From Mutant Genesis MUX
Birth of the Cosmos
The survivor of a dead universe arrives as the universe is born. This being will be known as Galactus. An inordinate amount of time would pass until the first species would flourish to become what are the modern Watchers, a race that has sworn never to involve itself with the dealings of others, destined to remember all that they would survive.
Although the Kree and the Skrulls would form empires and wage war against each other for millions of years, it were the enigmatic Celestials who would first take notice of Earth. Their arrival would mark the birth of mankind as the Celestials "seeded" the planet with their technology and gifted humans with the latent potential for god-like power.
Rise of Humanity
Great human civilizations arose on Earth, later to be known as Atlantis and Valusia, only to be destroyed by the Great Cataclysm. After the Cataclysm there was a new age of warriors and wizards on the surface, in which a sword-wielding barbarian rose to become the greatest monarch of his time. In the sunken remnants of Atlantis the water-breathing ancestors of today's Atlanteans would settle. A mutant was born at this time, alone in the world, and she was named Selene by her tribe.
It was a time of gods and monsters, and although these godly beings would show their faces and mark history as myths and legends, their reigns would be brief, or their attention would turn elsewhere. Humanity was allowed to forget the superhuman lived amongst them.
Dawn of the Superhero
Captain America is born from the efforts of the United States most brilliant minds to fight the war overseas, where a sinister breed of evil was being made in Nazi laboratories. The war would stir the waters enough to bring Atlantis into the fight, but the most shocking event of that time wouldn't be Captain America's valiant sacrifice to stop the Red Skull, but the mushroom clouds of Hiroshima and Nagasaki. In a matter of years the high command of the SHIELD would be coining the term 'Children of the Atom' to refer to the strain of humanity most affected by that event.
Rare others, knowing fully well what these individuals meant, made decisions of their own. Project Rebirth, responsible for Captain America's creation, is renamed Weapon I.
Over the course of the next years the world would see a shift of paradigms. Although it started slowly and subtly, suddenly the supermen had arrived. A family is shot in space only to be granted incredible abilities when struck by a solar flare. Formidable, then-hidden nations in Africa reveal themselves to the world and empires of sorceror-kings rise in the heart of the European Balkans. Canada starts its own superhuman program, and young children shoot lasers from their eyes in Alaska.
But for every superhuman wonder, there are dozens of young mutants. For every Captain America, a Magneto, and humanity is scared.
Ever since the first superpowered human governments have tried to make them useful to their interests. Around the world initiatives like the Alpha Flight (sponsored by the Department K), the Avengers (sponsored by S.H.I.E.L.D.) and the Winter Guard (sponsored by Russia) have been a thing for years now. These groups are rarely advertised to the public (with the exception of the Avengers) and are often draconian in the handling of their members. To mutants in these groups, the alternative to their membership and obedience was often death, or worse.
IMPORTANT: The myriad of superhero teams from the comics, like the Champions, Defenders and so on, have not been a thing in this reality yet, and might never be. Metahumans tend to be highly independent, and often get absorbed by the teams that control their countries, like the Avengers.
There are two mutant teams in the US from S.H.I.E.L.D.'s perspective: the Brotherhood and the X-Men. While they are both unknown commodities, the Brotherhood was led by mutant terrorist Magneto and it is regarded as the most dangerous one, often portrayed as bloodthirsty savages. The X-Men are a strange band of young men and women that, although don't invoke fear like the Brotherhood, are seen with suspicion by the public at large thanks to their organization, tech and fearsome powers.
Other mutant teams are: the Hellfire Club, the Marauders and the Horsemen of Apocalypse.
IMPORTANT: There are no other mutant teams in this timeline. No X-Factor, X-Force, Excalibur, and so on.
As with any population that has ever existed, mutants have developed, by some mean or others, drugs that appeal to their particular tastes. The most famous are Toad Juice and MGH, with a third, named Kick, slowly growing in popularity inside Mutant Town. Toad Juice is sold in thin sheets of paper that one has only but lick to feel the powerful psychotropic agents take effect, giving what users describe as an unbelievable high and sensorial enhancement. MGH, the Mutant Growth Hormone, is popular among the few humans who know about it, can afford it, and are willing to face the side-effects. The MGH gives normal humans mutant powers for a while, and it is known to be highly addictive. Hypercortisone D, better known by its street name Kick, is a highly addictive aerosol used by metahuman individuals to attain boost in superhuman/mutant powers and a mental high. It makes one feel "angelic and violently insane for five hours," and it has become more and more prevalent in mutant communities.
Mutant fashion was, for the longest time, considered to be skintight suits and flamboyant colors, but it was only thanks to an enterprising young mutant called Jumbo Carnation that fashion took its next step forward. Opening the X-Factory, the young man started to craft clothes designed specifically for the mutant body, and not only for the sapiens-seeming types. Four-legged pants and adaptable coats became the new fad, and rumors abound that Jumbo is going to co-judge RuPaul's Drag Race soon.
Humans tend to be tribal, but they are nothing like mutants in that regard. Mutants only gather under banners of philosophy, as flimsy as they are, and even the most violent gangs protect their communities against outside predation and gather to discuss purpose, race and future. They often dress in ways humans might think is flamboyant and many refuse their sapiens names, instead calling themselves by codenames, as their heroes in the Brotherhood and X-Men. Usually seeing themselves as cells in a larger army, in a greater war, their names usually denote subservience to the two famous groups, or individuals in them. Mutant Town is a known hotbed for mutant gangs, but as an unfortunate side-effect of ideological clashes, mutant gangs are known to engage in violence not for territory or goods, but over ideas. Many human specialists point to this phenomena to prove mutantkind is inherently violent and predatory.
Mutants are not mainstream so most of their artistic production is counterculture, a lot of it meant to be offensive against the established powers or bizarre to allude to their exotic bodies and powers and make their strangeness a badge of honor. In Mutant Town, the cradle of much of these new bands, a mutant can find bands such as Juggernaut and Sentinel Bait playing regularly, while Dazzler, although having experienced a crashing of her career years back, has now resurged as "The Mutant Lady Gaga", with her army of Little Freaks. Most humans either don't understand mutant songs or are outright not bodily equipped to hear the right frequencies and pitch.
Friends of Humanity
The Friends Of Humanity is a group of political activists founded by Graydon Creed that has in its mission statement to keep America human. Whether by inflamed rhetoric or polemic protests, the Friends have earned themselves a spot on the cable news cycle as humanists, acting whenever there is a possibility of "mutant rights" encroaching on actual, human rights. Although their social media presence is massive, their main advantage is the ability to push lawmakers into reconsidering policy like The Mutant Registration Act. Groups inspired by or splintered from the Friends of Humanity include the survivalist Humanity's Last Stand and the religious fundamentalist Church of Humanity. To the Friends of Humanity the Avengers are national heroes, and failing to stand when they fly by is considered a grave offense akin to disrespecting the troops.
The TransSpecies Movement is a fringe group that uses John Sublime's The Third Species book as gospel, believing the emergence of Mutantkind as the dominant species on Earth is inevitable but that humans need not be left behind, and that there is a way to adapt to a changing, unclean world. Now profoundly tied with the U-Men, the TransSpecies is considered by most to be a dangerous and psychotic group of individuals that hunts mutants parts. Weird and elitist, given the expensive suits of armor donned by its high-end members to avoid contact with a world that isn't ready for their pure bodies, the TransSpecies has gone under and hides in the dark, still out there, somewhere, but having fallen the lofty heights it once reached.