August 2, 2014 by comparativelypsyched
Got lactose tolerance?
Milk has long been extolled as a great source of calcium, protein and vitamin A, and is considered so crucial to early development that since the 1940s children in the UK have been able to receive one government-funded milk carton each day. It may be surprising, then, to learn that the majority of the world’s adult population are unable to drink the recommended three daily glasses. Those able to consume milk have an enzyme that can break down the sugars present in the drink (lactose), and while a large proportion of North American and European populations possess this important enzyme, in large parts of Asia, Africa, and South America, milk cannot be consumed without suffering a number of unpleasant digestive ailments. Interestingly, the ability to digest lactose has something in common with an immunity some individuals have to malaria.
The symptoms of malaria are significantly more serious than those induced by lactose intolerance, and include fever, headache, sweats and nausea. The illness is caused by a parasite transmitted by mosquitoes and while preventative vaccines exist, malaria kills over half a million people each year. Some people, however, are immune to the parasite that causes malaria. The basis of malaria immunity and lactose tolerance is genetic, and the root of these adaptations is quite unique. There are many genes that protect against disease and infection, but genes for lactose tolerance and malaria immunity share a particular origin. Not an ancestor, a tribe or continent. These genes have been favoured by culture. This is amazing because as far as we know, we are the only species whose cultural practices have influenced biological evolution. We have broken with a tradition of almost four million years.
Three and a half billion years of tradition
Every living thing on this planet is the result of evolution by natural selection. Natural selection works by favouring certain traits above others. A trait may be physical (the enamel in our teeth or lenses in our eyes) or behavioural (our predisposition as children to quickly pick up languages), and are transmitted through genes.
Natural selection is creative; sometimes a gene changes, it mutates, and that mutation can alter an existing trait or create a new one. If favoured, this trait may be passed to the next generation and possibly (over time) spread throughout a population. To be favoured a trait must bestow some benefit to the individual that possesses it. This benefit usually derives from improving the individual’s chance of surviving and reproducing. An individual that does not reproduce won’t pass on a trait and no animal alive today had an ancestor that did not leave offspring. Here is a hypothetical example of how natural selection might work:
A genetic mutation has provided a prehistoric human with better eyesight than other hunters. Improved eyesight allows greater accuracy when hunting and improves the chance of spotting and avoiding a predator. Improved eyesight will favour this hunter’s ability to survive until he can reproduce, and his offspring, by inheriting the same trait, should benefit too. Of course, if food was so plentiful and predators so scarce that improved eyesight afforded no advantage, the trait would be no more likely to spread than any other. However, few environments are so forgiving.
No species evolves in a vacuum and the environment in which they live has a massive influence. A trait that is favoured in one environment may be harmful in another. Gills are great adaptations for living in water but are not so good on land.
The basic process of natural selection has shaped all life on earth. At a point in our evolutionary past our environment favoured individuals who walked upright, who could communicate through speech and use tools. Natural selection also favoured individuals who had culture.
There are an almost infinite number of ways in which an environment can shape the genetic makeup of a species and the result is the diversity we see around us in nature. However, humans have a unique talent for altering our environment. The human species has spread to all corners of the earth, comfortably inhabiting Arctic wasteland and parched desert. Culture, the communication of ideas and skills, is what enables this diversity and defines us as a species.
Learning skills by observing others, teaching our group-mates, and communication through language facilitate the detailed transfer of information, allowing us to pass on traditions we once learnt ourselves. We are able to build on the achievements of past generations, enhancing our knowledge in a cumulative way. Genes are nature’s way of transmitting information, but through culture humans have found a second method. And culture evolves fast. Consider how different your life is to a human living two or three centuries ago- the blink of an eye in biological terms. In our short time on this planet, humans have arguably become the most successful species. All because of culture.
This pathway of dual inheritance is unique to humans and recent research suggests that these pathways interact in fascinating ways. To examine this relationship we will revisit milk and malaria.
The gene for lactose tolerance has evolved quite recently (in the last 300 generations). Like other traits, a gene for lactose tolerance must have been adaptive, but what environmental factors could have led to lactose tolerance being favoured? One theory is that as humans spread throughout the globe, leaving warmer climates behind, a lack of sunlight left us deficient in certain minerals and vitamins. Milk may have been a way of overcoming this loss.
Another hypothesis posits that lactose-tolerant genes started spreading after human cultures began farming. In an environment where dairy-farming was already being practised (e.g. for the production of products that contain lower amounts of lactose like yoghurt and cheese), a tolerance to lactose was beneficial. Populations with histories of dairy-farming consistently contain a higher proportion of lactose-tolerant members, while populations without this history do not, even when these populations exist near dairy-farming cultures. A similar effect may have led to malaria immunity.
Yams, or sweet potatoes, are a common food source in Africa and other parts of the world, but the highest level of farming occurred in West Africa. To facilitate the cultivation of the vegetable large amounts of forest were cut down leading to an increase in the presence of stagnant water where mosquitoes lay their eggs. Professors Michael J. O’ Brien and Kevin Laland believe that the rapid increase of deaths due to malaria, driven by agricultural practices, led to immunity being strongly favoured. By creating an environment that benefited mosquitoes, humans inadvertently altered their environment enough to influence the genetics of entire populations.
These examples are in a way not extraordinary. This is, after all, how natural selection works. But in both cases, genetic change was influenced by human cultural practises. And it does not end there. Some geneticists think that up to two thousand human genes could have been selected for in recent times; recent enough to have been influenced by human culture. Laland and his colleagues think it is not only genes for immunity and digestion that have been affected. Greater tolerance to varied temperatures, improvements to skeletal and muscle structures, could all be traits that were favoured by culture.
Culture has long been recognised as a source of our beliefs and traditions. We are only now learning how significant culture may have been in our evolutionary pasts. Further research will focus on the extent to which culture has influenced our genetic makeup and may identify ways in which culture is still altering our evolutionary path. We have long studied culture’s impact on behaviour, and evolutionary psychology focuses on the constraints our genes place on cultural influences. The study of gene-culture co-evolution presents a new perspective and shows us that there is plenty yet to learn about how culture, the way we are nurtured, may be affecting our nature.
O’Brien, M. J., & Laland, K. N. (2012). Genes, Culture, and Agriculture. Current Anthropology, 53(4), 434–470. doi:10.1086/666585
Laland, K. N., Odling-Smee, J., & Myles, S. (2010). How culture shaped the human genome: bringing genetics and the human sciences together. Nature Reviews. Genetics, 11(2), 137–48. doi:10.1038/nrg2734
Laland, K. N. & Brown, G. R. (2002) Sense and nonsense. Evolutionary perspectives on human behaviour. Oxford University Press: Oxford.