Primate social behaviour and genes

Some have stressed an environmental determination of primate social behaviour, but a recent paper suggests that social behaviour in primates is genetic, not environmental: “Stepwise evolution of stable sociality in primates” (2011). Also see Nature’s commentary, and Nicholas Wade’s article in the NYT.

By studying many different primate lineages (see image below), the important conclusion they reach is that closely related primate groups display similar social organizations even when in different environments:

Caption from the study: "Branches and tips are coloured for solitary (purple), uni-male (orange), multi-male (red), pair-living (pink) where the combined probability of the state and the branch is greater than or equal to 0.7. Where the combined probability is less than 0.7, the branch is grey. Histograms represent the posterior probability distribution of each social state at the nodes indicated (a, primate root; b, anthropoid root, c, catarrhine root; d, great ape root; e, Pan–Homo split; f, Old World monkey root)."

From the Nature commentary: “The existence of a strong phylogenetic signal spells trouble for socioecological models that aim to explain the evolution of primate social organization. The models hypothesize that food distribution shapes competitive regimes, and that these, in turn, shape dispersal patterns and the nature of relationships within groups. These models generally assume that phylogeny does not impose notable constraints on social organization, and that changes from one form of social organization to another are all equally likely. But there is a growing realization that history does have a role and the new results strengthen that view.”

Pygmies, life-history, and maturation

After writing in an earlier post about Rushton’s & Lynn’s theories of black precociousness, it occurred to me to write about Pygmies’ unusual maturation pattern.

Unlike many other groups for whom there is a suspicion of different genetic potentials for height, but no absolute evidence, for Pygmy’s the matter has been pretty confidently settled. Here’s a map of pygmy world distribution. The black circles are groups that are not technically pygmy’s, but are also fairly short (notice their close geographical proximity).

From: "Evolution of the human pygmy phenotype" (2008)

[Note: there is conflicting evidence on whether pygmy and nearby shorter peoples cranial capacity is lower than surrounding African groups (1,2)]

The evidence for a genetic cause to the pygmy height:

“…three lines of evidence suggest that this phenotype is determined principally by genetic, rather than environmental, factors. First, … genetic disruptions of the growth hormone (GH) and insulin-like growth factor I (IGF1) pathway are likely to have etiological roles … Second, the childhood growth rates of some rainforest hunter-gatherer populations are surprisingly fast… for at least some populations, small adult body sizes are a reflection of relatively slow growth in adolescence rather than childhood… which is inconsistent with a simple model of stunted growth from poor nutrition. Indeed, other populations that also endure frequent episodes of nutritional stress still achieve adult heights that are greater than those of rainforest hunter-gatherers… Third, the offspring of Efe mothers and Lese (agriculturalist) fathers have statures intermediate to those of the two parental populations…”
– “Evolution of the human pygmy phenotype” (2008)

[Update: a new study looking at the genomes of Pygmies and their Bantu neighbours found that, among Pygmy, having more Bantu ancestry was correlated to being taller.]

It’s also worth noting that the geographical distribution of this phenotype, across genetically diverse populations, makes it highly unlikely that genetic drift could be a factor. So then what drives the pygmy phenotype.

Various explanations have been provided: thermoregulatory, improved mobility (reduced metabolic costs), dietary limitations, and most recently, a life-history proposal of high death rates in young-adulthood (source).

Correlation between adult height and mortality in short-populations. (Evolution of the human pygmy phenotype, 2008)

The idea lying behind the life history account is that the high death rate amongst the young promotes early truncation of growth to permit earlier reproduction. A recent archaeological study of a Khoisan (not pygmy, but also small) skeletal record, however, does not support the thesis of increased juvenile mortality (source). However, they also did not find an early cessation of growth, which the earlier (Migliano, 2007) study did find, among actual pygmies. Likewise, the Migliano study did find a higher juvenile (and adult) death rate among the (contemporary) pygmy’s. So while the Khoisan study contradicts what one would expect, there is clearly differences between the two populations studied. Not surprising since they inhabit pretty different environments as well.

[In any case, fortunately for the Pygmy’s, height preferences in a mate may not be universal.]

Comments on Pinker’s History of Violence

In Steven Pinker’s “The Better Angels of our Nature: Why Violence Has Declined” he argues that we have experienced a radical decrease in violence from pre-state societies, to state level societies, and then again in the transition to modern societies. The importance of the transition to state level society, according to Pinker, was that the state ended the perpetual feuding and and small scale warfare that exists in the state of anarchy.

I’m most interested in the claims about levels of violence in pre-state societies. Pinker’s evidence comes basically from two source: archaological sites, where skeletons have been analyzed for signs of violence, and ethnographic data from recent times, where anthropologists have calculated death rates due to homicide. The below graph, taken from the book, summarizes the data from Pinker’s sources:

Pinker arrives at an average of 15% deaths due to homicide in Hunter-gatherer societes, 24.5% in Hunter-horticulturalists & other tribal groups, and for state societies the numbers range from less than 1% to 5%.

Pinker takes the Hobbesian view that life in the state of nature was “solitary, poor, nasty, brutish, and short.” He doesn’t say much about how far back this tendency goes, but given that he doesn’t never qualifies his statements, and his general perspective on the role of states in pacifying violence, it seems he views these numbers as representing long term patterns of human behaviour. Futher, in an interview, when presented with the view that evidence for extensive violence mostly arises in the last 10,000 years, Pinker seems to think this is largely a matter of the deficient skeletal record, which supports the view that he takes the high homicide figures to be representative of the distant past as well.

