Does the "Dragon Man" Skull Fit the Human Self-Domestication Hypothesis?

 

Is this a human skull?  Even if it is not human in the sense of belonging to the species Homo sapiens, could it be human in the sense of belonging to the genus Homo?  

The scientists who announced on Friday the discovery of this fossil in China have identified it as the skull of an adult male who belonged to a new human species named Homo longi.  Long means dragon in Mandarin Chinese, and the fossil was found in the Dragon River region of northeastern China.  Carl Zimmer has written about this in The New York Times (Zimmer 2021), and he has links to the three articles by the scientists announcing the discovery (Ji et al. 2021; Ni et al. 2021; Shao et al. 2021).

The fossil has been dated as being at least 140,000 years old, which means that it is one of the six Homo species that lived on the Earth at the same time as Homo sapiens, which includes Homo erectus, Homo heidelbergensis, Denisovans, Homo luzonensis, and Homo neanderthalensis.  Although they did not all meet one another, we do know that Homo sapiens had intimate contact with Neanderthals and Denisovans--so intimate that we today still have Neanderthal and Denisovan genes.

Studying this "Dragon Man" skull in comparison with all the other fossils of the ancient hominids raises two kinds of questions.  One kind of question is about deciding where this skull belongs in the evolutionary lineage of hominids.  Is it really a new species?  Or does it belong to one of the already identified species?  Some scientists are arguing that it's actually a Denisovan skull.

Another kind of question is about explaining the emergence of Homo sapiens to be the dominant--the only--Homo species on the planet.  Why did those other six hominid species go extinct?  What was so special about our human ancestors that they survived and spread across the globe, while the other hominids disappeared?  This is especially puzzling when one realizes that these other Homo species seem to have been well-adapted for life on Earth.  After all, Homo erectus lived on the planet for a long time--from about 2 million years ago to about 115,000 years ago--longer than our species has.

Explaining how we evolved from ancestral hominids who were similar to us, while also explaining how we evolved to be a unique species, would tell us something about the meaning of our human existence on Earth.

It is often assumed that what makes us unique in the evolution of primates is our large brains as measured by our large craniums, which indicates greater cognitive capacities.  But in fact some of the Neanderthal fossil skulls have a cranial capacity greater than ours.  The Dragon Man skull's cranial capacity is around 1,420 cubic centimeters, which puts it within the range of modern humans.  So the mere size of the human cranium does not by itself explain the human difference.

There are some differences in the fossil skulls that point to ways that Homo sapiens surpasses other Homo species.  One of the most evident differences is in the brow ridges.  Notice how far the brow ridge of Dragon Man projects from the face, much farther out than for a typical Homo sapiens skull.  A similar difference can be seen even within the evolutionary history of Homo sapiens.  The older human skulls show a more prominent brow ridge than the younger human skulls.  Men tend to have thicker, more overhanging brow ridges than women, which is caused by men having higher levels of testosterone than women during their development, particularly during puberty.  So we can say that the skulls of Homo sapiens are more "feminized" than the skulls of other Homo species like Dragon Man, just as younger human skulls are more "feminized" than older human skulls.  You can see this craniofacial feminization in these human skulls:

On the left, you see a 110,000 to 90,000 years-old human male in lateral (top) and frontal (bottom) views, compared to that of a recent African male (right).  The older skull on the left shows the large brow ridges and long and narrow, masculinized face characteristic of Middle Stone Age/Middle Paleolithic-associated humans, as compared to the more feminized face of recent humans.

Some evolutionary biologists--such as Brian Hare (Hare 2017; Hare and Woods 2020) and Richard Wrangham (2019)--have seen this as evidence for the Human Self-Domestication Hypothesis: just as some wild animals have evolved through domestication to become tame animals living around human beings, so have human beings domesticated themselves in that ancient human ancestors were selected for being less aggressive and more socially tolerant individuals; and thus human beings have evolved by self-domestication through what Hare has called "survival of the friendliest."  Some of the evidence for this is found in our anatomy, particularly in our faces.

