Let's just say for the sake of argument, there were common ancestors. With which common ancestor would the superstition, and God concept begin with?
I don't know and it's immaterial. If we assume common ancestors, then we can logically conclude that at some point one of them had the concept of god for the first time. If you want to assert that can only have been so if it was given to them by god, then I ask can you show that this is the case? If it could have formed without revelation from a god, then why is it unreasonable to consider that a possibility?
The only concern in the animal kingdom is food, sex, and sleep. Some animal infants are lucky if their parents nurture them instead of eat them.
So logically the further we would go back in the alleged evolutionary chain, the less likely of any concern outside of basic needs and pleasure.
This is again immaterial. Would you agree that humans are capable of having ideas? Would you say it's possible for humans to create complex fictions? If so, then why couldn't a humanity entirely devoid of the concept of gods imagined their way to that concept on their own? What relevance does our ability or inability to pin point when we became capable of conceiving of god have to do with the question of if babies are atheists?
Do claim babies believe in a god?
Do you accept babies are people?
If God didn't exist, I would say the earliest humans would have less thought of a creator than a john in Amsterdam
Sure, but that's your opinion. We know that people can imagine. We know that we can come up with ideas that are either true or false (intentionally or otherwise) and that people can believe ideas, regardless of if they are true or not.
Why is it so implausible a position to consider that early humans anthropomorphised the world around them? We anthroporphise a great deal after all.
As for why they'd take the time, the world was scary and dangerous and brutal. Don't you think the idea that intelligence and powerful beings that bring good and bad events into our lives wouldn't have been more comforting than the idea that disasters were simply beyond their control? At least with a being doing it we can hope to bargain or at least find reason to the events.