As a biology teacher who is a passionate proponent of teaching evolution as the backbone of my classes, I constantly get bombarded by students with tons of evolution questions. I do a lot of work correcting their misconceptions and clarifying some of the more nuanced aspects of evolutionary theory.
YES, evolution IS a theory and here’s why that is a GOOD thing.
Perhaps my favorite question from middle-school minds about evolution comes after we have studied the “Stones and Bones” of human origins. Each year, without fail, my students ask “Are humans still evolving?”
I answer with an unequivocal YES, and that leads to discussions of what are the selective pressures that might have influenced human survival over the past several thousand years. The main answers my students usually supply are a treasure trove of advancements that are all tied to improved tool use and the growth of human collective intelligence. As both of these are tied to healthy brains, I usually steer the conversation to feeding that brain, which demands more energy than any other single organ.
This focus on nutrition winds around to what is the most nutritious food that would have been available to modern humans over the past five or ten thousand years. We finally come to the conclusion that milk is a well-balanced food that provides all three critical macromolecules—protein, carbohydrates, and lipids—as well as important vitamins and minerals. Without fail, our discussion of milk as a food leads someone in class to point out that someone they know is lactose intolerant. This “revelation” allows me to introduce a great example of evolution in modern human populations!
To set the stage, we cover the fact that other than humans, no mammal species digests milk after they are weaned. Even domestic cats are lactose intolerant despite what you may have heard! Learn more with this great Q&A resource.
The reason for this inability to digest milk comes from the fact that mammals stop producing the enzyme lactase once mother’s milk becomes unavailable. For a quick overview of how lactase breaks up lactose into two simple sugars (galactose and glucose), take a minute to see how my friends the Amoeba Sisters explain it in their video about enzymes.
Once the undigested lactose sugar passes the small intestine (where glucose and galactose are usually absorbed), it makes it to the colon (large intestine), where it feeds gut bacteria. This makes those bacteria produce waste products (including lots of carbon dioxide), which cause the symptoms of lactose intolerance—gassiness, cramping, and diarrhea.
So what does this have to do with evolution?
If humans are the only mammals that can digest lactose, how come not all of them do? It turns out that approximately only 30% of adults worldwide can digest lactose, but this ability is distributed unequally across the globe.
As you can see from the map above, there are specific geographic areas where native populations are lactose tolerant (also called lactase persistent): Western Europe, the Middle East, the Indus Valley, and parts of both East and West Africa.
It turns out the areas where high percentages of people are able to digest milk are also the areas where archaeological evidence shows populations began to raise livestock and utilize milk as a food source between five and ten thousand years ago. From the perspective of natural selection, any biological change that allowed lactose intolerant humans to digest lactose in these areas would have a survival advantage which would then be passed down to future generations.
This survival advantage is being studied, and hypotheses focus on three major benefits of lactose tolerance.
- First is the obvious benefit of having access to a nutrient-rich food in addition to usual dietary items.
- Second, this slight benefit would be highlighted in times of seasonal scarcity such as between harvests of staple crops.
- Third, such a benefit would be highly significant in the event of famine, where the ability to utilize milk as a primary food source without the dehydrating and life-threatening effects of diarrhea would have promoted lactose tolerance throughout the surviving population.
Scientists are also looking at the possibility that lactose tolerance would have been beneficial to populations in Northern Europe as a vitamin D boost during winter, when lower levels of sunlight would make it harder for people to get adequate vitamin D from routine sun exposure.
These advantages suggest how the ability to digest milk might have become common in some populations where dairying existed, but how did we go from being mammals who are lactose intolerant to possessing the ability to successfully digest milk past toddlerhood? This is where we connect genetics to a study of the geography of lactose tolerance.
Studies of the gene that produces lactase in different populations show no differences between lactose-tolerant and lactose-intolerant populations, and that puzzled scientists. Upon deeper investigation, however, a series of mutations in the gene “switches” that turn the lactase gene on and off were discovered. These mutations wound up being different in European and African populations, which suggests they arose and were preserved when different populations adopted a dairy culture. Scientists refer to this combination of genetics and culture coming together as “Gene-Culture Coevolution.”
I always look forward to this example with my students because it allows us to tackle a great question in a truly multidisciplinary way. In answering this query, we encounter the following topics:
- genetics and gene control
- biochemistry and nutrition
- evolution and natural selection
- anatomy and physiology of the human digestive system
- human migrations and cultural change
Being able to tie these various threads together is a powerful model of how our students can appreciate that “science” is not a mere collection of facts to be regurgitated on a test, but rather a web of facts that are connected in meaningful ways across the curriculum in both time and space.
One of the great resources I recommend for teaching this topic is from the Howard Hughes Medical Institute’s BIOINTERACTIVE wing. Their work on “The Making of the Fittest: Got Lactase? The Co-evolution of Genes and Culture” includes a wide range of superb resources based around this short film narrated by geneticist Spencer Wells (@spwells).
Other resources that can allow you to dig deeper are:
- Sarah Tishkoff- Adaptation to Taste and Lactose Intolerance in Africa – https://www.youtube.com/watch?v=ZAJ9s2ePYHY
- How Did Ancient Societies Adapt to Dairy Consumption – https://dailyhistory.org/How_Did_Ancient_Societies_Adapt_to_Dairy_Consumption%3F
- How Milk Goes Down Around the World – http://theplate.nationalgeographic.com/2015/05/13/how-milk-goes-down-around-the-world/
- Lactose Tolerance in East Africa Points to Recent Evolution – http://www.nytimes.com/2006/12/11/science/11evolve.html
- Archaeology: The milk revolution – https://www.nature.com/news/archaeology-the-milk-revolution-1.13471
- The Biology Corner’s experimental demo -“Can Yeast Digest Lactose?” https://www.biologycorner.com/2017/11/14/demonstration-can-yeast-digest-lactose/