I can imagine one scenario under which war would have been rare or absent in the Pleistocene. W.C. Allee in the 1930s described a demographic phenomenon later called the Allee Effect. We often think of intraspecific competition in terms of the logistic equation in which competition increases monotonically with population size or density. Allee pointed out that this is unlikely to be true at very low population sizes or densities. For example, conspecifics in sexual species are a source of mates. If a species becomes rare enough, just finding mates is a problem. Humans likely have an exaggerated Allee Effect. We not only need mates but our original way of life was dependent on band level cooperation in hunting. We shared information on an even larger scale. Evidence suggests that it takes sharing information on a considerable scale to maintain a toolkit as sophisticated as those of the Upper Paleolithic. Yet population size estimates for Upper Paleolithic Europe, using mtDNA and guestimates based on archaeological site frequencies, are quite low. Indeed, I don’t think that there is any evidence that compels us to think that Pleistocene Homo were ever as common as Holocene hunter-gatherers generally were. To maintain the UP level of toolkit complexity probably required UP Europeans to be very efficiently interconnected. In other words, in the Pleistocene another band of humans approaching your band might have been more of a resource for cooperation than competitors.
In addition to population size estimates, some other evidence is consistent with an Allee Effect as late as UP times. First, symbolic artifacts that in the Holocene would mark tribes occurred over a very wide area. See Richard Klein’s figure depicting the distribution of Gravettian Venus figurines. It is as if all of Europe from the Urals to the Atlantic and from the Mediterranean to the ice margins housed only one tribe or culture. Neanderthals (and many populations of Anatomical Moderns) made simpler tools than the Gravettians, perhaps because they were even rarer and/or more poorly interconnected. Dale Guthrie, a specialist on Pleistocene large mammals, has written a treatise on UP cave art. He argues that the relatively abundant crude images in the caves were mainly made by adolescent boys. Many of the images are naturalistic depictions of animals seen from the perspective of a human hunter or an adolescent neophyte hunter. Depictions of collective violence and of defensive weapons are absent. If neophyte hunters were also neophyte warriors the lack of graffiti with warrior themes is surprising since such themes are common in the graffiti of modern adolescent boys.
Could human band and tribal scale cooperation have arisen without warfare? Paul Smaldino used an agent based simulation to test Peter Kropotkin’s intuition that harsh environments could select for cooperation without competition between groups. He found that it could. This is easiest to see in an extreme case. Imagine a species of tropical frugivorous ape in a world where deserts and savannas are expanding and forest is disappearing. These environments have resources that could be exploited by a tropical ape, but only if they can evolve more cooperation. For example, if males could cooperate to protect one another from predators, they might be able to exploit resources in the savanna in short forays out of the forest. If women could cooperate to take care of each other’s children, women could forage for resources in open forests unencumbered by children. Hunter-gatherers specialize in seeking resources that take cooperation to produce in environments where solitary foragers could not exist.
Many people find this argument implausible. It does seem to me to be important to entertain alternative scenarios regarding the evolution of human behavior. We do know that Pleistocene glacial environments were very different from the Holocene. Much evidence suggests that even modern looking Upper Paleolithic people were outside the envelope of our sample of ethnographic hunter-gatherers. The evidence from the deeper past is lamentably scanty and methodological breakthroughs, most recently the ability to recover ancient DNA, lead to quite surprising findings. The same is true for expanding the archaeological record to new areas like Southern Africa and North-east Asia. The temptation is to produce a coherent and true narrative account of how humans evolved. I don’t think we are yet in a position to do that.
Atkinson, Q. D., Gray, R. D., & Drummond, A. J. (2008). mtDNA variation predicts population size in humans and reveals a major southern Asian chapter in human prehistory. Molecular Biology and Evolution, 25(2), 468-474.
Bocquet-Appel, J.-P., Demars, P.-Y., Noiret, L., & Dobrowsky, D. (2005). Estimates of Upper Paleolithic meta-population size in Europe from archaeological data. Journal of Archaeological Science, 32(1656-1668).
Klein, R. G. (2009). The Human Career: Human Biological and Cultural Origins (2nd ed.). Chicago IL: University of Chicago.
