Joined: Sep. 2004
There is a huge amount of anthropological data starting from the time Western people encountered primitive peoples that shows that oral language can be quite sophisticated, with a long tradition of knowledge handed orally, and yet there is no written language. Among other things, there is no technology (like material to write on and instruments to write with) present in these cultures.
There is also lots of evidence about how written language was invented, starting with simple pictographs. Written language is one of the greatest of humankind's inventions, but many people in the past (both recent primitive societies and prehistoric people for tens of thousands of years) have lived without written language.