it would not be clear that modern caribou have not been selectively bred for this purpose. Likewise, it's not clear that ancient humans would have understood th

it would not be clear that modern caribou have not been selectively bred for this purpose. Likewise, it's not clear that ancient humans would have understood this behavior. The invention of the "animal pen" would have involved persistent living in an area, as well as the ability to construct pens out of wood/stone. The latter material would not have been as available without ice age drop stones.