What was the most important invention from the last 1,000 years?  A strong
argument can be made for the Guttenberg (sp?) movable-type press.  Why?
Most if not all great advancement in human knowledge have been made by a
very, very small percentage of the population.  The industrial revolution
wasn't the result of a suddenly smarter population, but of the ability of
humankind to preserve and build upon the knowledge of the few via written
language, rather than re-inventing the wheel over and over again.  Maybe
there was no need for written communication within small hunter-gatherer
tribes, nor even among loosely affiliated groups of tribes.  I'd guess
written communication developed only after population densities passed the
point where inter-tribal bartering became viable, probably as a budding
merchant's account ledger, and he was probably selling mead or beer.

Personally, I find the romanticized view of hunter-gather life appealing.
But with no medicine to speak of, and with large animals to compete with,
I'd guess it was usually a short life, if not nasty and brutish.  Perhaps
this was enough by itself to hold down population density for 30,000 or
90,000 years.  I'd bet the great Leap Forward had lots to do with the
ability to record knowledge and to disseminate it.  The easier this became,
the faster we leapt forward.  I'd also bet that if you could magically take
away our ability to record and disseminate knowledge, we'd be hunting and
gathering again within two or three generations.

For what it's worth (no, I don't have references; I making all of this up).

Robert