Opinion: Uncovering evolution’s footprint in the human body

Written by Daniel Sprockett Monday, 11 April 2011 17:47

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It seems like every few months there is another story that makes its rounds through the media about how humans have stopped evolving. For example, in a recent interview for the website BigThink, physicist Michio Kaku made the ridiculous claim that since saber-tooth cats are no longer a major threat to human health, humans have stopped evolving. Kaku has made some very important contributions to physics and is credited with being a co-founder of string field theory. He has authored multiple science books, starred in movies, radio and TV science programs. He is undoubtedly a great science popularizer — but in this case he is dead wrong. Although the way natural selection shapes human biology has changed radically since the advent of modern medicine, there is still a mountain of evidence suggesting humans are still evolving in many diverse ways.

One great example of recent human evolution deals with our ability to digest milk. Today, around 90 percent of Americans have the ability to break down lactose, a sugar found in milk and other dairy products. Conversely, nearly 99 percent of Chinese people are lactose intolerant — resulting in stomach pains, flatulence and diarrhea when they drink milk or eat cheese.

A person’s ability to handle dairy products stems from our ability to produce the enzyme lactase, which breaks down lactose. All mammals — including humans — feed their offspring protein-rich milk while they are young. However, most mammals lose the ability to breakdown lactose shortly after they are weaned off their mother’s breast milk.

In many agrarian societies, the ability to drink milk throughout adulthood gives individuals a substantial nutritional advantage over their lactose-intolerant neighbors. Dairy products are a valuable source of calories when food is limited, and milk can be an alternate source of water during periods of drought. Individuals who were best able exploit this nutritious resource produced more children who were also able to digest lactose into adulthood, and the trait quickly swept through the population by way of natural selection.

Archaeological evidence, along with analyses of bovine genetics, show that humans began domesticating cattle sometime within the last 8,000-10,000 years. In fact, multiple lines of evidence suggest that two or three separate groups of humans began breeding and domesticating cattle completely independent of each other. Populations in Sub-Saharan Africa and the Fertile Crescent region of the Middle East were the first to do so, and it later spread to parts of Europe.

Human genetic studies done across Asia, Europe and Africa have shown conclusively that shortly after we began domesticating cattle, random mutations occurred in our genome that expanded the amount of time that our lactase enzyme is active. These mutations were probably occurring even earlier, but they didn’t convey any sort of advantage until after the agricultural revolution and the advent of dairy farming. In fact, we know that lactase persistence arose at least three separate times among geographically isolated agricultural societies, each time caused by a mutation in a different area of our genome.

Lactase persistence and dairy cattle domestication is a great example of recent human gene-culture co-evolution. Humans have created our own unique niche in the world, but our environment continues to shape us nonetheless. Despite our advances in nutrition, hygiene and medicine, humans have not yet liberated ourselves from evolution, and I doubt we ever will.