The Timing of the “Symbolic Revolution” in Modern Humans

Dienekes did everyone a favor by posting videos of talks by two famous Bay Area professors, one from Henry Gilbert on Homo erectus, the other one from Richard Klein on the “symbolic revolution” in modern humans. Both talks were delivered at the end of 2011-early 2012 at Cal Poly, the favorite source of knowledge for Alvah (Pardner) Hicks who uniquely supports out-of-America I.

Klein’s presentation is a classical one that I heard on a number of occasions during my years at Stanford. He updated it to reflect the recent evidence for genetic admixture with Neandertals but otherwise it’s the same explication of his theory that around 50,000 YBP a genetic mutation resulted in the rapid transformation of “anatomically modern humans” well-attested in Africa from 195,000 to 100,000 into “behaviorally modern humans.” In a number of books, Klein documented the sudden appearance of unambiguous signs of modern human behavior in Europe, Asia, Africa and Australia. He believes that a symbolic revolution took place in Africa around 50,000 YBP, which involved enhanced cognitive abilities, previously unseen technological sophistication, language and culture (visible in the archaeological record through ritualized burials and abstract art). Armed with this new “package,” modern humans expanded out of Africa, colonized the globe and replaced all earlier hominins who lacked this evolutionary advantage.

It’s a very simple, well-argued and well-supported model that nevertheless has failed to command universal acceptance. One of the reasons for this lack of acceptance is that language and culture are a “black box” and it’s presently impossible to say how they emerged and was the evolutionary process leading to language and culture unique, rapid and irreversible, or long, stepwise and unsteady. At roughly 40:00 of the video, Klein makes a comment that some scholars include the emergence of nuclear family and the gender division of labor into the “modern human behavioral package” appearing at 50,000 YBP in Africa. By “some” Klein means, for example, Henry Gilbert’s colleague, Terrence Deacon, who argued that human symbolic capacities are derived from pair-bonding and that the first utterance was a “marriage proposal.” (This means it could have been a performative!) Klein disagrees because he thinks “nuclear families” had been already in place by the time the revolutionary genetic mutation kicked in. What he has in mind is that nuclear families date back to Homo erectus who is assumed to have emerged in Africa at ~ 1.8 MYA. Homo erectus is believed to be a watershed between primate and human sociality. With Homo erectus, sexual dimorphism in human decreased dramatically which must have resulted in the decrease in competition between males and made stable pair-bonds between males and females possible.

Importantly, pair-bonding (a.k.a. monogamy, nuclear family) doesn’t come alone. As the comparison between New World monkeys and humans suggests, pair-bonding is just one element in a social “package,” which I call “calculus of humanity.” It forms a functional unity with cooperative breeding (allomothering, alloparenting) and high rates of paternal investment. Behavioral homoplasies between some species of New World monkeys and humans (similar rates of infant abandonment, infanticide by mothers, etc.) are so striking and specific that there can be little doubt that several key properties of sociality utterly missing in chimpanzees and Australopithecines co-evolved in the lineage Homo some time between Homo erectus and Homo sapiens. Unfortunately, Henry Gilbert didn’t address this question in his talk. New World monkeys consistently outperform individually breeding sister taxa on the following attributes (from “Cooperative Breeding and Human Cognitive Evolution,” by Burkart, J. M., Hrdy, S. B., and C. P. van Schaik).

Scholars increasingly consider pair-bonding, cooperative breeding and paternal investment as having made direct contribution to the evolution of language, cognition, music and other aspects of culture (see, e.g., Fitch, W. Tecumseh. 2007. Evolving Meaning: The Roles of Kin Selection, Allomothering and Paternal Care in Language Evolution. In Emergence of Communication and Language, edited by C. Lyon, C. Nehaniv, and A. Cangelosi. Pp. 29-51. New York: Springer.).

Consistently with this thesis, the closest parallels to human language in primates (babbling in infants, turn-taking, lexical syntax, etc.) and, arguably, music (duetting) come from New World monkeys. It seems clear that pair-bonding and duetting, pair-bonding and turn-taking, cooperative breeding and communication via abstract signs are related. When added to the hardware inherited from the chimpanzees, this “calculus of humanity” finalized the growing difference between humans and chimps 1.8 MYA.

Or, between humans and earlier hominins at 50,000 YA?

If Klein claims that archaeologically there are no unambiguous signs of modern human behavior anywhere around the globe prior to 50,000 YBP, then it’s hard to imagine that humans had possessed all the ingredients for the “symbolic revolution” for more than ~ 1.7 MY without leaving any traces of them anywhere in the archaeological record. True, Homo erectus managed to leave Africa and then exposure to varying environments and resulting unpredictability of resources must have put cooperative breeding to good use, but if “the calculus of humanity” married to the chimpanzee heritage in our ancestors is all we need in order to radically transform the environment and put us on a fast track to sending a monkey to the moon within a geological instant, then how come we haven’t found evidence for this new adaptation earlier than 50,000 YBP?

Out-of-America II offers a perfectly sensible perspective on this conundrum. The “calculus of humanity” must be what propelled the “symbolic revolution” in modern humans at 50,000 YBP. There was nothing similar to this going on before that time, as archaeology is firmly telling us. But, assuming that the greatest linguistic diversity is in the New World (140 stocks vs. 20 in Africa) and ancestral human forms of systems of kinship and alliance are best preserved in American Indians (see The Genius of Kinship), this “symbolic revolution” must have led to the migration out of America, not out of Africa and the replacement of all hominins with significantly weaker symbolic capacities.

If we, however, choose to consider decreased sexual dimorphism in Homo erectus as a sufficient observable sign of the presence of the “calculus of humanity” at ~1.8 MYA, we may still need to consider an unusual alternative. As Robin Dennell and Wil Roebroeks write,

“In Asia, the recent discoveries of H. georgicus and H. floresiensis should make us very wary of assuming that H. erectus s. was the only player on the Asian stage in the Early Pleistocene. Third, Asia might not have been the passive recipient of whatever migrated out of Africa but might have been a major donor to speciation events, as well as dispersals back into Africa. Such two-way traffic is well documented for other mammals in the Pliocene and Early Pleistocene, such as Equus and bovids, with more taxa migrating into than out of Africa. There is no reason why hominin migrations were always from Africa into Asia, and movements in the opposite direction might also have occurred, as has been suggested for the Olduvai OH9 and Daka specimens. We should even allow for the possibility that H. ergaster originated in Asia and perhaps explain its lack of an obvious east African ancestry as the result of immigration rather than a short (and undocumented) process of anagenetic (in situ) evolution.”

If pair-bonding, cooperative breeding and paternal investment are indeed foreign to African primates, as they are unattested in great apes and Australopithecines, but are found with increasing systematicity and rate among Asian and New World primates, one may wonder if indeed Homo erectus became a Homo outside of Africa. Evolutionary convergence may have been facilitated by exposure to the same environment.