There were lots of other people around when Adam and Eve were born.
But Adam and Eve were the first 2 people with a human soul.
That is, they were the first 2 spirits who were in a covenant relationship with God to have their souls redeemed by the blood of Jesus.
The other creatures, "almost human souls", did not have knowledge of good and evil as Adam did. That means that they didn't need redemption for sin as Adam. Animals have spirits, but do not need to be redeemed for sin as the human souls in the covenant do.
When Cain left Adam he married into one of the many tribes on the planet that did not know or worship Jehovah.
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