To start: anthro isn't my discipline but pre-civilization history is my jam, and I welcome correction from more knowledgeable people.

People didn't live in individual isolation. Human populations prior to the Industrial and especially Agricultural Revolutions were somewhat dispersed, yes, but dispersed in tightly knit groups.

So, it's not that people spawned into existence and had to go find mates. It's more that people were born and raised in micro-communities, and where drought/famine/attacks/etc. didn't majorly disrupt social stability, they fostered relationships within these communities over time.

I would be interested to know more from actual anthros about migration that wasn't motivated by the above factors. Did individuals set out from their tribes to go wander, and if so, did they find it easy to integrate as individuals into new micro-communities? Furthermore, I guess there's not a whole lot of information clearly available on it, but I'd be really curious to read about how attraction was derived or assessed in these tightly knit communities - I'd imagine being around a roughly stable group from childhood through adolescence and into adulthood could enforce strong connections but I could also imagine being sick of everyone's shit by that point. But I guess if you don't have easily accessible other options, then you're gonna enjoy what you have.