Biology follows culture follows biology….

Humans are a pretty big deal. I’m human, you’re human. We’re a very successful large mammal. A substantial proportion of the earth’s biomass is us, or, is due to us. So the field of human paleoanthropology gets a lot of attention in comparison to something like materials science, even though materials science is far more practical, and a much bigger deal in our day to day life. Most people have heard the name Richard Leakey. They might not even know there’s a field called materials science.

Because people are interested in paleoanthropology there’s a demand for discoveries, and people promising understanding. The root of a lot of this is ontological. Why us? Necessarily us? But more prosaically many scholars have responded by generating models of the rise of humanity with silver bullet explanations. To give a few examples, fire, tools, gossip, and meat. One phenomenon that has interested scientists for years is the rise to dominance of an African lineage 50,000 years ago, and the subsequent manifestation of “behavioral modernity” in the archaeological record. To illustrate behavioral modernity researchers might present you with the image of ostrich shells which may have been painted, or the artistic caves of France and Spain. I think it is simply enough to contrast behavioral modernity with what came before. The Acheulean tool culture persisted for over 1 million years. Can you imagine a cultural tradition today persisting for 10,000 years? Obviously something changed.

One elegant model proposed by the paleoanthropologist Richard Klein is that human culture is a product of a punctuated evolutionary change, which resulted in a revolution in capacities, and a rapid marginalization of all other populations. The phenotypic manifestation of that neurological shift may have been the capacity for fully featured language. In this scenario the chasm between archaic and modern human populations is enormous. At the opposite extreme you have theories which posit a gradual shift across the Homo lineage over time, with behavioral modernity being only the latest manifestation of a trajectory which was initiated long ago. A possible implication in this framework is that something like us was inevitable at some point, or, the Homo lineage would just go extinct (yes, we are going to go extinct at some point in any case).

In 2005 or so I would probably have been close to the Klein model of relatively quick biological driven changes that gave rise to H. sapiens. Today I am much nearer the second scenario. One of the reasons is that a few years back the geneticist Luke Jostins produced the result that Homo as a whole was undergoing encephalization. To some extent this is obvious in hindsight. Neanderthals had larger brains  than their ancestors. But Jostin’s plot suggested to me that the path which led to our big brained lineage had roots somehow early on in the emergence of our broader lineage, and not just in the recent past of H. sapiens sapiens. Only that could explain a world wide pattern across disparate lineages which were genetically isolated. I don’t have a specific outline of what I’m thinking of, but in the generality there are cases where evolutionary processes exhibit path dependence. One could argue then that Homo was going to get big brained, or go extinct. We do know that apes as a whole have been less successful in the evolutionary game if being speciose is a guide over the Cenozoic. Monkeys have taken over many of the niches which were previous held by the apes. There used to many more of us. Homo is then the exception to the rule, as it broke out of the niche which monkeys were taking over, and made lemonade from lemons.

So how might this inevitable process have played out? As implied in the title I suspect that early in the Pleistocene Homo got “trapped” in a unidirectional ratchet where biological changes allowed for the elaboration of complex culture, which then drove further biological changes, again resulting in culture transformations, and so forth. The evolutionary process did not explore the whole parameter space with equal frequency. H. floresiensis stumbled upon a unique, but rare, niche (you can speculate about what it was, but its small size and peculiar anatomy indicate sit differed from others of the lineage quite a bit in its circumstances). In contrast other groups of Homo were getting bigger brained. In fact the shift toward bigger brains leveled off at about ~100,000 years ago, before behavioral modernity. This is probably a consequence of the fact that there are various biological limits in terms of how disproportionate our brains and heads can be in relation to the rest of our bodies (e.g., we are born at a rather fetal stage because otherwise our heads would get too large to fit through the birth canal, whose widening is achieved at some cost to female locomotion). At yet changed continued.

A new paper in Current Anthropology attempts to bring various threads together to explain the rise of modern humans in a manner Charles Darwin would appreciate, Craniofacial feminization and the origin of behavioral modernity. Like most of you I can see that the figure above at the top of this post illustrates two individuals where one is more gracile or feminized than the other. But that’s about it. I don’t know skeletal anatomy well enough to comment with great force on the data within the paper, though it does seem that the results are somewhat confused, with data scarcity and combining agriculturalists and foragers in the Holocene confounding the signal. Nevertheless, they conjecture is that over the past 100,000 years humans have been subject to the domestication syndrome. Less aggressive social behavior at higher densities due to reduction in androgen levels produces the more feminine features in the fossil record. At least according to their model.

The major problems I have is that the signal in their data is not that clear to me. There is a lot of talk about why agriculturalists seem more masculinized than Holocene foragers. Obviously that sort of result is not as “neat” as they might like across the time period of interest. They acknowledge that limitations on their data set might explain this, but that makes me wonder what conclusions they can draw then in the first place from their data. Or, their model is just too simple. It’s a bit rich for me to say that, because compared to Richard Klein’s thesis that one mutation produced modern human behavior the argument outlined in exceedingly elaborate. But, if behavioral modernity arose via processes which were gradual, and often worked upon standing genetic variation, I don’t see why the selection should have been through the same processes across hundreds of thousands of years. It could be that in some epochs higher population density was due to biological changes in humans triggered by culture shifts, while in other phases the higher density itself was driving biological change. This is not congenial from a scientific perspective because it isn’t parsimonious. But it isn’t wrong on the face of it.