Possibly not Angels at all.
I am a religious man, I have read the bible, several times, but not much is said about the condition of the earth before, and during, and immediately after Adam and eve in the Garden of Eden. Some speculate that the NEPHILIM weren't actually angels, but beings from outer space come to this earth to help the human race. All cultures, if you look at the pictographs, paintings, writings on cave walls etc. depict what looks like spaceships and alien beings in some form or another. They came down to the earth and intermingled with humans and spread their seed among us. I don't know the exact truth od this, but since God created the universe, worlds without number, It is certainly possible that there are more beings than us in this universe and they could have visited earth.
The earth is about 14 BILLION years old, we only have the history going back about 5000 years. What happened on earth before that??