One can imagine that, very early Man, in spite of habitually moving upright on 2 legs, behaved no differently from the animals when best by awesome, terribly frightening thunder and lightning accompanying heavy rain. As well, there may have been devastating explosions caused by a few deadly sporadic impacts by meteors, or even larger lumps of cosmic rock.
For instance, reportedly, there is a vast vertical tunnel about 20 kms deep into the sea adjacent to Chile. That might explain the sudden rise of the Andes mountain range; as well as the presence of sea shells at the top of this range. How would mankind – early or advanced – cope with that, or similar bombardments from the sky?
Did the feelings of awe and fear lead, somehow, to attempts to propitiate those imagined to be responsible for the clamour and catastrophes continually cast upon them? However, to conceive of an act of propitiation would surely require a brain far superior to that of Early Man. The latter may have evolved from, or been based upon, our alleged near-cousin in the animal kingdom.
Could random mutation somehow expand the conceptual capacity of the human brain? Or, did the extra-terrestrials who had introduced the extra 223 genes into Man been responsible? Or, did a cosmic cataclysm, in the form of an exceptional burst of radiation, alter the human brain, increasing its capacity for conceptualisation? This issue has nothing to do with brain size.
An interesting thought: did not THE Adam (refer the Bible), whether created, wrought, or evolved, have to learn about morality, about right or wrong behaviour? It does take a certain level of brain function to learn, as evidenced by animals (as well as people, of course) being trained. For instance, I trained a German Shepherd pup in 2 months, but failed with a pedigreed pup of another breed over 4 months.
Did the eventual move from propitiation to a full-blown religiosity require a further development of the human brain? If so, what were the physical causes? Or, were there social relationship causes; that is, learning to do things in a collective manner?