The origins of religion are thought to be rooted in animism and other primitive notions
which our pre-literate ancestors chose to accept as part of the explanation for our
very existence. Archaeological evidence demonstrates that early humans possessed a religious
consciousness going back to the Middle Paleolithic period. This predates agriculture,
writing, science, or civilization. Possibly before our use of tools, fire, or language.
Our early existence was predicated on interacting with nature at it's most rudimentary
level: as foragers and hunters. Acquiring food and securing shelter was the
most primary duty of our primitive ancestors. This quest for sustenance from
our surroundings made it necessary that all forms of plant and animal matter should
be necessarily scrutinized as a possible food source. In this constant search for
edible food sources it was inevitable that early men and women would accidentally
ingest certain plants that contained chemical compounds which would affect their central
nervous system. This accidental ingestion of a hallucinogenic plant (a mushroom?) would
be the earliest record of man's encounter with the numinosum [jungiancenter.org/essay
/healing-power-numinosum] and could of possibly led to the formation of the concept of deity and the supernatural. This imaginary proposition was suggested by the ethno-mycologist R. Gordon Wasson during the late fifties/early sixties.
It does have a logical appeal in the sense that a person scanning nature for sources of
food would quickly single out the unusual shape and appearance of the lowly
mushroom. Given a couple thousand years of random experimentation our early ancestors
would eventually discover and ingest psychoactive fungi that would induce a
hallucinogenic experience and provide the spiritual connection to the numinosum.
Wasson and his colleagues determined that a religious cult, centered around the ritual
ingestion of hallucinogenic mushrooms, existed in the high country of central Mexico
since before the time of the Conquest/Conquistadors. How long before is not certain.
Whether it was the ingestion of a fungus or other plant life there remains a distinct possibility
that the hallucinogenic experience/altered state of consciousness played a direct role in the
formation of transcendental thought. Perhaps the 'religious experience' has it's origin
in the consumption of psychoactive plants by our forefathers.
Knocks on the head aside, I believe the hallucinogenic experience could of been one avenue for our early ancestors to ponder about a supernatural realm. I don't want to imply that it was the definitive starting point for religious concepts but it might have had an influence. As someone who has experienced, albeit many years ago, the effects of hallucinogenic compounds in the form of LDS and psilocybin I can attest to the 'creative juices' that flow when under their influence. The chemical imbalance of one's blood system resulting from these substances allows the mind to explore areas previously never considered.
The effects of psychoactive substances to stimulate our thought processes should not be underestimated. The question is whether any validity should be attached to those thoughts. Regarding the supernatural I think not; there always remains the need for verification
It's also interesting to consider the interplay between what you propose and the theory of the "oceanic feeling" put forward by Romain Rolland (popularized by Freud). The theory suggests that feelings of oneness with the universe are actually remnants from the infantile period in which there was no distinction between the self and the outside world.