Genesis 6:1

‘And when men began to multiply on the face of the ground, and daughters were born to them.'

This is the connecting link with Genesis 5. It assumes a gradual growth in the human race, and thus connects back directly to the descriptions of the growth of mankind there, and especially to the references to daughters. That is the only place, with the exception of Naamah (Genesis 4:22), that we have learned of daughters being born to men.

Furthermore the suggestion of daughters to Noah has probably been deliberately excluded precisely because of the connection with these next verses. So this section is an integral part of the covenant record commencing in Genesis 5:1 b and contains the covenant which is central to this particular record, in a passage that is leading up to the flood. It is not a very pleasant conclusion. It suggests that what is to follow was largely the result of the activities of women, although probably encouraged by their menfolk, which occurred almost right from the beginning, including at some stage the daughters of the line of Seth.