As a rule of thumb, I only give six stars to non-fiction books that have significantly changed my world view in some way. Using that rule, this book deserves all six stars. I always wondered why, in the past few millennia, there have been several periods of hundreds of years with seemingly no progress at all. This book offers the view, and I see no possible way of denying it, that those "quiet" years were actually critical in preparing humankind for the following advances. So what happened in those times? In short, various diseases and human populations both adapted, changing those diseases from epidemic to endemic, and altering the future of the world radically in the progress. For more details, read the book, which you should do anyway. It's a bit repetitive in places, but it's bearable.