The Deaths of Enosh, Kenan, Mahalalel and Yered
Four generations of Shet's line pass in turn: Enosh (1140), Kenan (1235), Mahalalel (1290) and Yered (1422) — the last who still remembered the old world.
Within some three hundred years four generations pass in a row. Enosh dies in AM 1140 aged 905; Kenan in 1235 aged 910; Mahalalel in 1290 aged 895; Yered in 1422 aged 962. All of them lived to see Noach: when he was born in 1056, Enosh, Kenan, Mahalalel and Yered were still alive. The man who would survive the Flood personally knew his great-grandfather's great-great-grandfather.
Yered is the last to go. After 1422, of the ten generations only Methuselah, Lemech and Noach remain — three men in the world whose memory reaches back to the beginning. It is precisely into this gap, between Yered's death and the Flood, that Genesis 6 places the final decay: the «sons of God», violence, «the earth was filled with corruption».
Read in sequence, these dates make a simple point. The Torah does not present the fall as a sudden catastrophe. It lists generations, each living nearly a thousand years and dying, and only after the last witnesses are gone does judgement arrive.