One similarity between Souls and humans is that they both crave living on Earth. The humans have no other place to live and while their destruction of Earth has helped to justify the souls from taking it over, they do love living on Earth, as evidenced with the resistance movement against the Souls. The Souls love Earth, themselves, believing that their takeover of the human beings is justified because humans have done much in way of destroying Earth.
Another similarity is that they both need the human mind in which to live. This ends up being the basis for the mind that Wanderer and Melanie share with one another. Both seek to live in the human mind and the battle for supremacy indicates a desire for the mind that both the Soul and the human share. Finally, a similarity that both the Souls and humans share is the propensity for experiencing human emotions. While most of the Souls seek to wipe out the human consciousness of the past, Wanderer shows why this is the case. The Souls and the humans both can experience emotions that cause attachment to being in the world. The fact that both Souls and humans battle for supremacy in the world is a reflection of the similarities that both share.
I think , Aliens are what we called GOD, and Human are workforced produced by Aliens.
( The following are all based on my personal opinions okay)
Before humans, there were also early humans existed. When the Earth went through its cycle of glaciation. These early humans , who had lived on the Earth for centuries, either moved out from the Earth with some sort of space flying device ( just like our rocket), the remained one were dead and burried, or perhpas they did not even evolved skeleton or bone strucutre at all.( So nothing remained)
So after centuries, they came back to the Earth , knowing the Earth had again recovered from it's glaciation period or dark era whatever. They came for resources, to do that because they could no longer used to the atomphere composition or gravity , they used the technology ( cell clone) , with DNA engineering . Modified and made something with lower intellegent from their own cast. ( We had big brain capacity, but little percentage is used)
Speaking of resources, one possible raw material might be Gold, magnetic force...etc ( Those rare radiactive elements) , Just take look at Gold, it is something that people have been searching for without knowing the exact reason. If calling it a rare and limited element , thus gave its values to be collected. I guess, there are way more elements from the periodic table we can collect. But why people have bee mining gold since the stone age ( Some show introduced the evidence of the earliest humans already taught of using complicated tool for refining gold and mining goal before. ( I think the property of Gold is what we should be considered rather than looking at its shinny appearance)
So, aliens came back to the Earth, and speed up the evolution by combining their own genes into our Chimpanzee , booom. they made " Humans" ( so some part of our genes are same) ..people started to evolved and leaped in civilization that was taughted by aliens so we could help them to build devices and work as labors until everything was prepared, they then left.