milk, since low levels can be obtained from the sun. In contrast, part of the selective advantage for persistence in places like west Africa and the Middle East may have arisen from the fact that milk was a source of water in an environment where water can be hard to come by (Henrich, 2016).