In comic books published by Marvel Comics, a mutant is an organism (usually otherwise human) who possesses a genetic trait called an X-gene that allows the mutant to naturally develop superhuman powers and abilities. Human mutants are considered to be of the subspecies Homo sapiens superior or simply Homo superior, an evolutionary progeny of Homo sapiens, and are considered the next stage in human evolution, though whether this is true or not is a subject of much debate in-universe.
Unlike Marvel's mutates, which are characters who develop their powers only after exposure to outside stimuli or energies (such as the Hulk, Spider-Man, The Fantastic Four, and Absorbing Man), mutants are born with the genetic potential to possess their powers, although the powers typically manifest at puberty.
Within the Earth X universe, the powers of the vast majority of Marvel's human superheroes are the result of genetic manipulation by the Celestials millions of years in the past.
Background
A March 1952 story in Amazing Detective Cases #11 called "The Weird Woman" tells of a woman describing herself as a mutant who seeks a similarly superhuman mate.
Roger Carstairs, a mutant who can create illusions, is shown in Man Comics #28, dated September 1953.
A character with superhuman powers, born from a radiation-exposed parent, was seen in "The Man with the Atomic Brain!" in Journey into Mystery #52 in May 1959; although not specifically called a "mutant", his origin is consistent with one.
A little-known story in Tales of Suspense #6 (November 1959) titled "The Mutants and Me!" was one of the first Marvel (then known as Atlas) stories to feature a named "mutant".
The modern concept of mutants as an independent subspecies was created and utilized by Marvel editor/writer Stan Lee in the early 1960s, as a means to create a large number of superheroes and supervillains without having to think of a separate origin for each one. As part of the concept, Lee decided that these mutant teenagers should, like ordinary ones, attend school in order to better cope with the world, in this case Xavier's School for Gifted Youngsters. These mutants first appeared in the superhero series X-Men, which debuted in 1963. Marvel later introduced several additional mutant superhero teams, including The New Mutants, X-Factor, Excalibur, X-Force, and Generation X.
Officially, Namor the Sub-Mariner is considered the first mutant superhero whom Marvel Comics ever published, debuting in 1939. However, Namor was not actually described as a mutant until decades after his first appearance. The same is true of Toro, partner of the android Human Torch introduced in 1940.
Secondary mutations
Some mutants have shown the ability to develop a secondary mutation.
Omega-level mutants
An Omega-level mutant is one with the most powerful genetic potential of their mutant abilities. The term was first seen in the 1986 issue Uncanny X-Men #208, but was completely unexplained beyond the obvious implication of it referring to an exceptional level of power. The term was not seen again until the 2001 limited series X-Men Forever. Some abilities depicted by mutants described as Omega-level include immortality, extreme manipulation of matter and energy, high psionic ability, strong telekinesis, and the potential to exist beyond the boundaries of the known physical universe. No firm definition has been offered in comics. Examples of mutants that have been confirmed as Omega-level include Jean Grey, Quentin Quire, Vulcan, Rachel Summers, Iceman, Legion, Elixir, Franklin Richards, and Psylocke.
"Homo superior superior"
Introduced in Chris Claremont's X-Treme X-Men, a character known as Vargas claims to be humanity's natural response to mutants. Vargas was born at the epitome of peak physical skill, having superhuman levels of strength, speed, reflexes, agility, stamina, and durability. Vargas also seems to be immune to various mutant abilities (such as Rogue's absorption and Psylocke's telekinetic blast).
Externals
Created by Rob Liefeld, Externals are immortal mutants whose powers have allowed them to exist for centuries. Eventually, most of the Externals are killed by Selene. Gideon, Selene, and Apocalypse are examples of Externals.
Cheyarafim and Neyaphem
Cheyarafim and Neyaphem first appear in Uncanny X-Men #429. According to the character Azazel, the Cheyarafim are a group of angel-like mutants who were the traditional enemies of the Neyaphem, a demonic-looking group of mutants who lived in Biblical times. The Cheyarafim were fanatics who had a strict, absolutist view of morality which led them into conflict with the Neyaphem. This escalated into a holy war, causing the Neyaphem to be exiled into an alternate dimension. What happened to the Cheyarafim after this has not been revealed.
The X-Man Angel is said to be descended from Cheyarafim, while Nightcrawler is supposedly the son of a Neyaphem, Azazel. It is unknown if Joshua Guthrie, known as Icarus, is a descendant of the Cheyarafim.
Dominant Species/lupine
Maximus Lobo claims to be a part of a mutant sub-species of feral, wolf-like mutants, whom he calls the Dominant Species. He later tries to recruit Wolf Cub into his ranks, to no avail. A few years later, another mutant, Romulus, claims that some human mutants evolved from canines instead of primates. Mutants who are a part of this group include Romulus, Wolverine, Daken, Sabretooth, Wolfsbane, Wild Child, Thornn, Feral, and Wolf Cub, with X-23 and the Native as other likely candidates. These groups appear to be one and the same.
Changelings
Introduced in the second series of X-Factor, a changeling is a mutant whose powers manifest at birth. Jamie Madrox (Multiple Man) and Damian Tryp are examples of this sub-class.
Extraterrestrial mutants
Humans are not the only species to have mutant subspecies. Ariel, Longshot, Cerise, Ultra Girl, and Warlock are examples of mutant aliens.
Mutants as metaphor
As a fictional oppressed minority, mutants are often used as extended metaphors for real-world people and situations. In 1982, X-Men writer Chris Claremont said, "[mutants] are hated, feared and despised collectively by humanity for no other reason than that they are mutants. So what we have here, intended or not, is a book that is about racism, bigotry and prejudice."
Danny Fingeroth writes extensively in his book Superman on the Couch about the appeal of mutants and their meaning to society:
An obvious parallel between homosexuality and mutation is drawn in the feature film X2, where Iceman's mother asks, "Have you tried not being a mutant?" This question (or various forms thereof) is common among parents who find out their children are gay. In the 2011 film X-Men: First Class, Hank McCoy (later known as Beast), upon being outed to a colleague as a mutant, responds, "You didn't ask, so I didn't tell."
In his article Super Heroes, a Modern Mythology, Richard Reynolds writes:
Ultimate Mutants
In Ultimate Origins #1 it is revealed that super-powered "mutants" in the Ultimate Marvel universe were artificially created via genetic modification by the Weapon X program in a laboratory in Alberta, Canada in October 1943. The project was an attempt to produce a supersoldier, inspired by the existence of Captain America. James Howlett was the first individual to be so modified. At some later point, possibly during a confrontation between Magneto and his parents, the mutant trigger was released into the environment worldwide, leading to the appearance of mutants in the general population. Following the events of Ultimatum, information concerning the origins of mutancy was made public and steps were taken in the US to make being a mutant illegal. While the move apparently has majority support among the non-mutant population, a vocal minority has voiced concern that it will lead to witchhunts and genocide.
