The 'Human Revolution' Thesis

June 11, 2010

Abstract: I examine the recent debate over the "human revolution" thesis, which posits a genetic mutation that enabled the transition from archaic to modern Homo sapiens at around the time of the Middle to Upper Paleolithic transition. I review arguments for the competing positions including inferences from the archeological record and observations about effects of demographic variables on technological achievements, then provide an overview of current thinking about the evolution of human cognition. I conclude that the revolution thesis is not well supported at this time.

This paper reviews the debate over the "human revolution" thesis popularized by Stanford paleoanthropologist Richard G. Klein. Klein has proposed that a genetic mutation occurred in a population of humans somewhere in Africa, perhaps sometime around 50,000 years ago, as a result of which they acquired some ability that enabled their descendants—"modern humans"—to displace the Neandertals and all other "archaic humans" through the world (Klein, 2000; Klein, Avery, et al., 2004). "Arguably," says Klein, "this was the most significant mutation in the human evolutionary series" (Klein, 2000, p. 18). He acknowledges that the exact nature of this change is not obvious, that testability could be a problem for his proposal, and that the evidence he adduces is subject to other interpretations. With those reservations, he suggests that the saltational event could have been our acquisition of speech (Klein, 2000).

An examination of the thesis, to see whether such a revolution really happened, must examine several subsidiary questions along the way, and many articles in the anthropological literature and writings in related disciplines have addressed them. This essay will look at what the author hopes is a representative sample of the most recent commentary.

Archaic and modern humans—assuming the distinction to be real—are referred to collectively as anatomically modern humans (AMH). They constitute the species Homo sapiens. Because of a convergence of genetic and fossil evidence, there is a current consensus that our species existed by sometime between 150,000 and 200,000 years ago (100-200 kya) and that the origin occurred in Africa (Bouzouggar, Barton, et al., 2007; Mellars, 2006; Powell, Shennan, et al., 2009). There were early migrations of H. sapiens out of Africa into Europe and the Far East, apparently localized and short-lived (Mellars, 2006). Then a more rapid and permanent population expansion began around 60-80 kya (Mellars, 2006). Very roughly (give or take a few tens of kya), this was coincident with a sudden proliferation of various technological and apparent behavioral changes as indicated by findings in places such as Blombos Cave, Howiesons Poort, and several European sites (Ambrose, 2001; Mellars 2004; Mellars, 2006). Progress in the Orient meanwhile, at least according to the archeological record to date, was more gradual (Powell, et al., 2009). Technological advances apparently followed the invasion of modern humans, but not immediately (James and Petraglia, 2005).

The archeological record thus suggests that H. sapiens, once evolved, persisted in a kind of technological and behavioral stasis for at least 100,000 years before acquiring the propensity for innovation that has persisted to the present day. This raises the question in some minds: What took so long (Bickerton, 2007; Powell, et al., 2009; Richerson, Boyd, et al., 2009)? Proposed answers tend to fall into at least two groups. One sides with Klein in holding for a sudden change in human cognitive abilities, while another appeals to climatic or other environmental influences that motivated our ancestors to exercise intellectual skills that had simply lain dormant for a hundred millennia or so (Mellars, 2006). A third group appeals to the paucity or unreliability of the archeological data to argue that the lag between ability and achievement could be either exaggerated or more apparent than real (Clark, 2002; McBrearty, 2007; Richerson, Boyd, et al., 2010). In general, though, the participants in this discussion have taken it for granted that the archeological record as we have it is approximately correct in what it reveals of human progress and when that progress occurred. There comes a point, they argue, when absence of evidence must be construed as evidence of absence (Kuhn and Stiner, 2006). Thus, at some point significantly less than 100 kya we started doing things that we had not been doing previously. One side in the debate says we were not doing them because we could not do them. The other says we could have but just did not, that the human revolution, if there was one, was social, not biological (McBrearty and Brooks, 2000).

The inference of primitive minds from primitive technology is obviously hazardous. Technological regression has happened in historic times (d'Errico, Henshilwood, et al., 2003; Kline and Boyd, 2010; Richerson, et al., 2009), so actual use of technology does not track the ability to use it (Cain, 2006; Kuhn and Stiner, 2006). The attempt to correlate the artifactual record with humanity’s intellectual progress has raised concerns about genetic determinism and what some perceive as its potentially racist implications (McBrearty and Brooks, 2000). Perhaps more charitably, others have pointed out the sheer complexity of the problem and the unavoidable constraints of theoretical presuppositions (Bahn, 1998; Bickerton, 2007; Clark, 2002; de Boer and Fitch, 2010; Shea, 2006). A partial antidote to the latter, some think, would be contributions from other disciplines (Ghazanfar, 2008; Gibson, 2007).

