In Pre-Crisis history, there were a couple different futures that dealt with a post apocalyptic world. It was never specified what happened, but a great disaster took place. There were hints that the disaster happened on different fronts. They talked about some sort of war that might have taken place, probably nuclear in nature. There was also mention that nature had gone haywire also. After the disaster, the earth became very different. Groups of intelligent dogs, gorillas, tigers, and other animal species ruled the world now. There were also some robot cities that continued to flourish. Most humans however were basically de-evolved, like in Planet of the Apes. This future earth became known as Earth A.D. (After Disaster).
Kamandi is probably the most known character from this earth. He was one of the last intelligent humans left since he escaped the disaster by living out the first half of his life in an underground shelter called Command D, which is where he gets his name from. Kamandi is also the grandson of Buddy Blank, the original OMAC. An interesting thing to note is that there was also an alternate future where the disaster never happened and the boy who grew up to become Kamandi instead grew up to become a space hero named Tommy Tomorrow.
The Atomic Knights are another small group that survived the disaster. They use giant mutated dalmations like horses and protect what's left of humanity, in order to rebuild someday. They are kind of like a post-apocalyptic Knights of the Round Table.
What I find really interesting is that if you look at what DC is doing now, they are starting to seed in things like this. The Atomic Knights appeared in Bludhaven to protect the people after Chemo destroyed it. After what was left of Bludhaven was destroyed by Captain Atom, they retreated into a secret underground bunker called Command D.
There was also an ad for Countdown which showed the DC characters in front of a destroyed Statue of Liberty. This is very similar to the cover of Kamandi's first book.
Last, if you look at the ads for Countdown. One of them has Darkseid with the words "Unto Man Shall Come... A Great Disaster.)
Pretty interesting stuff.