The point I’m trying to get to is that, in reading Pinker’s book one gets a definite sense that extreme levels of violence (by today’s standards) have been with us throughout human history, and is only diminishing in turns by increasing state intervention and the discovery of more civilized norms.

Never referenced are, for example, anthropologists like Brian Ferguson or Douglas Fry, who take a very different view of the evolution of violence and war. Actually, there is overlap between the two views, but the fact that Pinker never even mentions them or their evidence in a history on the decline of violence is pretty remarkable. At least one would think that, since Ferguson and Fry (and others) see strong evidence for a drastic increase in violence beginning around 10,000 years ago, this would be worth a note in a history on the rates of inter-personal violence.

Pinker suggests that there simply isn’t enough skeletal evidence from before 10,000 years ago – but I find this a bit weak. Brian Ferguson in Archaeology, Cultural Anthropology, and the Origins and Intensification of War (2006), goes over the worldwide evidence of violence, both before 10,000 years ago, as well as tracing the emerging evidence of war and violence in different regions.

While there is certainly evidence of violence, it is not very extensive before 10,000 years ago. For example Brennan (1991) studies all skeletons available from the 100,000 – 10,000 BP from southwestern France – fragments of 203 individuals – and only 5 (2.5%) showed any signs of fractures, but even these were compatible with accidental injury. In the Middle East, 370 skeletons from the Natufian (10,800 – 8,500 BC) were analyzed, and only 2 showed signs of any kind of trauma. There are outliers of course – Nile Site 117 from around 12,000 BC, has 24 out of 59 skeletons are associated with projectiles. As Ferguson notes: “This is a true outlier without continuation…” But, naturally, it’s been included as one of the sites used as archaeological evidence for high rates of violence in Hunter-gatherer societies in Pinker’s book.

A general problem with the kinds of data sources used in Pinker’s book is noted by Ferguson  – that is, people often measure the populations, whether archaeological or ethnographic, because something about the violence catches their eye. Then someone else compiles a list of groups / sites with definite numbers. This is not exactly a random sample.

Ferguson also makes a strong argument for the very misleading nature of ethnographic data, in that it is collected in a world already highly distorted by the impact of Europeans or other more complex societies. Though there is pretty good evidence for a decided lack of peacefulness in the pre-colonial period, we still can’t take numbers from the 20th century and extrapolate them backwards indefinitely, or even numbers from the pre-contact period.

It is also worthwhile to distinguish Simple Hunter-gatherers from Complex Hunter-gatherers if we want to extrapolate very far back in time to when all populations would have been fairly simple. For example, today the less violent hunter-gatherer societies also tend to be the more simple ones. Looking back up towards the graph at the beginning of this post, one can see the huge variability in violence rates even among these perhaps not entirely representative samples. This suggests just how important it is that we not skew the data with unrepresentative or inappropriate data.

I don’t doubt there has been a general decrease in violence. I suspect however the picture is something more like that presented by Ferguson and Fry than the impression one gets from ‘The Better Angels of Our Nature’ – at least in relation to the ‘origins and intensification of war.’

Ferguson presents a picture of initially low-level of violence (relatively), and then, in the last 10,000 years war emerges and violence intensifies – then, with colonialism there is a dramatic dislocation of culture which in many cases increased violence, for a time, before pacification. Where Pinker has a point is that, despite increasing ‘warfare’ that went along with state development, there seems to have been a drop in actual interpersonal violence. Take the study of thousands of skeletons from North & South America over the last several thousand years. They found a 13.4% violent trauma rate among hunter-gatherers, and a 2.7% rate among pre-Columbian city and village dwellers. They also found an increase of violent trauma in the colonial period, which supports Ferguson’s thesis (the highest rates of violent trauma were actually found on black males, 25-34, at rates of 18.53%). However, and this is important, the 13.4% hunter-gatherer rate is within a  relatively recent past (6,000 years), and can’t be extrapolated far beyond that. In fact, data from North and South America from the earliest periods supports the hypothesis that violence was not originally nearly as high. For example, in the Great Plains region, of 173 individuals from all periods up through Woodland, there was only 1 with signs of violence, whereas after 500 AD, 74 of 447 skeletons show signs of violence.

This kind of pattern of violence coming into the archaeological record occurs over and over again – but, strangely, despite his whole book tracing the historical development of interpersonal violence, Pinker does not treat pre-state societies historically, instead lumping them all together. I think that the reason he does this is probably because of his Hobbesian view of man in the state of nature and the connected belief in the decline of violence as being heavily tied to the development of the state and social complexity. Whereas, if this alternative view is the case, there were fairly low levels of interpersonal violence and warfare in a pre-state, anarchic condition – until increasing complexity, and whatever other conditions pertain to the last thousands of years, caused violence to increase (before state level complexity reverses the trend). When stated like this, it is clear why Pinker might simply sidestep the whole issue.

– See also the review by Douglas Fry

– Others have brought up issues with the medieval data as well

Online videos touching on these topics:

If you want to watch lectures/interviews of the ideas mentioned here, watch Steven Pinker summarize his book; watch an interview with Brian Ferguson in which he discusses his issues with attempts to paint human history as extremely violent. Also of interest, a conference on on ‘coalitionary violenence and warfare‘ (2 videos).