Humanity and Climate Change
Human like biped species proliferated over the next 3-4 million years, but their brain sizes stayed pretty close to chimp size (400cc) - roughly 1/4 the size of the modern humans. At that point a period of violent climate change began, with the climate swingly wildly from tropical jungle to near desert over very short periods of time (on the thousand year scale). Lake Victoria size lakes would form and then completely, or nearly completely, dry up. That period is poor in human fossils, but at the end of it a new type of human was making stone tools and packing a double-sized brain (800cc).
Climate change on the scale of thousands or tens of thousands of years has continued apace, and of course modern sized brains emerged 100,000 years or so ago. Meanwhile we seem to have done away with our last remaining fellow human species (Neandertals and Hobbits).
If this view is correct, whatever the disadvantages of climate change, it does seem to have made us versatile and smart.