This prehistoric dead ringer is simple but fundamental because the English idiom to “stick in” at something means to keep going at something, to keep trying at something or to keep keen at something, and the dead ringer is that in prehistory a lot of sticks were used for hunting and fire etc. Also, the English idiom “stay sharp” means keep alert or be clever or be intelligent and the dead ringer is that in prehistory, sticks and blades were kept sharp.