The fact that many creation stories from around the world have been preserved (see here) shows that there is something behind them all. It shows us that something happened at the very dawn of time that human beings were aware of and considered important and tried to pass on.
Now imagine trying to preserve a story for thousands of years. How would you do it? If you wrote it on paper, the paper would disintegrate over time. If you inscribed it on stone, the stones could be buried over time. If you taught it to your children, how would you know if they passed it on correctly? All this and language changes so that the story might be misunderstood.
That is to say it’s not surprising that there are many variations of the creation story. It’s impossible now to combine them into one account because they are so different. For example, the role of water in the various stories makes some of them creation stories and some more like flood stories. How can we ever sort this out?
When you read of the creation and flood in Genesis, you can start to see how the other creation stories are all related to it. Both creation and flood are covered but in separate narratives. Instead of rivalries between purported gods, there is the orderly creation by the divine creator of a universe that we know is very ordered.
There are unique aspects to the Genesis account, too. This author notes several:– The Identity of God, No Rival Gods, Creation out of nothing, The Value of Humanity, and The Sabbath.
What about creation stories that are older than the Bible? Writing was not invented in the early days of humanity so stories were passed on orally. It’s of little significance which story was written down first. What counts is the reliability of the source, whether oral or written. The Bible has been zealously preserved for thousands of years. Other written accounts languished in caves and underground, forgotten and rejected.
To preserve a story for thousands of years you’ll need a group of people dedicated to preserve it intact. The Hebrew people have a well-deserved reputation for preserving their oral and written traditions. They (and later the Christians) have been fanatic about preserving the Bible over thousands of years. The Bible contains the original story.
Where does that leave ancient mythologies? They are a combination of corruptions of Genesis and legends about people such as Adam and Noah that sometimes made them into gods or superhuman beings. They indirectly reflect what actually happened, of which the key events are preserved in Genesis.