Why Human Evolution Is a Fact
Despite religious controversy in the U.S., the science is settled.
Posted Jul 30, 2019
The U.S. is the only developed country where controversy continues about teaching evolution in schools. Yet, there is little doubt that evolution happens, or that humans are products of evolution by natural selection.
Evolution Is a No-Brainer
While evolutionary theory engendered a great deal of controversy from a theological perspective, it is not inherently questionable. Indeed, it is self-evidently true in the same way as Pythagoras's theorem is. The underlying logic is flawless and inescapable.
If one accepts that animal breeders can modify target features of their subjects, then the only additional claim of evolutionary theory is that the natural environment functions like an animal breeder.
Darwin was a pigeon fancier and knew that breeders could select some trait, such as unusually long neck feathers and selectively breed individuals with this trait so as to exaggerate it yielding the many varieties of different pigeons available then and now.
Selecting for inherited traits is the basis of all animal breeding and there is little question that the natural environment selectively breeds certain types. For example, animals living in cold climates have exceptionally warm fur. Human populations in cold climates, such as the Inuit, have unusually short stature compared to people living at the equator as a mechanism of conserving heat.
Humans Are a Product of Evolution by Natural Selection
Evolutionary biologist Theodosius Dhobzanski concluded that nothing in biology makes sense except in the light of evolution. If so, then it follows that humans are products of evolution just as other species are.
In practical terms, this means that, like other species, humans respond adaptively to differing environments and changing ways of life. Examples of such adaptation are many and varied. As people made more sophisticated stone tools, they developed handedness. This meant holding a stone core in the left hand and working on it with a hammer stone wielded in the right. Consequently, the brain developed lateralized function (1).
As tools became more sophisticated, manual dexterity and fine motor control improved. This phenomenon illustrates how changes in behavior alter selection pressures leading to diverse biological adaptations.
Diverse changes have occurred even in the evolutionarily brief time period since the agricultural revolution. In dairy farming regions, human adults developed lactose tolerance that allowed them to consume cheese and other milk products during adult life. Conversely, adults from non-dairying regions, such as most of Africa experience bloating and diarrhea when they consume dairy products.
There are many examples of adaptation to living with domestic animals, including resistance to some of their diseases.
The light skin color of early European cereal farmers can be explained as an adaptation to reduced levels of vitamin D in the diet. Greater exposure to sunlight due to less pigmented skin facilitated greater synthesis of Vitamin D (1).
While natural selection is easy to demonstrate in modern human populations, human adaptability exposes a mystery that epitomizes many claims of human exceptionalism. This concerns the fact that humans migrated out of Africa to inhabit the entire globe whereas chimpanzees and other primates mostly stayed put.
The Mystery of Migration Out of Africa
The divergent histories of humans and other primates is highlighted by the fact that our species expanded to dominate the entire planet whereas most others occupy the same places as their distant ancestors. These differing historical trajectories are often interpreted as a sign that our species has somehow stepped outside the biological constraints that rule other species.
Yet, this may be a misperception. After all, there is nothing unique about expansionist species. Good examples are provided by rats and cockroaches that follow humans and subsist on their leavings.
As to why humans migrated north through Africa and ultimately expanded throughout Asia and the Americas, there is no widely-accepted explanation.
Even so, most scholars identify a central role for increasing technological sophistication. Our ancestors became increasingly skilled at killing large prey animals from a distance. (This capacity reached a climax around 40,000 years ago when large prey animals became extinct around the globe in a phenomenon known as the Pleistocene overkill).
Even before humans completely overwhelmed their large prey animals, it is likely that our activities reduced their numbers to the point that we did better by moving on to new territories. This might help explain our restless migration around the globe.
Of course, that project was facilitated by unusual intelligence and a capacity to adjust quickly to new places with varied plants and animals, differing climate, and new opportunities and challenges.
The human trajectory may have been unusual for a primate but it is also understandable as a product of evolution.
1 Henrich, J. (2015). The Secret of our Success: How Culture is Driving Human Evolution Domesticating our Species and Making us Smarter, Princeton, NJ: Princeton University Press.