Genesis seemed like it was two stories mashed up to one.

One had a mass of people already created, like you said but another story where Adam is first person.

My punishment is greater than I can bear! Today you have driven me away from the soil, and I shall be hidden from your face; I shall be a fugitive and a wanderer on the earth, and anyone who meets me may kill me.

People already existed or is it assumed Eve populated the world in some way?