While the evidence has not generated anything like a consensus, the trend of discoveries in the past decade has not been friendly to Klein. Evidence for the lateness of modern behavior has eroded as more and earlier evidence for it is discovered (Henshilwood and Marean, 2003; Mellars 2004). Evidence of modernity appearing in southern Africa has been dated to 70-80 kya (Richerson, et al., 2010). Bone technology, frequently taken as a marker of the revolution, is being found in ever-earlier strata (Cain, 2006). Symbolism, another marker, could go back at least 200,000 years (d'Errico, et al., 2003). The oldest known burials, which possibly presuppose language, have been dated to 100 kya (d'Errico, et al., 2003).

That the revolution occurred earlier and more gradually than Klein supposed does not mean it did not happen, but as McBrearty observed, it does make it look less like anything we should call a revolution: "It seems inappropriate to label changes accumulating over a period of 200,000 years either a revolution or a punctuated event" (McBrearty and Brooks, 2000). Never minding how the event should be labeled, though, it is in our human nature now to wonder what it took for it to happen.

It would help, in finding an answer, to be clear on just what "it" was, but this, too, is part of the argument. "Modern human behavior" has never been defined to everyone’s satisfaction (Henshilwood and Marean, 2003), but the capacity for speech seems to be on everyone’s short list of necessary, if not necessarily sufficient, characteristics. Speech, obviously, does not fossilize, which means that practically nothing is certain about how or when it originated (d'Errico, et al., 2003; Sherwood, Subiaul, et al., 2008). On the basis of gross anatomy alone, we might suppose that it has always existed at least in our own species (Dunbar, 2003) and could have been present in some predecessor or sister species (McBrearty, 2007). Our linguistic ability indisputably required some genetic mutations, but their correlation with advanced cognition is not yet a settled issue.

The whole history of science in general has eroded most of our suppositions about human uniqueness. There is little if anything we can do that some other animal cannot also do to some degree. But even a degree of difference needs some explanation, and some differences of degrees—such as our tool-making and linguistic abilities—have become differences in kind (Gibson, 2007), and several researchers think something about the way our brains are wired accounts for both.

Comparisons between ourselves and chimpanzees is suggestive. Even to the extent that they can be trained to imitate some human activities, the question remains whether they could ever have acquired the behaviors on their own initiative. They exhibit patterns of behavior and imitation among themselves that merit the label "culture," but whereas humans can accumulate cultural modifications, chimpanzees do not seem to manage this. Chimpanzees and other primates can use symbols, but none create any (Chase, 2007).

One difference in kind between human and other languages is the virtually unlimited utility of the former. Nonhuman language lacks compositionality—the number of things a user can say with it is fixed, typically at a very low number (Arbib, Liebal, et al., 2008). Apes’ ability to acquire symbolic communication is limited. Their learning is slower and limited. They lack compositional ability and apparently have no notion of syntax. Ape "speech" is little more than rudimentary gesturing (Arbib, et al., 2008). The effectiveness of human language is due among other things to its modality and stimulus independence, the former exemplified by our multitude of distinct languages as well as writing and sign language, the latter by our ability to talk about things we cannot see or never could see, as well as matters irrelevant to our immediate situation (Sherwood, et al., 2008).

All this seems unlikely to have come to us all at once (Corballis, 2010; Ghazanfar, 2008). Language seems instead to be related to a suite of changes driven by our sociality interacting with an unstable environment. Human social organizations are more diverse than those of other primate species (Richerson, et al., 2010). The archeological record suggests that complex culture arose during Ice Age climatic fluctuations (Richerson and Boyd, 2001), and the complexity would have created pressure for new social instincts (Richerson, et al., 2009). A need for effective communication could have driven that adaptation of mechanisms, both cognitive and physical, not originally evolved for language, to produce originally a proto-language that could then have undergone further development (Arbib, et al., 2008; Bickerton, 2007; Boyd and Richerson, 2009; Hauser, Chomsky, et al., 2002).

