No one really knows.
Ancient peoples appear to have had some sort of religion. Neanderthals buried their dead with flowers. Babies who were either killed because they were born too soon after a mother had just had one, or who had died, appeared to have been more than just put into a hole.
Gods and shaman have played roles in every tribe, in every society, and each made sure that he held power over the others in the tribe.
Christianity is only 2000 year old... a mere blink in the life of humanity.
And, you are assuming that Christianity is the only true religion. But any follower of any religion that has ever existed has always felt that theirs was the only true religion...
In a word or two, same dog, different collar.
And as an obtw, few assume any deity created the universe.
You'll get some intelligent discussion even on Yahoo Answers.
(Tho usually those occur in the evenings. Teens and housewives with children usually are on during the day.)
Science may not know every answer. (such as "What did indeed happen before The Big Bang? And were there other Big Bangs before the last one? ", But as well, few of them shunt unknown answers off into "god did it.'
For a scientist, an answer "we don't know yet." is perfectly okay.