Dogs were the first animals, and the only large carnivores, to be domesticated.  They were domesticated sometime before 15,000 years ago, and perhaps as long as 30,000 years ago.  They were the only animals domesticated for life with nomadic hunter-gatherers.  They were descended from wolves, particularly from individual wolves with less aggressive temperaments who could live around humans, perhaps feeding on human waste, including human feces.  Dogs have been selected for their friendliness to human beings.  But this selection for friendliness brings with it a suite of developmental, anatomical, and physiological changes, which has been called the "domestication syndrome."  

In his famous experiments with silver foxes, Dmitry Belyaev showed how we can directly observe the evolutionary process by which wild animals become domesticated animals, because he selected wild foxes that showed friendliness to human beings and bred them.  Within 40 years, his breeding for friendliness had created foxes that were as loving to human beings as dogs (Dugatkin and Trut 2017).  As I have written about this in previous posts (here and here), this has suggested to some scientists the possibility that human beings have domesticated themselves in evolutionary history because their ancient ancestors were selected for friendliness just like Belyaev's foxes.

The first test of this Human Self-Domestication Hypothesis came in 2014, when Robert Cieri and his colleagues made some predictions about what would be found in the record of human fossil skulls if this hypothesis were correct.  

They were attempting to solve what paleoanthropologists have called the "problem of behavioral modernity."  The evidence suggests that the first fully human members of Homo sapiens--as indicated by their anatomy--appeared sometime between 200,000 and 150,000 years ago.  But prior to about 80,000 to 50,000 years ago, there is only a little sporadic evidence of "behavioral modernity"--that is, complex symbolic and cultural behavior as indicated by art, ornamentation, hunting and fishing technology, music, and long-distance trade.  The evidence for behavioral modernity manifest in complex symbolic and cultural artifacts becomes prevalent and persistent only after 80,000 years ago.  So why did it take so long for anatomically modern human beings to achieve the behavioral modernity that is unique to human beings?

It has become common for paleoanthropologists to explain this as a result of increasing human populations with dense social networks, so that more people interacting with one another promoted the generation, retention, and diffusion of cultural innovations.  But for this to happen, Cieri and his colleagues argued, there needed to be an increase in social tolerance, so that people in densely populated groups would cooperate with one another rather than fall into conflict.  There had to be some evolutionary selection against aggressive individuals and favoring cooperative individuals.

We should expect to find fossil evidence for this, because the neurotransmitters and hormones that mediate aggressiveness have effects on skeletal development, particularly in craniofacial growth and development.  So if there has been evolutionary selection for social tolerance--for survival of the friendliest--we can expect to see changes in skeletal morphology.

"In addition to moderating social tolerance, androgens play osteogenic roles and are important to the development of sexual dimorphism in craniofacial features . . . . Thus selection against the antisocial behavioral traits associated with high androgen reactivity would be expected to cause a reduction in average androgen levels or receptor density and result in craniofacial feminization in a population over time" (Cieri et al. 2014, 422).

The evolution of craniofacial feminization in Homo sapiens over the last 200,000 years could be seen as a skeletal by-product of an evolution for reduced aggressiveness and increased friendliness similar to what can be seen in domesticated silver foxes, domesticated dogs, and in bonobos.  Lyudmila Trut continued Belyaev's work after his death in 1985.  In 1999, she reported that the domesticated lines of silver foxes showed changes in skull shape.  The domesticated foxes had narrower skulls, with less cranial height, than the farm foxes, so that "the skulls of males became more like those of females" (Trut 1999, 167).  

If human fossils show a similar evolution of craniofacial feminization, this could be evidence for human self-domestication.

To test this prediction, Cieri and his colleagues studied measures of brow ridge projection, facial shape, and endocranial volume in samples of human skulls in three groups.  The samples included 13 skulls ranging in age from 200,000 to 90,000 years ago (Middle Stone Age/Middle Pleistocene), 41 skulls ranging in age from 38,000 to 10,000 years ago (Late Stone Age/Late Pleistocene), and 1,367 skulls of recent humans (Holocene Epoch). 

