wow, some wild responses in my thread.

It's entirely possible that if the ancient aliens theory is anything close to what really happened then maybe some terraforming of earth was done and some genetic engineering of us.

The idea of a quarantined earth is interesting too. I don't know about toxicity but in as far as our human culture relates to itself and the other lifeforms on this planet, I totally agree that extraterrestrials would not want us spreading ourselves beyond the earth before we reach a semblance of maturity.

It's impossible to say for sure what could have happened a thousand years ago let alone 300 million years ago. I think it's also possible that we have never encountered an alien but maybe a civilization developed here on earth that was very very advanced and they were able to achieve space-flight and leave the earth. They could have built outposts on the moon and mars and the the earth was rocked by natural disasters and the advanced humans came back to find very few survivors living the life of tribal hunter-gatherers. These advanced humans would have changed anatomically by living in reduced gravity they would be giants. They would be human though. This is how the stories of the fallen angels mating with women. How could they mate and have offspring unless they were close genetically. Being genetically very similar but slieghtly different could have caused a hybrid vigor which made the nephelim offspring superhuman leading to legends of demi-gods.