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The sweet genetics of spit

Humans have an intimate relationship with food. We’ve evolved ways of consuming toxins like alcohol, ways of detecting and avoiding potentially harmful chemicals, and we’ve done all of this to make sure we have the energy and nutrients we need to survive. One of the most energy rich foods we can eat is starch. It’s in our rice, in our bread, and in our french fries. Starch is jam-packed with energy, but it’s locked away in a complex chemical structure. Undeterred, animals, bacteria, and plants have all evolved ways of liberating that energy. For us, it starts with our spit.
It’s no coincidence that many people feel like they have a sweet tooth—sugars are important molecules that our bodies recognize as being very rich in energy. When we consume foods that have sugar in them, our cells take those sugars and break them apart. In doing so, the energy from their chemical bonds is released and can be used to power other processes in our cells. This is such an important job that our cells have permanently hired a bacterium to help us with the energy extraction process (we now call that bacterium a mitochondrion). Starch is a long chain of interconnected sugars that can be difficult, but rewarding, to break down.
Digestion is usually thought of as a job for your stomach. We eat the food, and send it down to the stomach where it will be broken down and absorbed. But digestion actually starts long before that. We mechanically digest food by chewing it, and our saliva begins the arduous task of breaking chemical bonds in the food. Saliva contains multiple proteins that help in this process. The most abundant protein in saliva is the alpha-amylase protein, which has the important job of breaking down starches. This protein, and the AMY1 gene which makes it, have drawn considerable attention in the past few decades because some researchers suggest the AMY1 gene may have helped us evolve in some big ways.1

The AMY1 gene may have helped us evolve in some big ways

The AMY1 gene has ancient origins dating back more than 100 million years ago. Originally, the AMY2 gene was the only amylase gene. But a gene duplication event took place which caused there to be two copies of the AMY2 gene. Over time, this duplicate gene accumulated small changes in its DNA sequence which caused it to become its own gene. Now, we know AMY1 as salivary amylase, and AMY2 as pancreatic amylase. As their names suggest, the two genes are turned on in different parts of the body, but they both serve a function in digesting starches.1
Genetic research into the evolution of the AMY1 gene suggests that humans have undergone many more AMY1 gene duplication events over time. In fact, on average, most people now have upwards of 6 copies of the AMY1 gene in their DNA, but there is a wide amount of variability among people. The past few decades have seen extensive research looking into what effect this has on our physiology with some evidence indicating that more copies of AMY1 means more alpha-amylase activity in the saliva. This finding has drawn attention, because some hypothesize that gaining the ability to digest starch rapidly in our mouth may have given us an evolutionary advantage when it came to finding and extracting sugars from food. However, this idea has been challenged by other scientists because the amylase proteins are only part of the starch digestion process, and similar evolutionary trends are not observed with other sugar extraction genes. Nonetheless, there is considerable effort being invested in understanding how variation in the AMY1 gene may be affecting our diets, sugar seeking behaviors, and our ability to metabolise sugars.1
It’s likely that we’ll see more news about the AMY1 gene in the coming years as technology advances, making it easier for researchers to expand their studies while still accurately analyzing these genes. It will be interesting to see what comes of this research, but in the meantime, there’s plenty waiting to be discovered in our DNA already!

1Fernández, Catalina I., and Andrea S. Wiley. “Rethinking the Starch Digestion Hypothesis for AMY1 Copy Number Variation in Humans.” American Journal of Physical Anthropology, vol. 163, no. 4, Jan. 2017, pp. 645–657., doi:10.1002/ajpa.23237.