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The Evolution of Lactose Tolerance in Humans

Carlotta Ceccarelli

24th May 2023

The primary means of nutrition for an infant human is through breastfeeding, a tailored supply of milk with a mixture of properties that promote infant growth. Mammalian milk mostly contains lactose sugar, the main source of energy for developing babies (Boyd, 2021). Lactose is only found in mammalian milk, constituting its modern implications in ‘dairy products’. Innately, mammals such as ourselves are born with the ability to digest and assimilate lactose but lose this once weaned. This is due to the synthesis of lactase-phlorizin hydrolase (LPH) enzymes ending, caused by the ceased exposure to lactose in fresh milk. Without LPH, lactose can’t be hydrolysed into glucose and galactose, its corresponding monosaccharides that are able to cross the epithelial cell membrane and provide energy (Gerbault et al., 2011). Instead of lactose being cleaved, it remains in the colon to ferment with bacteria, producing gases that arise to the symptoms of bloating, flatulence and abdominal cramps (Gerbault et al., 2017). It is thought that humans evolved to follow this pattern of LDH down-regulation; because lactose is only present in milk and exposure is not supposed to occur past weaning, making it energetically unfavourable for the body to continue synthesising LPH. The origins of our modern diet containing dairy aside from breastfeeding will be analysed, specifically how populations in northern Europe and western Africa have been able to convergently evolve lactose tolerance despite its maladaptive effects.

The Neolithic period has been marked as when dairy products became complementary to the human diet, the transition in which hunter-gatherers became farmers, resulting in a more settled lifestyle (Gerbault et al., 2013). Humans that were once seeking food in 8400 BP began producing it 2000 years later, prompting an environmental change. Due to this, fresh milk has been determined as a bridging food between the dietary compositions of hunter-gatherers and modern humans. Neolithic culture domesticated animals such as sheep and cattle, as shown by evidence of lipid data from residual milk fat found in pottery. The domestic animals found had a ‘kill-off’ profile, which meant their age and sex were indicative of females being kept alive and offspring being slaughtered, which is typical in early dairy practice (Gerbault et al., 2017; Zervos, 1962). Consequently, for pastoral neolithic who kept their livestock and did not farm, being able to digest lactose was advantageous. Mammalian milk is highly caloric and a substantial source of fat and protein, whereas the milk of a prehistoric cow is comparable to its meat for energy yield (Gerbault et al., 2011). Despite its properties, lactose tolerance has not evolved due to the use of milk after breastfeeding, it was naturally selected by the struggles of survival. Incorporating milk into the diet was a form of protection against the risks of seasonal crop failures and disasters. In environments where lactose is more significant in the adult human diet such as pastoralists, better health and reproductive vigour were established by increased LPH activity, which evidently prevailed in genetic pools (Simoons, 1970).

Figure 1

Neolithic archaeological sites where lipid residue analysis was performed on discovered potsherds (Gerbault et al., 2013).

How the practice of dairying originated and diffused in different regions is essential in determining the question of why lactose was consumed. Lactose tolerance is a trait recognised to have evolved independently in both Africa and Europe, with the same selective advantage of being able to synthesise LDH resulting in convergent molecular evolution. In each instance, a single nucleotide polymorphism (SNP) occurs in a non-coding region close to the gene that controls the LPH enzyme called LCT, located on the enterocyte cells of the intestinal lining. The mutation of a single nucleotide from cytosine (C) to thymine (T) is associated with the mutant allele T-13910 in Asians and Europeans, enabling individuals to tolerate lactose past the point of weaning due to the prolongation of LDH synthesis. In the African domain, traces of up to 4 similar mutations in the genome close to the LCT gene have been discovered (Boyd, 2021; Simoons, 1970). It is thought that variation in cultural processes is what led to respective human selective environments, where exposures that gave rise to mutations are what determined which genotypes were inherently able to survive and reproduce. However, not all societies that traditionally incorporate dairy in their diets 'adapted'. Techniques have been used by Indian sectors of Punjab and Pakistan to utilise dairy products while keeping lactose levels negligent, such as heat processing or using specific lactic acid bacteria or yeasts to produce sour milk products such as ghee, lassi and paneer. High lactose-intolerant populations such as these are able to reap the nutritional benefits of protein and fat in fresh milk, which confutes the idea that lactose has a specific subsistent advantage (Flatz and Rotthauwe, 1973; Ahmad and Flatz, 1984). This casts doubts on whether African and European populations were able to culturally adapt or if these techniques were simply developed for longer-lasting dairy preservation.

