Gracilization and Human Self-Domestication
Or, why do so many people get impacted wisdom teeth?
During the last decade of the 20th century and up until a 2005 court ruling banning it in US public school curriculum, Intelligent Design was heavily promoted by creationists as a pseudoscientific alternative to evolution. I always found the central tenet of ID, that human bodies were created in the current form by an intelligent creator, preposterous, not only due to the incredible amount of archaeological, fossil, and genetic evidence that belies this ID dogma, but because the human species is so clearly NOT designed intelligently (see also the last Pure Science post Why Do Hereditary Pathogenic Genes Exist?)
Exhibit A is the plethora of human orthodontic issues, and in particular, the fact that a large percentage (24%) of the worldwide human population have impacted wisdom teeth (if not removed) that occur because, as the Mayo Clinic explains, “ they don't have enough room to emerge or develop normally.”
Scientists have long recognized that the physical characteristics of our genus, Homo, and in particular the human face, have become more gracile (less robust) over time. This reduction is most clear when comparing Neanderthal (a minor Homo sapiens genetic contributor mostly for those with European ancestry) and modern human skulls (as shown at top above), but it also occurs in less obvious fashion when the comparison is restricted just to anatomically modern humans of the last few hundred thousand years.
For example, the chart below shows the total tooth surface area from a large number of human fossil specimens over the last 100,000 years, demonstrating significant, continuing and accelerating reduction in total tooth size.
The observant reader may wonder: “If our teeth are shrinking wouldn’t that make wisdom tooth impaction less likely?” It would if our jaw size stayed constant - human jaw dimensions have decreased faster than the corresponding dimension of our teeth!
But why are our species’ jaws and teeth shrinking? Changes over the last few hundred thousand years in both food preparation (cooking with fire and pounding with stones), along with the adoption of softer, grain-based diets since Neolithic times, have reduced the mechanical requirements on the human masticatory complex but these factors cannot alone explain its reduction in size and robustness.
Scientists have advanced numerous, unsupported and tenuous theories, from the Probable Mutation Effect to epigenetics. Others have speculated that a smaller human face improved vocal communications or improved head stabilization while pursuing big game. All these theories fail because they miss the bigger picture.
There is significant and mounting evidence that the gracilization of the human body including the face is primarily caused by human self-domestication (HSD).
Duke University professor Brian Hare Ph.D. provides compelling evidence of HSD in his 2020 book “Survival of the Friendliest: Understanding Our Origins and Rediscovering Our Common Humanity” and earlier research papers. As Dr. Hare writes:
“The human self domestication hypothesis proposes that these early-emerging social skills evolved when natural selection favored increased in-group pro-sociality over aggression in late human evolution. As a by-product of this selection, humans are predicted to show traits of the domestication syndrome observed in other domestic animals.
Many species that have been domesticated show morphological changes that correspond with a feminization or juvenilization of physical characteristics. This has been most clearly demonstrated with the domestication experiments of foxes started by Dmitry Balyaev in Russia. For over 40 years, generations of farm raised foxes were bred selectively for friendliness and lack of aggression to humans similar to how dogs were likely domesticated from wolves.
As a result, the “domesticated” foxes are not only much friendlier to humans but also have undergone (unselected) physical changes including: shorter snouts, curly and sometimes shorter tails that frequently wag when exposed to humans, and ears that stay floppier much longer after birth (3 months vs a few weeks). Similar physiological and morphological changes are inferred by Dr. Hare in the evolution of dogs from wolves, bonobos from chimpanzees and now Homo sapiens.
The physical changes have all been linked to the fact that domesticated animals have fewer of a certain type of stem cell, called neural crest stem cells. A recent Science paper describes:
Giuseppe Testa, a molecular biologist at University of Milan in Italy, and colleagues knew that one gene, BAZ1B, plays an important role in orchestrating the movements of neural crest cells. Most people have two copies of this gene. Curiously, one copy of BAZ1B, along with a handful of others, is missing in people with Williams-Beuren syndrome, a disorder linked to cognitive impairments, smaller skulls, elfin-like facial features, and extreme friendliness.
This new research complements the physiological and morphological evidence of HSD with strong genetic evidence that genes linked to friendliness like BAZ1B are also linked to human facial development and likely result from HSD.
Human self-domestication has not been as widely accepted in the scientific community as the evidence would dictate. This is due in part as a conscientious response to the fact that numerous civilizations, from Chinese dynasties to Western Civilization, have supported racist views that their own people and societies are “more domesticated” than out-group barbarians. Unfortunately (due to the way it might be misused), there is evidence of varied human physiology dependent on the geographic region where each individual’s ancestors lived, and some of these differences possibly are due to small, but differing, rates of domestication in those source populations. Fortunately, the differences between group/races are negligible compared to the variation among the entire current human population - all the more reason to treat everyone as individuals and to de-emphasize race.
Never knew what HSD was before reading this! Great article.