A lot of things form spontaneously, art, technology, language have all developed spontaneously and independently many times in the past 100,000 years.
Religion isn't special in this regard.
> If you didn't start with that belief, what observation could you make to come to the conclusion that there is an afterlife?
Clearly, lots of observations lead people to the conclusion that there is an afterlife, independent of starting with that belief.
A lot of things form spontaneously, art, technology, language have all developed spontaneously and independently many times in the past 100,000 years.
Religion isn't special in this regard.