There are two life phases that are found in Homo sapiens and not in Neanderthals. One is the period between weaning and the loss of baby teeth. My father used to call that plateau, reaching the age of reason. After it, there were no excuses. We were supposed to knnow the difference between right and wrong. The second period begins at puberty and ends with the halt some years later of skeletal growth. Neanderthals grew up much quicker, so there three year olds look like our five year olds and their thirteen year olds look like our eigteen year olds.
This finding suggests that we are built biologically to learn the cultural ways of an adult even after we have reached sexual maturity. Chimpanzees typically have their first child at age 13, humans at age 19. Apparently Neanderthals were much closer to the chimpanzee style than the human one.
All other things being equal, we would expect the species that starts reproducing at an earlier age to outmultipy the later species, but apparently all things were not equal What was the difference? I can make up an answer, but I would dearly love to know jjust how different the lnaguage of an 18 year old is from a 13 year old.