Imagine there’s no religion

When you say "tribe" are you literally referring to tribes, or are you talking about people in general? Or are you asking whether there is a religion that doesn't have a deity?

If it's the latter, Buddhism doesn't have a deity, though there are Buddhists who choose to worship deities of other origins (especially Hindu deities, given Buddhism's origins). Siddhartha Gautama was a man - he was a prince who gave up everything he had to find a way to end suffering. He was NOT fat (contrary to many people's mistaken belief - the so-called "fat buddha" is Ho Tei, not the founder of Buddhism) and he was just a man, who never claimed any divinity whatsoever. There are a few people who choose to worship him, but that is their own prerogative, and not central to Buddhism, nor what Siddhartha Buddha suggested in any way. 

I don't know of any - in fact, the more primitive, it would seem the more likely they have some concept of divinity, since it's the easiest way to think of to explain things that seem beyond comprehension. If we were able to bring someone from the past to the present, say someone from two thousand years ago, they would marvel at our mobile phones and think they were magic because it's so far beyond their understanding. (Well, at least until they had a natural explanation for its workings.) So no - I don't know of any tribe/culture/group that doesn't have some sort of concept of divinity. Sorry I can't help.

Scientists