Mutants (also known as Homo Superiors) are people who posses the X-Gene and developed superhuman powers and abilities that most could not control properly or at all or even change their appearance. This gene manifests at puberty during a teen's life.
Sebastian Shaw: We are the children of the atom. Creation gave birth to us. What weakens the humans will only make us stronger.
Jean Grey: Ladies and Gentlemen, we are now seeing the beginnings of another stage of human evolution. These mutations manifest at puberty and are often triggered by periods of emotional stress
Jean Grey and Professor X: Mutation: it is the key to our evolution. It is how we have evolved from a single-celled organism into the dominant species on the planet. This process is slow, normally taking thousands and thousands of years. But every few hundred millennia, evolution leaps forward.
Mystique and Magneto: Mutant and Proud
Magneto: Tomorrow, mankind will know that mutants exist. They will fear us, and that fear will turn to hatred.
Storm: Neanderthals, we once believed that they were wiped out by years of conflict with a much more advanced branch of humanity called Cro-Magnon, but recent research in to our own DNA suggests that these two species may have inter-bread involving into modern humans (asks class) into what? (Class:us) Us.