5.4.3 Natural Selection and Human Variation: Are Humans Still Evolving?

Human variability is attributed to a combination of environmental and genetic factors, including social status, ethnicity, age, nutrition, quality of life, access to healthcare, work and occupation, etc. As mentioned in Chapter 1, anthropology contributes many insights into both the social construct of race and the impacts racial categories have on people’s lives. The focus in this chapter is the role of natural selection in human variation.

A number of changes are associated with the Neolithic era and the rise of agriculture around 10,000 to 8,000 years ago. Many have noted that changes during this time period did not have positive effects on human and environmental health. The evolutionary mismatch hypothesis proposes that our bodies are best suited to the environments we have spent much of our evolutionary history in, which are very different from the environments we inhabit today (Li, van Vugt, and Colarelli 2018).

Humans evolved for one million years as hunter-gatherers. Today, human bodies are still trying to adapt to the largely grain-based diet brought about by agriculture, a diet characterized by less diversity and lower levels of nutrition than that of a typical hunter-gatherer. Incomplete adaptation to this change has made people susceptible to a number of diseases and nutritional deficiencies. Lactose intolerance is a prime example. The domestication of cattle and the drinking of cow’s milk began during the agricultural age, not very long ago in evolutionary history. Currently 65 percent of humans are unable to digest cow’s milk. Dental caries (cavities) are another problem linked to the change in diet associated with agriculture. The grain-based and high-sugar diets associated with agriculture are very different from the diet of hunter-gatherers. Neither our bodies nor the bacteria in our mouths have had time to fully adapt to this change.

Another adaptation that took place during the Neolithic era is related to variation in skin pigmentation. Humans who left Africa and settled in Europe about 40,000 years most likely had dark skin with high levels of melanin, which provides protection against ultraviolet radiation New data confirms that about 8,500 years ago, early hunter-gatherers in Spain, Luxembourg, and Hungary also had darker skin. Skin pigmentation is an adaptation to ultraviolet radiation, with different tones offering different advantages, depending on one’s distance from the equator. As humans migrated to the Northern Hemisphere, they were exposed to less ultraviolet radiation, which also meant less absorption of the Vitamin D needed for strong bones and other important immune functions. In order to compensate for this loss and to allow for greater exposure to ultraviolet radiation, skin pigmentation became lighter.

Another example of human variation as a result of adaptation to the environment can be seen in Indigenous populations in the Andes, Tibet, and the Ethiopian highlands. Each of these three groups faces the same environmental challenge, living in a low-oxygen environment, and they have responded with unique adaptations. Tibetans compensate for low oxygen levels by taking more breaths per minute than people who live at sea level. Those living at high altitudes in the Andes have been found to have higher concentrations of hemoglobin in their blood than other people. Ethiopians living at altitudes of 9,800 to 11,580 feet have neither of these adaptations. The explanation as to how the Ethiopian highlanders thrive in their environment is still a mystery.

This chapter has explored just some of the immense biological and cultural diversity of the genus Homo. This diversity has emerged in response to highly complex and variable environments connected to factors such as exposure to UV radiation, low oxygen levels at high altitude, changes in diet as a result of hunting or agricultural practices, geographic isolation in island populations, and climate variability and temperature. The genus Homo has proven to be resilient and adaptive in response to whatever environment or challenge it has faced. Variation is the key to survival. While scientists recognize that biological and cultural variation has greatly contributed to our human evolution, the human species is now facing a moment in which we must contemplate a difficult question: To what extent has our success as a species jeopardized the survival of other species and the health of the planet we all call home?