Rate of Innovation in Material Culture

It has become very clear to archaeologists that—based on changes in stone tool technology over the last 2.5 million years—the rate of innovation in material culture has been constantly accelerating. For the first 2 million years’ change was almost non-existent, with the only major innovation being the appearance of hand axes about 1.8 million years ago. Then, prepared cores first appeared around 400,000 years ago, followed by blade technology, which first appeared around 100,000 years ago (but didn’t become widespread until about 40,000 years ago). After this the rate of change accelerated very rapidly. Can you think of various explanations for why things appear to remain unchanged for so long and then begin to change at a faster and faster rate?