Post-apocalyptic futures were all the rage back in the 1970's, both in comics and movies with feature films like Logan's Run, Soylent Green, The Omega Man and several movie sequels to Planet of the Apes, just to name a few.
Jack Kirby crafted one of his most successful post-Marvel series by mining this genre, when he introduced Kamandi #1 in 1972. The Last Boy on Earth's world is set on a future Earth following an event known as "The Great Disaster", where humans are a persecuted minority in a world ruled by intelligent, highly evolved animals. The evolved animals who stand on their hind legs and have human-level intelligence and humanoid hands; include gorillas, tigers, dogs, lions, cheetahs, and other mammals. Some animals have not changed their physical appearance but are still intelligent and can speak; including snakes, dolphins and killer whales. Certain previously smaller animals have acquired gigantic size or been mutated in a variety of ways; such as insects and crabs. Horses have not been affected.
Although there are some exceptions, like Kamandi's mutant friend Ben Boxer, most humans in the series do not talk and are dependent upon the intelligent animals. The nature of the "Great Disaster" was never fully explained, but it "had something to do with radiation". In Kamandi # 16 an explanation is given for the talking animals. A gorilla doctor reads the diary of a dead human physician that was written at the time the Great Disaster occurred. As a battle rages between the gorilla and the tiger factions within the ruins of Washington D.C., the gorilla doctor reads about how Dr. Michael Grant invented a chemical called cortexin. The chemical apparently spilled into the water supply, and when the animals ingested it, it gave them greater intelligence. These effects have been passed down to the animals' descendants. Many of the original intelligent animals came from the Washington Zoo. The gorilla doctor successfully recreates this chemical. While he is dying he sees the same effects occur with the formerly animalistic humans; this implies that perhaps humans will someday regain their lost intelligence.
Jack Kirby remained with the series through issue #40, but Kamandi continued until issue #59 (two partially completed additional issues were collected later in DC's Cancelled Comic Cavalcade).