Humans experienced a brain expansion after they developed stone tools to augment their rather pitiful natural endowments. What provoked that shift? Chimps hunt without stone tools so it wasn’t hunting by itself that caused the change. Some 2.8 mya a supernova seems to have gone off close enough to the solar system to shower the planet in iron-60 and probably battered the place with cosmic-rays – chimps and gorillas seemed relatively untouched, but seemingly coincidentally Australopithecus garhi appears 2.6 mya with Oldowan stone tools. Then genus Homo springs forth at 2.5 mya.
Something fishy is going on, but just what can’t be pinned down. If human intelligence, “sapience”, on Earth was so contingent on odd events – like Aaron Filler’s macromutation 20 mya that caused the ape spinal change to preadapt them for bipedalism, a supernova at 2.8 mya, and notable spikes in cosmic-ray flux just prior to other shifts in intelligence – then I’ve got to wonder what other weird events are needed to push alien species towards sapience and not mere animal intelligence, which Earth has aplenty. Perhaps we really are an utter fluke and sapience is a “one in a trillion” event, turning up maybe once per Galaxy. Or Hubble volume?