Literally taken, the first twelve chapters of Genesis could have been written by just three generations (Adam > Methuselah > Shem), all who overlapped each other more than 100 years and lived 500 years after the flood. Even without written language, it would be pretty easy to convey verbally, per most primitive cultures. The remaining 42 chapters Genesis would require only one or two additional generations ending with Jacob (Israel) and son Joseph.*
Also read literally, lifespans of Antediluvian peoples were 900+ years, while no one born after lived half that. In fact, Noah outlives his next seven generations, and his son, Shem, the next ten. I've never found any literary logic behind the rapid age changes of Postdiluvian peoples, the text seems to simply record this as technical fact. (Unlike Genesis 1, which has a pretty strong literary structure... three days of environments > three creatures for each of those environments, in order.)
* Genesis 5, 11, 21:5, 25:7,26, 35:28, 41:46,53, 45:6, 47:8-9, 50:22, and Exodus 12:40-41.