They found that, on average, skulls from the Late Pleistocene had a 40 percent reduction in how far the brow ridges projected from the face; and they were 10 percent shorter and 5 percent narrower than the skulls from the Middle Pleistocene.  The faces of modern hunter-gatherers were even more feminized than those from the Late Pleistocene.

The fossil evidence of friendliness can be seen not only in skulls but also in fingers.  Emma Nelson and her colleagues (2011) measured the ratio of the second digit (the index finger) to the fourth digit (the ring finger) in ancient skeletons.  For all primates, mothers with high levels of androgen while they are pregnant have babies with ring fingers that are longer than their index fingers.  Men typically have a lower second digit/fourth digit ratio than women.  Chimpanzees have a lower second digit/fourth digit ratio than bonobos.  In both humans and other primates, a lower (more masculinized) second digit/fourth digit ratio is associated with a higher potential for aggression.

Nelson found that the digit ratio of Middle Pleistocene humans was lower (more masculinized) than that of modern humans.  The digit ratio of four Neanderthals was even more masculine.  So it seems that the more feminized digit ratio of modern humans appeared along with their more feminized faces, as indicators of their evolution for friendliness.

I will be writing a long series of posts on this Human Self-Domestication Hypothesis.  And while this might seem to paint a rosy picture of evolved human nature as the friendliest animal, we will see that the picture has a dark side, because our friendliness to others in our group is combined with aggression towards those outside our group.  We are at once the nicest and the nastiest species.

I will also argue that this Human Self-Domestication Hypothesis supports Lockean liberalism, because what Locke identifies as the natural propensity to punish those who violate the law of nature corresponds to the self-domesticating propensity to select against aggressive individuals and to favor cooperative and socially tolerant individuals.  So I will defend a Lockean Liberal Self-Domestication Hypothesis.

REFEENCES

Cieri, Robert L., Steven E. Churchill, Robert G. Franciscus, Jingzhi Tan, and Brian Hare. 2014. "Craniofacial Feminization, Social Tolerance, and the Origins of Behavioral Modernity." Current Anthropology 55: 419-43.

Dugatkin, Lee Alan, and Lyudmila Trut. 2017. How to Tame a Fox (and Build a Dog): Visionary Scientists and a Siberian Tale of Jump-Started Evolution. Chicago: University of Chicago Press.

Hare, Brian. 2017. "Survival of the Friendliest: Homo sapiens Evolved via Selection for Prosociality." Annual Review of Psychology 68: 155-86.

Hare, Brian, and Vanessa Woods. 2020. Survival of the Friendliest: Understanding Our Origins and Rediscovering Our Common Humanity. New York: Random House.

Ji, Qiang, et al. 2021. "Late Middle Pleistocene Harbin Cranium Represents a New Homo Species." The Innovation 100132.

Nelson, Emma, Campbell Rolian, Lisa Cashmore, and Susanne Shultz. 2011. "Digit Ratios Predict Polygyny in Early Apes, Ardipithecus, Neanderthals, and Early Modern Humans, But Not in Australopithecus." Proceedings of the Royal Society B 278: 1556-63.

Ni, Xijun, et al. 2021. "Massive Cranium from Harbin in Northeastern China Establishes a New Middle Pleistocene Human Lineage." The Innovation 100130.

Shao, Qingfeng, et al. 2021. "Geochemical Provenancing and Direct Dating of the Harbin Archaic Human Cranium."  The Innovation 100131.

Trut, Lyudmilla. 1992. "Early Canid Domestication: The Farm-Fox Experiment." American Scientist 87: 160-169.

Wrangham, Richard. 2019. The Goodness Paradox: The Strange Relationship Between Virtue and Violence in Human Evolution. New York: Pantheon Books.

Zimmer, Carl. 2021. "Discovery of 'Dragon Man' Skull in China May Add Species to Human Family." The New York Times, June 25.