recognition of "self" and "non-self"
at the level of groups
There is a societal "immune system" that isolates one group from another - so, a group can quickly distinguish between members and non-members and make evolutionary significant decisions.
In humans, there are instinctive mechanism of language that serve recognition of group identity - and this serves aposyndesis, the binding together of some individuals away from others.
The concept of a quick verbal test of belonging is exemplified in a story in the Bible that is well over 2000 years old. Note: one usage of the Hebrew word “shibboleth” was to refer to flowing water, such as possibly the Jordan River. In Judges 12:
“5 And the Gileadites captured the fords of the Jordan against the Ephraimites. And when any of the fugitives of Ephraim said, "Let me go over," the men of Gilead said to him, "Are you an Ephraimite?" When he said, "No," 6 they said to him, "Then say Shibboleth," and he said, "Sibboleth," for he could not pronounce it right. Then they seized him and slaughtered him at the fords of the Jordan...”News from a shopping mall in Kenya in September 2013 illustrates some of the concepts that much of religion serves for aposyndesis: binding Us together away from Them.
“Men, women and children were lined up and then gunned down with AK-47s after failing to name the Prophet Mohammed’s mother or recite passages from the Koran – sure-fire proof they were ‘kafirs’, or non-believers.” September 2013.
Ordinarily, the mechanism is more subtle.
Aposyndesis operates in (most) every human (most) every day.
Other social species, including insects, have signalling mechanisms serving aposyndesis.
Aposyndesis in Even Earlier Stories
The main thrust of aposyndesis in the evolution of humans (and other social species) was a subject of stories that were already ancient 2000 years ago:
We see in Josephus elements of aposyndedic fission, the reproduction of a social group. There is a flourishing of adolescents who act together in trouble-making ("seditious") groups. An existing group spawns another group, consisting mostly of adolescents, that leaves its home village to found a new village. From The Book of Judges, we see that it was understood that even a simple difference in language between two groups could result in antagonistic behavior of members of one group towards members of another.
We see that a version of the concept of aposyndesis was embedded in myths that existed long before the modern theories of evolution.
It is easy to speculate beyond the current reach of empirical knowledge and pile hypothesis upon hypothesis, hoping that someday someone will provide more empirical and theory-embedded evidence.
Every act of aposyndesic fission is preceded by a differentiation of two sub-groups via language. Parents, in well-fed societies, see elements of this in their children who, entering adolescence, begin to use words and phrases totally unknown to the parents. Being as how we are awash in this behavior, it ought to be easy to gather relevant data and form elaborating hypotheses.
The impulse to go forth and conquer territory was demanded by God, it seemed. People were separated (as well as bound together) by their languages and, via their groups, spawned colonies that pushed outward into new territories. Usually this meant pushing someone else off his land. The genes that had groups pushing most successfully, won. So, God was making evolutionary sense.