The best theory I have read is that the rise of the Isthmus of Panama and the periodic opening and closing of the Strait of Gibraltar caused the climate in Africa to experience wild swings from jungle to savannah. The stress of having to adjust from trees to savannah and back to trees favored the development of intelligence and tool use. Animals that didn’t have the potential perished in large numbers. Those that could handle the swing evolved.
I hold that hominids who eat WATER insects - crab, shrimp, lobster, etc. - have the biggest brain HP.