The geographic distribution of lactose tolerances today can be explained in cultural and historical contexts such as the calcium-assimilation hypothesis, arguing that lactose tolerance was naturally selected for the support of bone health. Strong selective pressures were found in regions of low-sunlight latitudes, where dairy was able to provide vitamin D for the assimilation of calcium (Gerbault et al., 2011). Many populations rely on ultraviolet (UV) radiations for the photo-conversion of 7-dehydrocholesterol to cholecalciferol (vitamin D3) in the skin, however, northern Europe did not have sufficient irradiation to obtain the required amount photochemically. On this account, depigmentation in the skin, hair and iris of individuals in these populations favoured the genes that naturally produced cholecalciferol in the skin as an advantage against the progression of vitamin D deficiencies. Yet because agriculturalists of these areas had a fibre-rich diet from cereals, the naturally occurring amount of vitamin D was still insufficient, unlike the European hunter-gatherers that collected marine foods. Lactose-tolerant individuals had an upper hand, being able to digest lactose and ingest larger quantities of fresh milk meant better calcium assimilation (Gerbault et al., 2017). Vitamin D deficiencies are known to result in severe deformities including rickets and osteomalacia, which likely impacted health and life expectancy, constituting lower reproductive rates in certain groups. This idea is supported by adult rickets being linked to the cause of deaths in archaeological Viking records found in Greenland (Flatz and Rotthauwe, 1973). The importance of attributing the calcium-assimilation hypothesis as a factor of cultural-historical changes is reinforced when looking at other European regions such as Iberia, which has sufficient daily UVB exposure for vitamin D synthesis yet lactose tolerance is high. Genetic surfing was likely the reason for genetic variants drifting in regions far from where they mutated, especially in the circumstances of population expansion (Gerbault et al., 2017).

Environmental pressures that may have coerced the natural selection of lactose tolerance in Africa include extreme conditions such as droughts, famines and epidemics. From approximately 5500-2000 BC is the earliest piece of evidence that dairying occurred in the Sahara, an illustration of a cow being milked. Many populations of African pastoralists today are said to have descended from this group, however not encompassing all of Africa due to ‘sleeping sickness’ killing domestic animals from disease (Simoons, 1970). When water became seasonally scarce, fresh milk was used by pastoralists for rehydration. To lactose intolerant individuals, fresh milk served the opposite purpose; there was the potential of experiencing diarrhoea which could exacerbate their dehydrated states. The absence of LDH meant that lactose reached the colon to be fermented by bacteria microbes, where a byproduct of short-chain fatty acids is produced alongside gas such as acetate, butyrate and propionate. These components are not absorbed by the body and therefore remain in the intestinal lumen, increasing osmotic pressure and triggering a water influx in the bowels known as diarrhoea. In these conditions, lactose intolerant individuals are selectively disadvantaged by not having access to the nutritional benefits of fresh milk as well as liquid amidst a drought (Cook and al-Torki, 1975). Although these ideas are able to explain adaptations taking place in arid climates, it does not explain why domesticated animal milk was utilised; the depletion of milk supply caused by extreme climate would impact both livestock and humans alike.

Figure 2

Distribution of lactase tolerance is present mainly in northern Europe and western Africa, data was gathered in the dot regions with units representing the fraction of lactose tolerance (Boyd, 2021).

It is clear that lactose tolerance is a prime example of gene-culture coevolution. Strong selective pressures of animal domestication shared by multiple populations can correspond with the convergent data collected. Genotypes that have been designated in history as maladaptive are just adaptations from our environmental changes, which leads to the belief that humans will continue to disperse and combine genetically. Although selective pressures such as water scarcity and deadly deficiencies may not be as relevant in the future, the autosomal dominant mutation of lactose tolerance in high LPH-resisting populations has the possibility to spread (Tishkoff et al., 2007). This would be much like the mestizo populations of Latin America, which have lower lactose intolerance due to the interbreeding of indigenous Americans with lactose-adapted European groups (Simoons, 1970). Lactose tolerance distributions remain undetermined in many areas, with a rise in alternative milk sources and research, future lactose adaptations remain equivocal too.



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