How Milk Changed Your Genes, and Cooking Grew Your Brain

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.

Figure 1: Nutritional information for whole cow milk. Source: Sainsbury’s

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.

Figure 2: Map of frequency of lactase persistence. Red areas indicate high frequency, while blue areas indicate low frequency. Source: UCL

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).

From Cows, to Cooking

The way that we eat our food has greatly changed our anatomy and physiology (Carmody et al., 2016). In general, our digestive systems are weaker compared to other primates. Food processing has made many of our digestive organs, like the stomach, mouth, teeth and lips, small compared to other primates; for example, our stomach’s mass is only 60% of the expected mass for a primate which is a similar size (Aiello, 1995). Our ability to break down toxins is poor relative to other apes. The cause of such great differences is because humans process their food (Carmody, Wrangham, 2009). The act of processing food reduces the work the digestive system does, as food is effectively partly broken down when it enters the body. The earliest form of which was likely pounding meat, which effectively works like external teeth, since the meat is tenderised (Glover et al.,1977), arriving already partly broken down, hence reducing the work the jaw must do. Processes then developed through cultural evolution to create the wide repertoire available today; pickling, grinding, smoking to name but a few. Perhaps the most notable, however, is cooking. Cooking plays many roles, not only does it break down food but it also increases the energy available from it (Groopman et al., 2015; Carmody et al., 2011) and gives us more free time (we spend less time chewing for example). What these mean is that there is a selective pressure for smaller digestive organs. Digestive tissue is very energetically expensive; many calories are required to maintain them (Aiello, 1995). So, having unnecessarily large organs which aren’t being used is not efficient. As we have seen, there is evidence that the change in genes necessary for this reduction has occurred in humans. This reduction in expensive digestive tissue paves the way for another form of expensive tissue, the brain. The savings derived from cooking are used to create larger brains, explaining why our nearest evolutionary relative, chimps, have a brain only one third the size of ours. This creates the basis of the expensive tissue hypothesis. No other animal cooks its food, and it is this which is argued to have unlocked the energy required to have the large brains we have today (Fonseca-Azevedo, Herculano-Houzel, 2012). The powerful effect that cooking has had is seen in the graph below, rapid brain growth has been observed since the advent of cooking.

Figure 3: Graph of growth in brain size, the highlighted area is where cooking took place in the Homo lineage (Wrangham, 2006Source

We can see that interactions between our culture, in this case the food we eat and the way that we eat it, has had huge effects on many aspects of our body. In the case of cooking, many believe that it has provided us with the brains that define our species. In fact, studies have shown that we can get a multitude of health problems if we don’t eat cooked food (Koebnick et al., 1999; Fontana et al., 2005). This may lead us to ask, how will our culture affect our genes in the future? Or even today? It could be that the use of robotics on humans, like prosthetics, will change our genes in the future. Could our cultural tendencies for high consumption of high sugar foods push our genes to make us better equip to deal with higher sugar intakes? It is almost impossible to predict, but even today, genes and culture are continually interacting, shaping the humans of tomorrow.

Figure 4: TED talk by Suzana Herculano-Houzel on why the human brain is so special

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