This chart of pre-Flood patriarchs should be fairly accurate, where year 1 equals the year Adam was created.
So Lamech, the 8th generation descendant of Adam and father of Noah, knew the First Man for 56 years or so. This is kinda like the situation between Fry and the Professor in Futurama, except imagine that Fry was awake the whole time making an ass of himself at every family get together and no one can do anything about because he's been the family patriarch for centuries. Incidentally, my number crunching on Old Testament patriarchy also suggests that old man Noah knew young Abraham, who was born in the 1900's. The secrets of Creation could be trasmitted thusly: Adam teaches Lamech, who teaches his son Noah, who instructs Father Abraham. Try to imagine this for a minute from Noah's perspective. Him saying "My dad told me this" is the same thing as "Here's what an eyewitness to the Fall of Man said".
Here's another way of looking at the issue. Below are some historical figures from the 1st millenium AD given longevities identical to the dudes in the first chart.
So Origen spends the better part of 800 years hashing out theology with St. Paul, who runs the church for almost its first thousand years. St. Augustine takes the reins directly from him. Rome never becomes the seat of church power because Alaric sacks it every 50 or 100 years just for fun. In my campaign set in 1139 Mani's faith would still be a threat to Christianity, as he spent almost 900 years organizing his own religion. And who would reign in England? King Arthur (i.e. Riothamus)? Perhaps he retired to devote his time to reading Caedmon's 400 year corpus of poetry, leaving Alfred the Great to contend with Charlemagne always trying to increase his empire.
Only slightly related thoughts: Elves don't sleep each night for the same reason you probably don't nap for 20 minutes of every hour. When you ask an elf "When?" don't expect an answer more specific than "Winter." for the same reason you don't tell people "I'm taking my lunch break at 11:32 and 47 seconds."
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