science

Looking around the article actually posted, I'd place my bets on more control/restraint on violence, for the coordination to be able to form social networks that could overcome any threats of skirmish level, inter-tribal violence:

Paradoxically, low aggression may have been a massive advantage in intertribal warfare. Low aggression could have helped us to form big social groups – tribes of hundreds and thousands. And modern humans don’t just form huge groups, we’re unique among animals in being able to form peace treaties between different groups, and alliances between groups to defend or attack territory. What made modern Homo sapiens so uniquely dangerous might not have been a tendency towards violence and aggression, but friendliness, and the ability to forge alliances. The ability to create groups and social networks, and hold off fighting – at least, until we’re in a position to win – could have given us a decisive edge.

It's an interesting article, worth reading in its entirety.