In my forthcoming novel, Was A Time When, there are portions of the "Book of J" which was the early oral record of the times after the Great Collapse. Here is the beginning.
From the Book of J:
In the beginning, the plagues of Crowding, Depletion and Warming enveloped the world, but humanity did not recognize their danger.
Like so many species before them, the human race outgrew the capacity of their environment to support their numbers. Only extinction could control their exponential growth.
Resource depletion meant inexpensive sources of power vanished faster than humans could find alternatives, and the cost of power grew out of control.
Insufficient water and fewer fields meant food larders could not be refilled quickly enough, and people starved, for their numbers grew faster than their reserves.
Diseases evolved that could not be cured by age-old remedies and only those who evolved new immunities fast enough survived.
Like some dominant species before them, the human race forced significant changes to their environment, and the attributes of the troposphere found a new balance, with increased temperatures, decreased ice, and rising sea levels.
Primary utensils of the world's technology could no longer be built when their parts disappeared, and large segments of the technical world became unavailable then obsolete.
Civilization died as it lost the knowledge and tools to make itself work.
People in all the lands starved, the weakest first, then the strong, and new diseases followed the starvation close behind.
Chaos enveloped all the humans of the world and starving, diseased hordes fought amongst themselves and overran the lands.
Only the elite survived, though not for long, along with the few who evolved the talents and immunities to adapt to the new world. The latter came to be called neu-humans. They are us, and we are the next sub-species of the human race.
