Remove Christianity and civilization would soon smell better.

[Confused Ape]

The discovery of the alter states of kind by early people isn't hard to explain at all. Hypnosis is how it works, and when some people are hypnotized and then left alone it can induce visions based on the stimuli in daily life. In fact we often self hypnotize are selves to some degree when are falling asleep hence sometimes your leg may randomly twitch or something like that.
Now some people are more easily hypnotized then others and will fall in a trance much faster. So what likely happened was someone was going to sleep one night and slide into a trance state that induced visions. Combine that with ignorant hunter gather peoples and the human predisposition to see patterns and Viola! You have Shamanism.

According to a fascinating article I found on Psychology today, we aren't the only animals to go after altered states of consciousness.

Animals n Psychedelics: Survival of the Trippiest

"In his 1983 book, From Chocolate to Morphine, University of Arizona physician Andrew Weil points out that children spin in circles to change their consciousness, while adults do the same thing with booze and drugs. So instinctive does this behavior appear that, Weil suspected, perhaps humans aren't the first species to actively pursue altered states . As it turns out, he was correct in his suspicions. In 2006, Jane Goodall and Marc Bekoff visited the Mona Chimpanzee Sanctuary in Spain. They met a chimp named Marco who dances during thunderstorms with such abandon that, as Bekoff explains it: "He appears to be in a trance."

I find this next bit of chimp behaviour even more interesting.

Goodall has witnessed other chimps, usually adult males, enacting the same rituals near waterfalls. According to an article Bekoff wrote for New Scientists : "She described a chimpanzee approaching one of these falls with slightly bristled hair, a sign of heightened arousal. ‘As he gets closer, and the roar of the waterfall gets louder, his pace quickens, his hair becomes fully erect, and upon reaching the stream he performs a magnificent display close to the foot of the falls,' she describes. ‘Standing upright, he sways rhythmically from foot to foot, stamping in the shallow, rushing water, picking up and hurling great rocks. Sometimes he climbs up slender vines that hang down from the trees high above and swings out into the spray of the falling water. This ‘waterfall dance' may last ten to fifteen minutes.'" But dancing, while an effective method for altering one's consciousness, is perhaps the long way round.

Humans and chimps shared a common ancestor around 6 t0 8 million years ago so maybe the common ancestor did this kind of thing, too. Humans have been throwing offerings into water for thousands of years and it's a common practise to throw coins into wishing wells today.

In October 2006, National Public Radio's All Things Considered considered Lady , a Cocker Spaniel spending a suspicious amount of time down by the backyard pond. "Lady would wander the area, disoriented and withdrawn, soporific and glassy-eyed," Laura Mirsch, Lady's owner, told NPR. Then there was that one night when Lady wouldn't come back. Eventually, she staggered back from the cattails and opened her mouth like she was going to throw up. She didn't throw up. Instead, recalls Mirsch, "out popped this disgusting toad." The toad was Bufo alvarius, a Colorado River toad whose skin contains two different tryptamines-the same psychoactive found in "magic mushrooms"-and licking Bufo produces heady hallucinations.

Birds do it, bees do it and maybe even educated fleas do it.

And toad tripping dogs are just the beginning. Everywhere scientists have looked, they have found animals who love to party. Bees stoned on orchid nectar, goats gobbling magic mushrooms, birds chomping marijuana seeds, rats on opium, also mice, lizards, flies, spiders and cockroaches on opium, elephants drunk on anything they can find-usually fermented fruit in a bog hole, but they're known to raid breweries in India as well-felines crazy for cat-nip, cows loco for loco grass, moths preferring the incredibly hallucinogenic datura flower, mandrills taking the even stronger iboga root.

As there are entheogens in Africa where humans originated it's likely that our distant ancestors were tripping out on them as well. The human brain can also do odd things all by itself, including when people are awake. As I've said in other posts, my own brain produces odd, subjective experiences such as feeling disembodied presences and seeing ghosts.

Anomalous Experiences

Anomalous experiences, such as so-called benign hallucinations, may occur in a person in a state of good mental and physical health, even in the apparent absence of a transient trigger factor such as fatigue, intoxication or sensory deprivation.

