Did they? I'm more inclined to agree with WJW. These "egalitarian tribes" do not seem to be the conclusion one reaches from an unbiased reading of the historical record.
Think it is a matter of the sliding scale of what we are calling aggression. There are less wars and killing now, there is a clear downward trend in the historical record.
A clear downward trend since when? 1945? I'm not sure you can paint history before then as a clear downward trend in wars and killing. (If you've got data, feel free to point to it.)
It's a funny one, because it depends on how you count. In terms of frequency of wars, we seem to be on a downwards trends, with some historical polities (say such as Rome) engaging in war pretty much constantly.
In terms of absolute casualty numbers, the trend seems to be mostly reversed. That's mostly due to the deadliness of modern weaponry as well as the increased state capacity of the combatants.
On the third hand, casualty numbers as a percentage of total population are probably still down, mostly driven by the very large population growth.
Did they? I'm more inclined to agree with WJW. These "egalitarian tribes" do not seem to be the conclusion one reaches from an unbiased reading of the historical record.