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.