All old world monkeys and apes have opposable thumbs. Some of them still live in trees. So, leaving the wood and hunting by throwing stones does not seem reason

All old world monkeys and apes have opposable thumbs. Some of them still live in trees. So, leaving the wood and hunting by throwing stones does not seem reasonable to me as a reason for thumbs. But maybe I did not understand what is meant by “we”, hence my question.