Genesis was created by the Hebrew priest in Babylon for an Israelite audience. Without knowing any better they assumed Adam and Eve were the first humans on earth, so they wrote a story about themselves connected to Adam. The ancient story of Adam and Eve was prominent within Mesopotamian lore, the Hebrews grafted it into their all important blood lines and religious authority. It's a fragmented and garbled mess.
The evidence doesn't suggest that the ancient Hebrew writer thought that Adam and Eve were the first humans. There is an indication that Adam and his family were aware of others nearby (Genesis 4:13-17).