I read the book and found it depressing as well. There was never any mention of what actually caused the disaster/calamaty but it was not quite logical. Seems the only thing that survived was humans. No plants or animals anywhere. Not quite credible, in my opinion
After the animals ran out, the cannibals came out.
Don't know if I'll ever see that film.
Caused me to conclude that my PSK should include cyanide.