Scientists first postulated that a genetic change occurred in our species resulting in increased intelligence. Another theory stated that there was a change in the structure of the brain which accounted for the boom in intelligence. The best explanation I've heard is the fact that technology was stymied by a limitation in population size.
If you live in a small group someone has an idea, there may not be anybody else to offer a follow up to help make that idea a reality. As the population of a community gets larger and larger, ideas are able to shared more freely and change can be achieved. As more and people come together, and communities get larger, evolution indicates that the field of information is greater and new ideas can be synthesized that wouldn't be possible in a smaller population. Right now, we as humans are experiencing emergence.