|Prof. Steven E. Churchill|
In continuation of our newly introduced, Researcher of the Month (RotM) series, we spoke to Professor Steven Churchill, professor of Evolutionary Anthropology at Duke University.
Prof. Churchill specializes in human paleontology and his research interests include studying archaic and modern humans of the Middle and Late Pleistocene era. In his recently published paper in Current Anthropology, Prof. Churchill and his team discuss the 'feminization' of human skull and origins of the life that we live today. Here's Prof. Churchill, telling us more about his findings.
CTS: For the benefit of our readers, please tell us about the fresh perspective that your recent findings have provided on the origins of Behavioral modernity?
SC: Humans are remarkably socially tolerant compared to most other animals. We often gather with strangers in large numbers without violence (something that would not happen with chimpanzees, for example), and we are exceedingly cooperative. Social learning, including the transmission of new skills and technology, is greatly facilitated by our social tolerance and cooperative nature. It is also the case that about 50,000 years ago cultural evolution began to speed up. The rate of new technological innovations increased rapidly at that time (producing the transition from the Middle Stone Age to the Later Stone Age in Africa, and from the Middle Paleolithic to the Upper Paleolithic in Europe and Asia), items of art and personal adornment began to appear, and populations of modern humans appear to have been growing.
In our recent paper we argue that part of this technological revolution involved an increase in social tolerance around this time, which facilitated social learning and cultural transmission (and which itself may have been brought about by increasing population density, which may have produced situations in which less tolerant individuals found themselves at a disadvantage, either because they could not enjoy the benefits of cooperation or they were punished or ostracized by groups of cooperators). We further argue that this increase in social tolerance can be seen in the fossil record of our species over the last 200,000 years.
Fossils of Homo sapiens pre-dating 50,000 years ago tend to have very large brow ridges and long upper faces, whereas these features are much less pronounced in humans after 50,000 years ago. These features develop during puberty as a result of circulating testosterone, and differentiate (on average) males from females in living populations today. Because males today have facial skeletons that are most similar to those of female Homo sapiens prior to 50,000 years ago, we have referred to this shift as one of feminization. Thus the fossil record seems to denote a reduction either in levels of circulating testosterone, or the reactivity of target tissues to that testosterone. Androgens like testosterone provide one physiological axis that mediates aggressive behavior, and thus we think that the trends we see in the fossil record are reflecting a reduction in aggressiveness and a corresponding increase in social tolerance during the end of the Pleistocene.
|Feminization of modern human skull on the right side as |
compared to 110-90 Ka old skull on the left.
Image source and copyright:
Interestingly, in animals that are selected for tameness (such as domesticated animals, or the Siberian silver foxes that were the subject of a study on tameness), male skulls become more like those of females, which parallels what we are seeing in the human fossil record.
CTS: If males today have facial skeletons that are similar to females that roamed the Earth 50,000 years ago, what differences can one see between facial skeletons of females today and those that existed 50,000 years ago.
SC: Both sexes have become more "feminized" over the last 50,000 years. Females today have smaller brow ridges and shorter faces than did females between 200,000 and 50,000 years ago. For example, there is an ca. 90,000-year-old female skeleton from Qafzeh Cave in Israel (the specimen is called Qafzeh 9). We can tell the skeleton is that of a female based on the morphology of her pelvis. But if her skull were found in a modern context, most anthropologists would conclude that the skull was that of a male.
CTS: When we say that the human males lost their ‘robustness’ over a period of time, would that be to accommodate the more modern way of life or could it be that the more organized way of life did not demand that he remain that robust/ aggressive?
SC: Anthropologists use "robustness" to mean different things. Mainly the phrase refers to the relative mechanical strength of the skeleton, and in this regard humans have become less robust as we have increasingly relied on technology to perform jobs that we once had to do with muscular strength. Less commonly, anthropologists may use the term to refer to the ruggedness of features of the face and cranium (for example, a skull with large browridges may be referred to as "robust"). But to be clear, in our paper we are not talking about robustness of the overall skeleton, but rather testosterone-linked facial features (this is why we prefer to think of this as a change in facial masculinity [feminization] rather than a change in robustness). At present we can't see any reason that faces would become more feminine as a by-product of technological evolution that was not related to changing temperment.
CTS: If males become more feminized in the past and continue to do that even today, would you say that that someday, humans might become sexually monomorphic?
SC: While it isn't really prudent to speculate about what course evolution may or may not take, it seems unlikely that humans would ever become monomorphic. Testosterone is important to male sexual functioning (and to that of females as well, but at lower levels), and it seems more likely that continued selection for greater social tolerance would like operate on neurological mechanisms that allow individuals to suppress non-cooperative tendencies, rather than continuing to select against androgens.
CTS: In the wake of wars being fought in many places around the globe today, would you say that the social tolerance is low now? How does anthropology explain such a scenario when our population is booming?
SC: Just because we are remarkably socially tolerant doesn't mean we can't act in aggressive ways. War involves lots and lots of cooperative behavior (soldiers act as teams, follow orders, train together, coordinate movements, etc), and soldiers don't shoot at one another because they lack social tolerance. Just to be clear, we are not arguing that humans became totally non-aggressive 50,000 years ago, just that they became more socially tolerant than they had previously been.
CTS: There is scientific evidence that the Y chromosome has been shrinking. Along with your findings that males became more feminized, is this leading us to a possible future where males might no longer be required at all?
SC: This is really outside of my area of expertise. The Y chromosome doesn't carry much genetic information (mainly things like testes determining factor, which are important in producing male sex), so one might ask how necessary males are even now!