Several researchers have suggested a connection between language and theory of mind, which is our ability to discern, within limits, other people’s thoughts and thereby to infer their likely behavior in the immediate future (Dunbar, 1998; Dunbar, 2003; Gibson, 2007; Tooby and Cosmides, 2005). The utility of such an ability for members of any social species like ours is obvious. And, as it happens, primate evolution has exhibited a correlation of neocortex size with typical group size and social complexity (Dunbar, 1998; Dunbar, 2003, 2007). One consequence of improved social cognition appears to have been multi-level intentionality, which is illustrated, for instance, when John thinks to himself, "Joe is thinking about Susan, and Joe knows that I know he is thinking about her." Whether or not it was necessary, there seems to have been a connection between this ability and our acquisition of language (Bickerton, 2007). The current consensus, then, seems to be for a co-evolution of our linguistic and cognitive abilities, both driven by the increased complexity of our social lives, in a feedback process that would not bode well for a punctuational theory of Klein’s sort (Fessler, 2006).1

Among the forces driving this process seems to have been the advantages of cooperation. While this strikes some modern minds as a no-brainer, it had its costs. Enjoying the benefits of a group means getting along with others in the group, putting up with their annoyances and coping with competition, among other things. Thus arose the need for coalition management, which ratcheted up the need for more brainpower (Dunbar, 1998). This has been true for all primates, but became more so as group sizes got larger (Boyd and Richerson, 2006). Information-processing capabilities had to improve (Gibson, 2007), and thus we became, among other things, more flexible, more able to adapt our behavior, not only to one another but to the world around us as it changed. Behavior per se was not subject to selection pressure, but to be adaptive, behavior has to track information, and selection could work on the tracking mechanism, which is what our brains are (Tooby and Cosmides, 2005).

The specifics remain elusive, but the basic idea is that among animals in general, brains evolved in order to process environmental data and generate a response consisting of appropriate movement (Hoffecker, 2007), and so insofar as we got better at that sort of thing, we should expect that it was because we were evolving better brains. But better did not mean just bigger. Evolution could have been working on the programming, too (Falk, Redmond, et al., 2000; Gibson, 2007). As a consequence, both genes and culture would have been evolving in response to environmental pressures (Mellars, 2006; Richerson, et al., 2009).

Technological progress of the sort Klein sees as nonexistent until about 50 kya was contingent on our having acquired the ability to collectively accumulate new knowledge. This entails the transmission of knowledge across generations, which is a complex process requiring several skills (Boesch, Tomasello, et al., 1998) and with fidelity contingent on a variety of factors (Fessler, 2006). There are few defensible hypotheses yet on the particulars of brain evolution, but they must have had something to do with the control circuits or "executive functions," none of the changes to which were likely to show up in the fossil record. Among the components of this faculty is working memory, the biological analogue to computer RAM, which is highly variable in humans nowadays. Although controlled by numerous genes, its capacity could be sensitive to changes in just one (Coolidge and Wynn, 2001).

Another focus of recent research has been on mirror neurons, brain tissues associated with imitative activities in nonhuman primates, first discovered in monkey brains in a region thought to be homologous to Broca’s area, which is vital to speech in humans (Corballis, 2010; Sherwood, et al., 2008). If, as Corballis and others suggest, the acquisition of speech is a kind of imitative activity and the execution of speech is a kind of gesturing, then we stand to learn a great deal about our origins from further research in this area (Arbib, et al., 2008; Corballis, 2010). It also further reinforces the thinking against our post-Paleolithic activities being due to anything evolutionarily unprecedented. It suggests instead that all the rudiments of our new abilities evolved quite early and needed only sufficient time to develop (Dunbar, 1998; Gibson, 2007).

They also needed the right stimulus to become manifest as a spurt of technological growth. This could have been the population explosion the followed the last glacial retreat. Small populations cannot sustain, or least historically have not sustained, technological complexity (Kline and Boyd, 2010; Richerson, et al., 2009; Richerson, et al., 2010).

What the evidence to date suggests, then, is that the human revolution was just evolution doing its usual thing. While the hard data still leave more room for speculation than cogent theorizing, at this point there appears to be not much reason to doubt that whatever we can do now, we probably could have done as soon as we became H. sapiens. But first we had a lot of learning to do, and it took what seems in hindsight like an inordinately long time. The reason for that holdup has yet to be discovered, but right now it looks as though it probably was not in our genes.


1There remains some debate about whether language is itself an adaptation or simply a by-product of our general intelligence. For this essay we simply note that several competent researchers, e.g. (Hannenhalli and Kaestner, 2009; Pinker, 2010; Tooby and Cosmides, 2005), think the weight of the evidence supports the adaptation hypothesis.

(The original version of this paper was written as an assignment for an anthropology class taught by Dr. Wesley Niewoehner at California State University, San Bernardino.)


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