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Men Became “Feminized”! Tameness Led to Modern Humans

Science Fields

Humans have domesticated themselves alongside the animals they have pressed into service as guards, transport, pest controllers or living ornaments in order to modernize. That’s the word from the symposium on self domestication of humans held at the Salk Institute for Biological Studies in San Diego earlier in the month, at which anthropologists and evolutionary biologists concluded that with our  reduced chins, flattened faces  and diminished aggression, we have become as tame as the animals we live with.  That is, we share many of the traits which appear in the process of animal domestication like they did in cows, horses or dogs. Behavioral changes that accompanied the taming, particularly in males, were seen as factors facilitating the development of a complex language, emergence of urban life in crowded cities and development of advanced cultures through cooperation.  

Researchers point to the fact that in the course of their evolution, some animals have rid themselves of the fear and stress they felt on encountering humans or members of their own kind, as seen, for instance, in bonobos being less aggressive towards each other compared to chimpanzees. The same things happened, say anthropologists, when the humans began to live closer to each other and form larger communal webs in order to survive. Favoring more tolerant, more peaceful individuals, natural selection has rearranged our behaviors as well as our appearance.

A famous experiment in the then-Soviet Union in 1950s for the taming of silver foxes unveiled the components of what has come to be called the “domestication syndrome.” It was seen that new generations of foxes selected and bred for tameness sported floppy ears, curved tails and white patches of fur on their shortened snouts which combined to give them a feminized look, going together with skulls not much different in males than females. 

Paleoanthropologist Robert Franciscus of Iowa University and his colleagues reported their observation of some of the same traits becoming visible in later stages of human evolution. The team studied skulls of 13 Homo sapiens ancestors who lived more than 80,000 years ago, those of 41 modern humans who lived between 38.000 and 10.000 years ago and 1367 recent people for changes in brow ridges, face shapes and cranial volume. It was seen that as our forebears began to display symbolic behavior and spread across the globe over the past 80,000 years, the ridges diminished and faces shortened. The cranial volume was also seen to have shrunk, particularly after the invention of agriculture 10.000 years ago.

All these bring the male faces closer to women’s and point to reduced levels of testosterone, Franciscus explained.  Selection for social tolerance, particularly among men, have led to lower levels of testosterone and stress hormones with the result that faces become “feminized”

During embryo’s development, cells which break away from a tube called neural crest extending along the spine, migrate to other parts of the body to initiate changes important for domestication.

Other researchers in the symposium, meanwhile, have pointed to a common basis for all these changes, be them hormone levels or crano-facial features, rooted in embryonic development.

According to Tecumseh Fitch of the University of Vienna, Richard Wrangham from Harvard and Adam Wilkins of the Humboldt University in Berlin, the domestication syndrome is run by neural crest cells which form a tube extending along the spine in vertebrate embryos.   As the embryo develops, these cells break away from the tube and migrate to spots from head to foot to form tissues involved in the formation of pigmentation, muscles, teeth, bones and cartilage, and adrenal glands which secrete testosterone and stress hormones.

The process takes time in domesticated animals, the reason being easier taming of slow-developing animals. Young animals, like, for instance, dog pups do not know much fear until they are a few months old and therefore their bodies secrete lower levels of stress hormones. If they interact with a friendly human in that critical period, they learn to cooperate with people. The longer this period in the development of animals, the longer becomes the time for learning which is essential for domestication.  

Another consequence of slow development is that less neural crest cells reach their intended targets, affecting a number of processes from the functioning of adrenal glands to pigmentation of facial and tail fur as well as the length of the snout which all are signs of domestication.  This process also explains why mature domestic animals retain the physical features of their young, a phenomenon called neoteny.

Researchers proposed that natural selection triggered slower development and diminished aggression in humans as well, resulting in proliferation of males capable of living together and cooperating with others. Pointing, however, to the fact that humans, slowest developing of all, do not carry all the symptoms of domestication, like,  for instance, floppy ears or white patches on faces researchers do not rule out the possibility that some yet-unknown factors might have contributed to our domestication. 

REFERENCES

  • 1. “How we tamed ourselves – and became modern”, Science, 24 October 2014