The posterity of Adam lived for hundreds of years and they begat many sons and daughters. So. was plural marriage practiced at this time? Thank you
Prior to Abraham, we only have one recorded instance of polygamy. “Lamech took unto him two wives: the name of the one was Adah, and the name of the other Zillah.” (Genesis 4:19). Lamech was a descendant of Cain and was Enoch’s (yes, that Enoch) 5th cousin. That is, both Enoch and Lamech were “the seventh from Adam”(Jude 1:14). Whereas Enoch was noted for his faith and righteousness (see Hebrews 11:5 and all of Moses 7), Lamech was infamous for multiplying Cain’s work of murder and secret combinations (Moses 5:47-54).
Since the Genesis genealogy focuses on lineal descent (the father/son line to Abraham), we don’t get many siblings or cousins. For instance, Cain’s line (Moses 5) expressly mentions “many sons and daughters” for each generation, but only lists one son by name until we get to Lamech. And even with Lamech, the named line ends with his children. The biblical author has drawn the desired contrast and now wishes to move along to Noah and then to Abraham. The same is true of Seth’s line (Moses 6), bearing “many sons and daughters”, but only listing one son by name for each generation until we get to Noah.
I bring up the narrow scope of the genealogies in the books of Genesis and Moses to point out that we don’t know for sure whether polygamy was a common practice of the time or if Lamech was an anomaly. We don’t know if it was only practiced in wickedness until it was finally granted to Abraham, or if it was something God encouraged so the earth could fill the measure of its creation. The only reason that we know that Lamech had two wives is because 4 of his sons are listed and they had 2 different mothers. So canonically we only have Lamech practicing polygamy, but we leave ourselves open to the possibility that others practiced it as well if a fuller genealogy is ever revealed.