What does Genesis 5:12 mean?

Kenan, the fourth generation from Adam, lived just 70 years before he fathered Mahalalel. In the context of this passage, this seems to be a fairly early fatherhood! Then again, genealogies such as these don't mention every single child. On the contrary, it's said over and over that these men had other children. These are not necessarily a list of firstborn, but are only those in the line from Adam to Noah.

Also interesting about this passage is how the different generations overlap. Based on what's described in this passage, prior to the flood, humanity had continued access to the very first and second generation of man, up until right before the flood. In fact, Noah's father, Lamech, would have been born 56 years before Adam died. In other words, Noah and Shem both lived at the same time as someone who was a contemporary of the very first man.

This is another important aspect of the Genesis genealogies: showing that mankind had an unbroken, clear understanding of their own history prior to the flood.