According to Guns, Germs, and Steel, what stage in human development can be called the "Great Leap Forward?"

The answer to this can be found in Chapter 1, pages 39 to
41 in the paperback edition.  The "Great Leap Forward," to Diamond, is a time during
which human beings started to create tools and art that went far beyond anything that
had been found up to that point.

Sometime around 50,000
years ago, this "Great Leap" occurred.  It can be seen in archaeological sites that have
jewelry and standardized stone tools in them.  These sites show that humn beings were
becoming much more sophisticated in intellectual terms.

It
is not known at this point what happened to trigger this great leap, but it is very
clear that something happened to allow people to create things that were much more
sophisticated than anything that had been created up to that
point.