Evidence seems to indicate that the size of the human brain has been the same for 70,000 years.
It is interesting to think about the implication that we humans have been *equally intelligent* for all this time and the only difference between us and them is that now, with writing and other information storage and communications abilities, we can build on already-discovered ideas and not have them die off in some tribal war or other death of an oral tradition.
It makes you look at all that archeological evidence of “advanced” technologies much differently.
To some degree I think you can make the point that as much knowledge has been gained over time, there is a significant amount that has also been lost. Except that most of the time we’re more aware of what we have gained then we are of what we have lost because of course you don’t know what you don’t know :)
It seems to me that we've probably only retained a small percentage of the useful ideas that humans have ever had. So many individuals have so many clever ideas (well, not everyone ;0) that nobody ever hears about.
The classic example is the Library of Alexandria and what was lost when it was destroyed with all its scrolls from the ages past. There are some theories out there that diminish the significance of the loss but I'd sure like a chance to read whatever was there!