evolution of the human species

ok, so in 1999 i saw disney's "tarzan" in theatres with title track "you'll be in my heart" by sting, and tonight is saw francois truffaut's "l'enfant sauvage". what these films have in common is a young removed from society living in the wild from a very young age. suddenly civilized man comes in and interesting moral questions are raised. what is "civilization"? what is "man"? my question is what is homo erectus. both these characters insist on walking on four legs. now if i'm not mistaken homo erectus came before homo sapien. why should non verbal homo erectus walk on two legs while non verbal homo sapiens tarzan and victor do not? for tarzan there is at least some explanation because he grew up with gorillas and persumably he learned to walk from them. but for twelve year old victor there is no excuse. why is he not walking upright? i don't know. francios, please enlighten me.

(homo habilis, erectus, sapiens neathderthalis, sapien)