The time from roughly 15,000 to 5,000 BCE was a time of transition, and swift and extensive environmental change, as the planet was moving from an Ice age, towards an interstadial (warm period).
Sea levels rose dramatically (and are continuing to do so), land that was depressed by glaciers began lifting up again, forests and deserts expanded, and the climate gradually became more modern.
In addition, the Pleistocene megafauna became extinct due to environmental and evolutionary pressures from the changing climate.
This marked the end of the Quaternary extinction event, which was continued into the modern era by humans.
Meltwater pulse 1A raises sea level 20 meters.