There is no years given as to how old Cain was when he killed Abel so we have to make a few assumptions from the text and from our own knowledge of population explosions.
Let's say Eve had a child every 2-3 years ..... over 50 years she could have had 150 children plus ...
Some of the older ones would have married the younger sisters and they would have had children dirng that time and would have been moving into different areas.
I could give the example of a family I know of for just 60 years. They had 13 children. they now have 52 Grandchildren and they have another 16 great grandchildren - the eldest great grand daughter is now 24 years old and having her own family .... so great greats are already here - about 4 I think it is now .... and the eldest GGchild is now 7 years old. Give it another 9 plus years and she could be having babies too ..... They are spread between Perth and north Queensland.
That makes 72 direct descendants not counting those who married into the families all in 60 years.
If you look at the records of births in Australia from settlement you can also see how it can happen. Keep in mind that we had about 5 million people at the beginning of WW1, and let's say abotu 1/3 of them had been born and raised in Australia, we are talking about 1.7 million people who were born in Australia in just over 130 years!
As for the Sumarian stories ..... they are in all the cultures around the world so there has been a basic story handed down for centuries ..... When I looked a bit more into it I found that the only reason a person can say the Sumarian records are older than the Jewish is because the Sumarian records were on stone while the Jewish were on skins and were perishible and had to be replaced ......
What if in 5000 years someone comes across some of our records - some on CDs and others on perishible paper that had to be replaced over the years to replace them ..... yet the originals may have been written about the same time ..... the only thing that proves is that the non-perishibles didn't have to be replaced all the time ... not that the records preceded the original information
Hope this helps a bit