I heard about Annunaki. They were children of some big dude nephilim. They apparently lived separated from others but were arrogant and even wanted to be worshiped. God didn't let it slide, of course, so they were wiped out during the flood. Many accounts that are left are half truths perhaps even written by people who had remaining knowledge of nephilim after the flood. Unless, of course, some manuscripts survived the flood which would be hard but I guess possible. Anyway, they claim a lot for themselves, those Anunaki, and many people around the world, including those waiting for aliens to appear on 21st of December this year, have been seduced by the mass media into believing this, I'll be blatant, bunch of lies. It's not an attack on Christianity, which is curious enough, but just some manuscripts a few guys found and some dude named Sitchin interpreted "ancient astronauts" from them and poof: there you have some X-Planet, 2012 and they connected the accounts of the flood of different civilizations and ended up with a hoax.
They even claim; guys like Bob Robert Dean; that they personally know Jesus and that they were taken up to second heaven or something like that or were even taken to space by some creatures who claimed to be angels of the Lord.
I find it interesting how this theory aligns with evolutionary theory of creation or theistic evolution in other words. They who believe that those Anunaki were aliens will often tell you that those aliens evolved billions of years before we did and that is the way they explain why are those "aliens" (demons in reality) more advanced than humans in a biological sense. That theory is also being used by those who claim that Roosevelt accident was about aliens or stuff like that. While there might be strange organisms living around they are by no means more advanced than us. I wouldn't wonder if demons could pervert the human genome somehow and make who knows what out of it but they, for sure, can't create anything. They are seducing people by signs and wonders and half truths into believing all sorts of strange things.
It's beautiful that we have the Bible that can definitely discern those "alien" spirits to be of the devil. I know this isn't what OP really asked but I just thought I'd share anyway.
Maybe they did something to genome, who knows, I personally doubt that the reason people were big was because of genome modification but there could be some truth to it.
Apparently the atmosphere before the flood was thicker because of the canopy of water that protected the earth by absorbing UV rays and personally I believe that theory holds water (sorry about that
). People might have been able to grow big too. Or a bit bigger than they are now.
Annunaki, whoever they were, are not that important today. There are some old manuscripts that hold Sumeria to be the oldest civilization but I don't know how anyone could know about it because if they are 7000 years old than they were among the first people who lived before the flood. They certainly wrote seductive lies on their manuscripts about all sorts of fake deities, Annunaki just being one of them.
Jesus is the way, the truth and the life. His word is sharper than any two edged sword. If you read the Old Testament again and try to see what people who believe Annunaki were aliens say, you will see that Annunaki manuscripts are probably a lot younger than Sitchin claimed them to be and that they were perhaps even an attempt to pervert the Old Testament. That stuff is demonic through and through.
God bless!
PS: Bible is NOT based on any other text, it is actually a straw argument. Bible might have been written, that is, Genesis, later than those manuscripts but that does not make it a copy. Do not fall for the lies of zietgiest or whatever you spell it, they are full of the devil.