“Early humans were genetically perfect, thus inbreeding was a good thing for them.”
Would only take a few generations for imperfections to crop up, however—which is why law against incest was written. It is notable though, there there were no laws on incest until Moses, about 1,500 BC. Abraham himself married his half-sister.
It is also interesting that Abraham’s nephew, Lot, was condemned....for sex with his own daughters (giving rise to the Moabite & Amonite nations)—so certain forms of incest were known as wrong (at least parent/child incest).
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By the time the Law was given, Hebrews had been mixed with the sinful seed of Ham in Egypt.
That was why it mattered.
I heard a talk a long time ago about brother-sister marriages in ancient Egypt (I think from the period when it was part of the Roman Empire). The speaker argued that the harmful genes producing defects would be eliminated from the population because of the close relationships—two carriers would marry and produce non-viable children or badly deformed children and the bad gene would gradually disappear from the population.