In the world today, the majority of adults are lactose intolerant. In certain populations, however, the exact opposite is true. In this essay I will consider the evolutionary explanation for this.
At birth, all humans produce the enzyme lactase. Lactase hydrolyses the disaccharide lactose into galactose and glucose monosaccharides. In humans, lactase is present mainly along the brush border membrane of the enterocytes lining the villi of the small intestine. It is essential for digestive hydrolysis of lactose in milk, as lactose itself cannot be directly absorbed into the bloodstream at any point along the gastro-intestinal tract. In non dairy consuming societies, lactase production usually drops about 90% during the first four years of life (after the weaning phase is over), although the length of this period varies between individuals. This is due to the ‘switching off’ of the lactase gene. This used to be true of the human race in general, but in dairy consuming areas, a mutation in the gene regulating the ‘switching off’ of lactase production, situated on chromosome 2, has now become very common. Such a mutation is known to have arisen among an early cattle-raising people, the Funnel Beaker culture, who lived in north-central Europe around 5,000 to 6,000 years ago. This lactase-persistence allele is found in more than 90 per cent of Danes and Swedes, and 50 per cent of Spanish and French – illustrating that the mutation becomes progressively less common in Europeans who live at increasing distances from the ancient Funnel Beaker region. The mutation is rare in non-pastoral communities such as the Chinese (only 1 per cent of the population have it). In pastoral areas in East Africa, there is a very low frequency of this allele, although many adults are lactose tolerant. An international team of researchers has found that lactose tolerance in East African adults is served by three newly discovered variants of the lactase gene, all of which are independent of each other and the European strain. As in the European allele the mutation is present in the control region of the gene. These African variants appear to have arisen several thousand years later than the European allele. This fits well with archaeological evidence showing that pastoral farming groups from the north reached northern Kenya around 4,500 years ago, and southern Kenya and Tanzania about 3,300 years ago. In both Europe and Africa, the mutations have arisen after a long period of sustained pastoral lifestyle. The mutations have simply developed differently in the two different areas.
There was at first much debate as to whether the consuming of dairy produce caused the increased frequency of the lactase-persistence allele – called the culture-historical hypothesis – or if in fact that a dairy diet was introduced after the mutation became more common – the reverse cause hypothesis – (very much a chicken/egg deliberation). An analysis of DNA samples taken from skeletons of early European farmers shows that the lactase persistence allele was not present, thus supporting the culture-historical hypothesis. This means that an increase in dairy farming caused the prevalence of the lactase-persistence allele to gradually increase over the generations. This also means that the current high prevalence of the allele must be due to natural selection. In ‘The Origin of Species’, Darwin explains natural selection thus: ‘Owing to this struggle to life, any variation, however slight and from whatever course proceeding, if it be in any degree profitable to the individual of any species... will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man’s power of selection.’ In this instance, once dairy farming had developed in Europe, the increased exposure to milk (and the benefits associated with drinking milk – see later) meant that the few people who had the mutated, lactase-persistence allele were more likely to survive, therefore more likely to mate and thus more likely to pass on their alleles to their offspring. Gradually over many, many generations this has increased the allele frequency and totally reversed the ratio of lactose intolerant:tolerant in the areas of the world with a significant pastoral history.
This suggests that having a lactose tolerance is an evolutionary advantage, where milk is readily available. In areas such as China, where milk and dairy products are not widely consumed, the mutated allele remains in a very small minority. There is much speculation as to why this lactase-persistence allele is so advantageous. Milk is uncontaminated by parasites, unlike stream water, making it a safer drink. Also, if those that were intolerant of lactose tried to drink the milk, they would develop diarrhoea and vomiting – this could be lethal in difficult living conditions and they could therefore die of dehydration in the most extreme cases. Another suggestion is the benefit of having a continuous supply of milk as opposed to seasonal crops – cows will give milk all year round whereas crops can only thrive at certain times in the year. Also, milk has many nourishing properties – it is high in fat and calcium, amongst other nutrients. All in all, the ability to drink milk gave some early Europeans and East Africans a big survival advantage. As a result, 30 per cent of European adults are still lactose intolerant (i.e. do not have the lactase-persistence allele). Of these people, 24 per cent actually have secondary lactose intolerance as a result of coeliac disease. In coeliac disease, the body produces antibodies to the gluten in wheat products. This causes inflammation and damage to the villi in the small intestine, especially to the apical region (where the concentration of lactase is highest). Thus a sufferer of coeliac disease could be temporarily lactose intolerant for a few years (eventually, with a gluten free diet, the patient’s gut is able to fully heal). This could mean that the percentage of the population without a mutation for lactose persistence could be even lower than 30 per cent.
In conclusion, the evolution of lactose tolerance is a very good example of natural selection that has occurred in certain populations in different parts of the world in relatively recent times. Lactose tolerance is caused by an inherited mutation that causes lactase production to continue throughout adulthood.