When the Bible was written, the world did not have an understanding of science as they do today. I agree that much of the Old Testament is mythical but there are also parts that were misunderstood by the observers. Heaven is not some place in the clouds with angels flying around with wings. Heaven is what we now know as space and the rest of the Universe. They would interpret anything that came from Heaven (Space) as angels or gods or even demons, but they were really visitors from distant worlds. Our creators were really advanced lifeforms that were thousands to a million years ahead of us. They had probably been seeding other planets with life for quite some time before they reached Earth. They had known the genetic code for quite some time and were knowledgeable in genetic engineering.
Along with this, there is intelligent design that created the Universe to begin with. The creation of the universe did not happen by chance. There is a Creator who planned and set up the physical laws that govern the universe and even created the DNA code by which life is formed. The Creator originally produced the matter that would form the Universe and yes, there was a "Big Bang" as that was when the Creator put all of this into motion.
So there are two realms, that being the physical universe that we inhabit bodies in and the spiritual realm where the Creator resides and where all intelligent lifeforms started out.

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