About 100,000 years ago human beings began to evolve in a way that was not true of any other species - our social practices began to replicate, mutate and accumulate just as genes had been doing for billions of years. The key was our own exceptional neural plasticity, which significantly increases our adaptability. It is the foundation for the skills which are unique to us such as language, self-awareness and self-control. The term we use for the ability to pass on social skills such as collaborating and communicating via speech or Facebook is a very simple one: culture.
In Darwin's Cathedral , David Sloan-Wilson offers a very persuasive example of cultural evolution with respect to religion, one of the most important cultural phenomena of all. He describes it as an adaptive product whose function is to facilitate co-operation in larger units. Religion, Wilson insists, appears to be 'designed' to minimize internal competition within groups and maximize the competitive advantage of a particular group. 1 It establishes social norms as well as penalties against those who break them; it also promotes group selflessness - in binding people together it makes them put the group's interests ahead of their own.
The same dynamic would appear to be also operative in the case of war. Like religion it had adaptive value - it protected us against the murderous in-fighting of tribal groups and it helped us to avoid characterizing out-groups as non-human, in contrast to the characterization of other tribes. The names that hunter-gatherer tribes call themselves tell their own story - 'the People', 'the Good Ones' or 'the Fully Complete Ones'. Primitive warfare is particularly murderous, for that reason, and mortality rates are astonishingly high. Some of the battles in hunter-gatherer societies have a death rate of 0.5% of the population per year. If that had been true of industrialized warfare in the twentieth century 2 billion people would have died instead of the 100 million who actually did. 2 Had that dynamic been maintained over the centuries you probably wouldn't be reading this book.
Generally speaking, the more co-operative a species is within the group, the more hostility there is between groups. When there is a very variegated society, such as in New Guinea, which has more than 800 languages, out-group enmity can be fierce. That is why Robert Wright believes that religion was essential for early states to keep war within bounds. The relationship between the two is a dialectical one. The gods (in his felicitous words) were 'geopolitical lubricants' who made possible rudimentary international law; divine authority policed treaties between tribes and later hereditary chiefdoms. The problem, of course, was that the gods also sanctioned war, and still do. 3 Think of the suicide bomber in today's Middle East. The point is that religion is still deeply entrenched in human life. Schopenhauer thought we would outgrow it as we do our childhood clothes, but the failure of the 'new atheism' to make much of an impact suggests that we need to go back to humanity's childhood to see why it still has 'an iron grip' on us all; and if that is true of religion, it is surely also true of war.
Now, of course, this is an unashamedly group selection thesis and it assumes that warfare is the natural state of human existence. Fortunately, however, group selection is now coming back into fashion after being challenged in the 1960s. The neo-Darwinian evolutionists who still reject it, claims Mary Midgley, have simply misread Darwin. 4 And although there are many anthropologists who still question whether aggression has always been rewarded and can often be inherited, here too the tide is turning (Jared Diamond's The World until Yesterday  makes short work of the traditional idea that primitive societies are innately peaceful).