Many linguists and historians used to believe that human language didn't exist until man started to form social units in the form of villages.  Before that, they theorized, man had no need of language.  They also figured out that since language and art are both symbolic, the start of art could be assumed to be the start of language as well.  Since the oldest art was thought to be about 25 or 30 thousand years old, that was assumed to be the start date of human language.
    Two recent changes have pushed back that date.  First, newer testing methods have shown that some of the ancient art may be ten or twenty thousand years older than previously thought.  Second, some linguists have determined that the basic assumption is wrong.
    What they found, is that human babies learn language much more quickly that they should for such a complex thing.  They also found that many retarded people in mental institutions who can't  feed or dress themselves can still use language to communicate; not efficiently and with proper grammar, but well enough to be understood.  These things have led to the new belief that language is essentially instinctive in humans so has been around in one form or another for many thousands, perhaps even millions of years.
    One problem with this new concept is that one common explanation for the fact that human society seemed to change very little for millions of years, than, in a few thousand years, went from stone age to space age, was that the development of language allowed for the rapid change.  If language has existed for millions of years, than a new explanation is needed.  Either some event occurred fifteen or twenty thousand years ago to stimulate rapid changes or, man has developed before only to be pushed back to a primitive state by major catastrophes.