Originally being believed to be a possible future for earth (which contradicts the existence of the Legion of Superheroes) , earth AD was stablished as a parallel universe where the adventures of Kamandi and OMAC, both Kirby creations, happenend.
The story of Earth-AD begins in the future of a parallel dimension. A race of aliens known as the Visionaries forecast the future destruction of Earth in a cataclysm that would come to be known as the Great Disaster. The visionary who discovered this, Prof. Z, addressed the Council of Science and mapped out the precise incidents that would lead to the planet's ruin. He told them that the planet's industrial barons would rise to power and a civil war would ensue which would lead to a calamity known as the "Great Disaster".
Seeking to prevent such a future from coming to pass, the Visionaries came to Earth and established a network known as the Global Peace Agency. In order to keep their alien origins a secret from Earth's populace, they used cosmetic spray to obscure their features. The people of Earth believed that the Global Peace Agency was made up of world leaders from various nations who intentionally hid their features so as to promote equality between races and cultures. The Global Peace Agency began policing the planet, incarcerating vast numbers of crime lords, corrupt politicians and potential megalomaniacs. Prof. Z however was dubious of their efforts. Though the G.P.A. was making great strides in their efforts to eradicate crime, he still believed that Earth was destined to fall in the Great Disaster.
The One Man Army Corps
One of the G.P.A.'s premier scientists, a human named Professor Myron Forest created a powerful satellite computer system called Brother-Eye to help oversee Earth's affairs. He also became provincial in transforming an army stockroom clerk named Buddy Blank into a one-man army corps. Using the name O.M.A.C., Buddy worked in the interests of the Global Peace Agency in fighting Earth's enemies. One of his most dangerous foes was the criminal scientist Doctor Skuba.
The Great Disaster
For all of OMAC's efforts, the Great Disaster could not be averted. Very little is known as to the true causes of the Disaster, but what scant evidence exists points towards a mysterious energy wall known only as the Vortex. Earthquakes and tidal waves erupted all across the globe and the nations of Earth found themselves at the mercy of nuclear Armageddon. Shortly before the final bastions of humanity were to fall, a scientist named Doctor Michael Grant invented a mutagenic chemical known as Cortexin. Although he would not survive long enough to witness the fruits of his labor, the Cortexin drug would succeed in carving out its own legacy in the wake of global annihilation.
Following the Great Disaster, Buddy Blank found himself cut off from the satellite Brother-Eye. He retired as OMAC and set out to eke a new life for himself as ordinary Buddy Blank in this new post-apocalyptic era.
The ambient radiation left behind by the Great Disaster interacted with the Cortexin levels in many of Dr. Grant's laboratory test animals. Over the course of two generations, the animals began to mutate, and their progeny eventually developed anthropomorphic traits including human intellect and the ability to walk upright. With time, these Animal Men inherited the Earth and became the planet's dominant life form. Humanity was left devoid of all culture and social structure, and slowly began to de-evolve into a lesser species (Whether the Cortexin Effect had anything to do with this process is unknown).
The Last Boy on Earth
Decades later, the aging Buddy Blank returned to an army compound located in the ruins of New York City named Command-D. By this time period, the tribes of the Tiger-Men under the rule of Great Caesar had erected the Tiger Empire. Their domain covered most of the Eastern seaboard of the United States. Buddy was careful to conceal himself or else risk becoming prey to the Empire's anti-human initiatives. Buddy sired a child who in turn later gave birth to a son named Kamandi. Kamandi's name was derived from the bunker in which he had been born ( Command-D ).
Post Crisis presence
After the crisis on infinite earths, the original earth AD was erased from continuity, both as alternate world and as posible future, with Kamandi beign revealed as Tommy Tomorrow. However, different elements from earth AD keeped popping in main continuity as in:
- The adventures of Superboy (Kon-El) in a Kamandi-like landscape.
- The OMAC project, prior to Infinite crisis.
- The existence of Brother Eye during Infinite crisis and in New52.
Alternates Earth AD
There are also alternate versions for earth AD:
An earth were OMAC stopped the great disaster and avoided the existence of Kamandi future. This OMAC would meet with Earth-AD Kamandi.
An Elseworld version earth-AD, where Kamandi: At Earth's End and Superman: At Earth's End happens.
Earth-51, the post 52 and New52 version, which is, officially a recreation of the original earth-AD, filling the space of the destroyed utopic previous earth-51.