Commentary for Genesis 6:1

AND IT CAME TO PASS, WHEN MEN BEGAN TO MULTIPLY ON THE FACE OF THE EARTH. When Scripture mentioned Noah and his sons and wanted to begin the account of the flood, it said that as soon as men began to multiply they began to sin, and they continued their sinful ways for many years until Noah was four hundred and eighty years⁴⁹⁰Noah begot children at the age of 500 (Verse 32 in Chapter 5). Twenty years before, G-d decreed the advent of the flood. (See Rashi here, Verse 3.) of age. Then the Holy One, blessed be He, decreed against them that “His spirit will not abide in them forever,”⁴⁹¹See Verse 3. but that He will prolong their years until their measure of sin is full, for such is the way of G-d’s judgment.

ויהי כי החל האדם, a reference to the human species.