In order of what I feel like would have been the most likely:
1) Ancient Alien Hypothesis
Now, I’m not sure exactly what the “Ancient Alien Hypothesis” would entail; probes with DNA, actual breeding with Ancient man, a complete reengineering of Ancient man. Take you pick.
I’m just saying that the massive increase in brain size of human beings is unaccounted for and the only thing I can think of that naturally could enhance that would be psychedelics, which may as well be from another planet.