What does Genesis 5:10 mean?

After fathering Kenan at the age of 90, Enosh lived a further 815 years. We know from Genesis 4:26 that it was during Enosh's lifetime that people began to call on the name of the Lord. He likely passed this down to Kenan and his other sons and daughters, along with the likeness of God he had inherited from his father.

By modern standards, these lifespans are extraordinary. However, modern science tells us that most living creatures are capable of living, in theory, much longer than they do. Environment, genetic decay, and other factors limit how long people can live. Early in the history of mankind, these factors would have been much friendlier to life. This would have made extensive lifespans much more probable.

These enormous numbers make it easy to miss the more relevant point, one made over and over in this passage: all of these men died. Long or short, each man's life is destined to end, one way or the other.

Another interesting effect of these long lives would have been the value of oral history. In modern times, eyewitnesses are at best available for several decades. In these earlier days, however, it would have been possible to speak with many people who had seen events of centuries past. This huge overlap of generations would have made it all but impossible for historical truths to be lost.