The Judeo-Christians did their best to find a creation myth in all the "Pagan" mythologies they encountered, and when they found none they just didn't get it and twisted every word of that mythology and in effect created a creation myth for it. They were not able to understand that not everything has a beginning and an end. They didn't fathom eternity.
You might say that we cannot blame them for this, because neither do we - modern men. We can understand eternity, but we cannot really fathom it. Because of that one of the most common non-European notions today is that everything has a beginning and an end. Even amongst good European polytheists we find this primitive idea hammered into their minds. It works as a foundation to all their other ideas and because of that definitely limits their reality.
The Ancient European man was better than us. Yes, we have to admit this; he was better in all respects. He was thus also better intellectually, and unlike us he could fathom eternity. Because of that he created myths that modern man has serious problems interpreting, because he doesn't really understand them. He is locked in linear time, so to speak. He fails to see beyond the limits of time.
As explained in "Sorcery and Religion in Ancient Scandinavia"
the European world view is not linear, it is indeed circular, and thus in fact eternal. It is based on that understanding of eternity that we cannot fathom myth in our mythology. There is no beginning and no end. All births described are mere re-beginnings, so to say; rebirths. What the Judeo-Christians interpreted as "The Twilight of the Idols" or "The End of the World", called Ragnarök, is in fact today. There is no creation ability to think and to see just such a re-beginning; a rebirth of everything in nature and in man too.
Modern man reacts to this and asks; but when did it start? Where do they come from? What came first? It never started; it was always there. They didn't come from anywhere; they were always here. Nobody came here first; everyone and everything exist beyond time, i.e. at the same time, in eternity.
The man who failed to fathom eternity created his own flawed theories, that we best know as "The Creation", as described in e.g. the bible. When the same man a few thousand years later got to know more he created a new and to him better idea; "The Big Bang Theory", which of course is in fact identical to "The Creation" in the bible, save that "God" has been replaced by a "Big Bang", and the seven days it took "God" to create the world has been replaced by "evolution". His mind is still stuck in time.
What we should do, even though we cannot really fathom eternity, is to think as if there was no beginning and no end. So even though we cannot really fathom an endless universe we can base our world view on the understanding that it is indeed endless. Even though we cannot really fathom eternity we can base our world view on the understanding that we live in eternity. There is no beginning! There is no end!
How man came to be is thus a silly question! We didn't! We have always been here, in some form.