Lions

Why do lions live in prides? To survive perpetual war:

Scientists used to believe that prides—groups of a few to more than a dozen related females typically guarded by two or more males—were organized for hunting. Other aspects of the communal lifestyle—the animals’ affinity for napping in giant piles and even nursing each others’ young—were idealized as poignant examples of animal-kingdom altruism. But Packer and his collaborators have found that a pride isn’t formed primarily for catching dinner or sharing parenting chores or cuddling. The lions’ natural world—their behavior, their complex communities, their evolution—is shaped by one brutal, overarching force, what Packer calls “the dreadful enemy.”

Other lions.

I think the same dynamic drove a great deal of human cooperation; we lived in tribes and then kingdoms to survive wars with other tribes or kingdoms.