I had milk on my breakfast cereal this morning, and I am writing a new lecture on evolution for my biological physics course. The connection between these two facts is the lactase enzyme. Baby mammals of all mammal species drink their mothers’ milk, which contains a lot of its calories in the form of the sugar lactose. The baby mammals produce the lactase enzyme in their guts to digest the lactose. But when they grow up they no longer drink milk, and as lactose is rare except in milk, the growing mammals stop producing the lactase enzyme.
Making this protein would simply waste resources if there is no lactose in their diet, which is true for virtually all adult mammals. But around 10,000 years ago some populations of humans domesticated some of our fellow mammals, such as cows and goats. The adult humans could then milk these cows and goats and so milk was now available to adult mammals.
So, several human populations were now farmers, with cows, goats etc. In or around Europe, a child born to one of these farmers, with a mutated lactase gene that was active even when the child grew to be an adult. The descendants of this child spread through the population, carrying the mutated gene, drinking milk, and out-competing the less well nourished non-milk-digesting weaklings without this mutation.
As a result, there is one mutation that promotes lactase production in adults that is common in people of European descent, i.e., people who if you go back a few thousand years, most of whose ancestors were in Europe. It is quite likely that I have this mutation, as I drink milk quite happily.
A couple of different mutations that have similar effects, arose in eastern Africa, and again spread through eastern African populations, presumably for the same reason as that which drove the success of the European mutation.
Of course, I have known we are the product of evolution for all my adult life, but a specific example of how evolution has shaped us, really brings it home. I think this is especially true when it is only thousands not millions of years in the past. I hope my students will find it as interesting as I do.