Possibly contrary to what you’d expect, drinking milk is only a very recent development in our evolution. Despite it being completely natural for the young of animals to drink their mother’s milk, after a period of weening these animals will never drink milk again, and that used to be the same for us. But why is this the case?
Milk from animals such as cows and humans (but not monotremes like the duck-billed platypus, or the pinnidae (saltwater clams called pen shells)) contains the sugar lactose. In order to digest lactose, we need the enzyme lactase, which breaks the beta-glycosidic bond between glucose and galactose. Without this enzyme the milk passes through the body largely undigested, and the sugar content then has an osmotic effect in the colon, so instead of water being absorbed, water travels by osmosis into the colon, making the contents more liquified. The lactase is also repired by bacteria in the intesetines, producind hydrogen gas. This is how it can be discovered if you have the enzyme lactase in your body – if the amount of hydorgen in you breath is above that in the atmosphere it means bacteria are respiring lactose, and therefore you don’t produce any lactase. Even though we are capable of producing lactase at birth, the production of this enzyme decreases rapidly, and is essentially non-existant by the time we reach adulthood. Is this true for all of us thhough?
Lactase persistant (where lactase enzyme production does not decline with age) is common in europe – for example in Ireland over 60% of the population have this ability, yet in areas such as asia it’s very uncommon, except umong pastoralists.
The cause of lactase persistance? It’s thought that a mutation of a gene neighbouring the lactase gene, -13,910*T, increases the chances f developing lactase persistance. To determine when this mutation first occurrerd, microsatellites were analysed. As they mutate fast, the greater their variation, the longer the gene has existed in the mutated form. From this information a link was made between the mutation and the domestication of livestock. Until this point, it would have been hard for our hunter-gatherer ancestors to collect milk. This theory was then backed up by archaeological evidence showing the existance of fatty acids specific to animal milk such as goat’s milk on pottery from the neolithic period. However the study of DNA from neolithic bones found no mutation of this gene.
The distribution of the gene mutations linked to lactase are also somewhat of a mystery as it is much higher in north-west europe despite the fist farmers, including members of the Linearbandkeramik culture, being from much further south. Furthermore, the first mutation occured only about 10,000 years ago, yet the mutation has spread rapidly, so instead of genetic drift, extremely strong natural selection of between 10 and 15% has occured – an asonishingly high rate. This is were the calcium assimilation hypothesis comes in – it suggests that in high latitude regions there is not enough sunlight to get the daily vitamin D requirement and the vitamin D in milk supplemented this defficiency. Drinking milk would also be a much safer, regular soucre of fluids and proteins, in much the same way as beer can be thought of as liquid bread.
All this is very well, but it is not always true that where a gene is most abundant is where it’s origin lies. Computer simmulations have suggested allele surfing is the cause of the gene’s unusual distribution, which began in central europe (it’s not known exactly where). The way a population spreads can effect the distribution of genes and is a gene appears near the edge of a wave of population expansion, the gene can essentially surf on the population expansion and get pushed far away from it’s origin.
I hope you’ve found this interesting, and I’d like to thank Mark Thomas for his recent lecture at the London IOE on the evolution and origins of milk drinking, which inspired me to write this post. To find out more about his research in this area click here.