It is now widely recognized that hallucinatory experiences are not merely the prerogative of those suffering from mental illness, or normal people in abnormal states, but that they occur spontaneously in a significant proportion of the normal population, when in good health and not undergoing particular stress or other abnormal circumstance.

The evidence for this statement has been accumulating for more than a century. Studies of benign hallucinatory experiences go back to 1886 and the early work of the Society for Psychical Research,[1][2] which suggested approximately 10% of the population had experienced at least one hallucinatory episode in the course of their life. More recent studies have validated these findings; the precise incidence found varies with the nature of the episode and the criteria of ‘hallucination’ adopted, but the basic finding is now well-supported.[3]

One very common experience is the grief hallucination.

the experience of sensing the presence of a deceased loved one is a commonly reported phenomenon in bereavement. It can take the form of a clearly sensory impression or can involve a quasi-sensory 'feeling' of presence.[30] conducted a study of 293 widowed people living in a particular area of mid-Wales. He found that 14% of those interviewed reported having had a visual hallucination of their deceased spouse, 13.3% an auditory one and 2.7% a tactile one. These categories overlapped to some extent as some people reported a hallucinatory experience in more than one modality. Of interest in light of the previous heading was the fact that 46.7% of the sample reported experiencing the presence of the deceased spouse. Other studies have similarly reported a frequency of approximately 50% in the bereaved population.[31][32]

Sensing the presence of the deceased may be a cross-cultural phenomenon that is, however, interpreted differently depending on the cultural context in which it occurs.[33] For example, one of the earliest studies of the phenomenon published in a Western peer-reviewed journal investigated the grief experiences of Japanese widows and found that 90% of them reported to have sensed the deceased.[34] It was observed that, in contrast to Western interpretations, the widows were not concerned about their sanity and made sense of the experience in religious terms.

Grave Goods

In Homo sapiens burials beginning about 100,000 years ago, the body of the deceased was sprinkled with red ochre, and offerings of food, tools, and fresh flowers may have been deposited in the grave.[3]

Beads made of basalt deposited in graves in the Fertile Crescent date to the end of the Upper Paleolithic, beginning in about the 12th to 11th millennium BC.[4]

The distribution of grave goods are a good indicator of the social stratification of a society. Thus, early Neolithic graves tend to show equal distribution of goods, suggesting a more or less classless society, while in Chalcolithic and Bronze Age burials, rich grave goods are concentrated in "chieftain" graves (barrows), indicating social stratification.[5]

The importance of grave goods in archaeology cannot be overestimated. Because of their almost ubiquitous presence throughout the world and throughout prehistory,

It's likely that hallucinations of people wandering around after they'd died led to the belief of an afterlife where they would need possessions. The real problems began when humans invented theology and organised religions.

Richard Dawkins asked an interesting question towards the end of God On The Brain.

DAWKINS: If you ask the question 'what's the survival value of religious belief?' it could be that you're asking the wrong question. What you should be doing is asking what's the survival value of the kind of brain which manifests itself as religious belief under the right circumstances.

The documentary ends with the following -

NARRATOR: What is beyond doubt is that the origins of religion are even more complex than had been thought. The science of neurotheology has revealed that it is too simplistic to see religion as either spiritually inspired or the result of social conditioning. What it shows is that for some reason our brains have developed specific structures that help us believe in god. Remarkably it seems whether god exists or not, the way our brains have developed, we will go on believing.

DAWKINS: The human religious impulse does seem very difficult to wipe out, which causes me a certain amount of grief. Clearly religion has extreme tenacity.

NEWBERG: Because the brain seems to be designed the way it is, and because religion and spirituality seem to be built so well into that kind of function, the concepts of god and religion are going to be around for a very, very long time.

So, back to the topic question. If Christianity was removed, the majority of ex-believers would find something else. Quantum mysticism is a fairly harmless belief system at the moment but it could change if it attracted a lot of fanatics who insisted that their interpretation was THE TRUTH and everyone else's interpretation was heretical.