Ah, I pine for ye days of old...

My original model, gleamed from everything I stole
from everyone else, was a Glacial/Interglacial pump.

It worked like this:

The glaciers grew, sea level dropped and vast stretches
of land opened up along the coast and out to sea. our
ancestors could suddenly walk from continent to
continent, (what used to be) island to island. Our ancestors
spread everywhere, following these new coastlines as if
they were highways. But then...

The the interglacial period would arrive. The glaciers
melted, sea level rose, vast stretches of coast were
swallowed up... links between continents and islands
vanished. This drove our ancestors inland where they
settled, adapted and then later interbred with the fresh
arrivals the next time this all happened again.

It helped that the glacial periods -- the time spent on
the coastline, the beaches -- was significantly longer
than the interglacial periods.

So I saw a pump: Sea level drops, drawing people out
onto to newly formed "Highways." Sea level rises,
pushing everyone back inland...

I'm the first to admit that I'm over simplifying here. That,
breakaway groups had to be pushing inland from the
start, long before even the Quaternary Period (and the
glacial/interglacial cycle) began. This was partly due to
catastrophes, such as super volcanic eruptions, and
partly just happenstance.

I miss the simplicity of the olden days...

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https://jtem.tumblr.com/post/725527049879552000