one might note that as recently as 12,000 years ago the seal level was four hundred feet lower than it is today. Today 80% of the worlds population live within 100 miles of the seacoast, that percentage approached 100 percent before there was modern transportation. Is there any reason to think that people twelve thousand years ago were any different.

In other words....there were likely civilizations that prospered for eons...on the continental shelf that is now four hundred foot underwater. Doggerland comes to mind.

How do we know what they could or could not do?