Given the scientists mention around 13,000 years ago, that would be about the time of the great northern hemisphere bolide strike(s). It is suggested this hastened the extinction of the very large land mammals, like mammoths, etc. Loss of the large mammals would force the remaining humans to eat smaller items like fish, rabbits, deer, etc. Also if many humans were also killed the survivors might have formed much small bands not able to easily kill the remaining large animals such as bisons.
Time to show the Firestone book.
Oooh, good idea!
The Cycle of Cosmic Catastrophes:
Flood, Fire, and Famine
in the History of Civilization
by Richard Firestone,
Allen West, and
Simon Warwick-Smith