Kline, M. A., & Boyd, R. (2010). Population size predicts technological complexity in Oceania. Proceedings of the Royal Society B, 277, 2559-2564.
Powell, A., Shennan, S., & Thomas, M. G. (2009). Late Pleistocene demography and the appearance of modern human behavior. Science, 324, 1298-1301.
Smaldino, P. E., Schank, J. C., & McElreath, R. (2013). Increased Costs of Cooperation Help Cooperators in the Long Run. The American Naturalist, 181(4), 451-463.
Pete, I actually agree. I think the main mode of multilevel selection in the Pleistocene was not direct competition, war, but indirect competition between groups. In other words, they competed like high jumpers, not like boxers. Groups that couldn’t clear the plank, got eliminated by the harsh environment.
However, once they all got very good at surviving the Pleistocene chaos, they must have increased in density and started impinging on each other. Certainly it happened once the climate moderated in the Holocene. But I don’t see why it couldn’t happen earlier, during the lengthy periods of relatively stable climate.
I guess I am arguing against dogmatism – insisting that there was no warfare at all in Pleistocene. Otherwise, I agree with everything in your post.
How many human skeletons with evidence of weapon-inflicted trauma are known from Pleistocene?
Atkinson suggest that human populations increased irregularly in all over the world after 50,000 ybp. Europe had a sort of plateau around an effective population size of around 10,000 until after the last glacial max.Southern Asia had the largest population in their study, eps around 100,000.
So far, there is no evidence that anything interesting happened in the pre-Holocene interglacials that I’m aware of. This is one of the more troubling problems of interpreting what was going on in the Pleistocene. Anatomically modern people were around in MIS 5, and there seem to have been thousands of years long stable periods. Why didn’t they invent agriculture, evolve states, and so on? If they had, it should be hard to miss in the record. The only thing I can think of is that humans were exploiting culture to adapt to high frequency environmental variation. In quiet times our populations might drop because non-human predators out-competed us. 50 kya, coincidentally or not, marks the beginning of a period of hypervariable climates that may have been what led to the post 50 kya population increase. Maybe we then came out of the last glacial maximum with larger populations and a more sophisticated toolkit than at the end of MIS 6. We parlayed that into plant rich H-Gering and agriculture.
I second your thought about dogmatism. I do think it is useful to have what some philosophers of biology call “how possibly” hypotheses. We can generally tell a range of plausible stories about events in situations where the current evidence is very scanty. They can guide the search for better data. For example, I’d really like to see paleontologists work on trying to get estimates of the population sizes of the large predator guild that preyed up on and competed with us. Did we really get the jump on them after 50 kya?
As far as I recall, the earliest evidence for war inflicted casualties is a find in the Nile River Valley about 14 kya. This would have been during the Allerod-Bolling warm period when climates were as warm and moist as the Holocene. At least in some places like the Levant, people were already getting into plants in a major way. This trajectory seems to have been reversed during the last great cold snap of the Pleistocene, the Younger Dryas cold snap that lasted about 1,000 years from 13.5 to 12.5 kya. For earlier times, truth be told, the skeletal sample we have for the last glacial is pretty poor. Even if warfare was common, our ability to detect it in skeletal samples is, alas, very limited.
My latest essay on these topics is here: http://www.des.ucdavis.edu/faculty/Richerson/Climatewasstrange.pdf
I wonder if it’s possible to prove negative correlation between population density and the level of violence in historic hunter-gatherer societies. As far as I remember (I might be wrong), there was a lot of warfare in the Far North, but the most militant groups (i. e. the Chukchi) were at least partially reindeer herders, and the bloodiest conflicts happened when market-oriented reindeer herders displaced true hunter-gatherers such as the Yukagirs.
@ Peter Turchin
I would imagine that were each struggling independently against the environment and those that succeeded would have grown, while those that didn’t would have been eliminated by the environment. That is, the success of one group would not negatively impact the success of another group; it would be like high jumpers each trying to get a high score, without concern for final ranking. Thus, there would have been no reason for conflict, since holding someone else down doesn’t help you get up.