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Ancient humans lived for a LONG time, anywhere between 600-1,000 years. Noah lived for 950 years. Meanwhile human pregnancy is only 9 months.

Could be wrong but lets say the time to maturity remained the same as modern humans but the prime of ancients humans were 16-300 years. Imagine how many children could be conceived in the span of 284 years. If ancient humans kept their fertility to 500 years then thats 484 years of multiplying from a single couple.

After only 18 years your first children would be married and multiplying. You’d witness generations after generations grow up in your household over the course of centuries.

Its extremely likely that during the life of Cain and Abel there were multiple generations who spread across the land. The Bible also does not state the number of Adam’s descendants so we may never know how big the first family was before Adam’s death.
 

https://en.m.wikipedia.org/wiki/Serpent_seed

Nah, all humans are descended from Adam and Eve, it skips a few generations from when they were kicked out of the Garden to when Cain killed Abel, and God permitted a certain degree of incest so humanity wouldnt completely die off. So the other people around are also descendants of Adam and Eve, and Cain eventually went on to produce his own race apart from Adam's line that was evil and quickly degenerated into devil worship. Seth's (Adam's son who replaced Abel) line were called "Sons of God" for their piety, while Abel's spawn were called "Daughters of Men" for their wickedness and base nature. It was the intermingling of these two races and the profanation of the worship of God by uniting it to the worship of devils that led to God flooding the earth.

Yes the scary implication is that there were “humans” but not “People”.
I’m not sure of the significance but it’s something to ponder. Also, the New Testament makes copious allegories of bloodlines and trees. And that some are cursed and some are blessed.