Your argument is profound.
To say that the world is predictable and unchanging for generations is just plain wrong.
One year the winters are mild, the next year extremely harsh. Some summers are warm and mile, some summers are blazing hot and filled with violent storms. Some years winter comes early, some years it comes late.
There are droughts, floods, plagues of insects, hurricanes, tornadoes, typhoons, any number of natural phenomena that make life completely unpredictable; especially so for Neanderthals.
L