Between 3 million & 200,000 years ago the average cranial capacity of this planet’s dominant hominids increased along a upward trendline, in starts and stops. Bipedal apes went from having nearly chimp sized crania to one similar to modern human beings (Neandertals had larger brains that H. sapiens sapiens). Symbolic culture as we know it though seems to really explode (a.k.a. “The Great Leap Forward”) between 50,000 and 30,000 years ago. There have been many books on these issues. The Mating Mind, The Prehistory of the Mind and The Dawn of Human Culture hit many of the controversies from a popular angle. Nevertheless, what do you think happened??? Was it climate change (I’m skeptical)? Was it some sort of cultural ratchet? Was it God?
Update: Brown guacho has a long post with graphs and pictures. Highly recommended.