Just a theory

[Angrboda]

It's a simple extrapolation of an idea that we are all born with.  We only see the physical form of another person, yet we project the idea of an invisible, immaterial mind behind the face of the other person as a way of explaining their behavior.  We don't view people just as bodies, but bodies which possess minds that move and coordinate their actions.  Minds which we "believe" are the same as our own, possessed of thoughts, beliefs, and so on.  From there, it's a simple step to imagine minds without bodies, such as ghosts, or people continuing on after death.  From there, it's but a hop skip and a jump to imagine minds which are able to make things happen simply by willing them.  And thus gods were born.  They had plenty of reference to immaterial minds with the power to will things to happen prior to the invention of gods.

There's an interesting experiment by Jesse Bering, in which he put on a brief puppet show for children of various ages.  In the puppet show, an alligator eats a mouse.  The researchers then asked the children various questions, such as does the mouse still need food?  Does the mouse still want to go home?  And so on.  From a young age, the children could understand that the mouse no longer had physical needs and actions, but they continued to posit mental qualities to the mouse after its death.  It was only later children who acknowledged that those, too, ended with the death of the mouse.  Interestingly enough, they repeated the experiment with groups that might have a bias towards belief in the afterlife, such as children at a Catholic school, and they found that those children did not give up the belief in the persistence of the mouse's mental attributes after death as readily as more secular children.

As for the first part of your post I disagree, it would be far more complicated than you seem to believe.

How so? Do we not already have the idea of immaterial minds that are able to cause things to happen by willing them?

As for death people understood what death was and at some point someone would have wanted to live forever because of their fear of death, but this in no way could lead to an idea of gods.

In order for people to be able to project the mental beyond death, they must first have the ability to separate the mental from the physical.

In the first part of your post you are basing and assuming from a modern man's perspective, from ideas we have today not what they were missing in the ages past.

No, little children do what you're claiming only comes from a modern perspective. I highly doubt the concept of minds being separate from bodies is a modern invention.

The only reasonable answer is that from the beginning God made himself know to man through Adam and Eve.

That's quite the hypothesis.