Darwin (Armando Muñoz) was created by writer Ed Brubaker for the mini-series X-Men: Deadly Genesis (2006). He first appeared in issue #2 of the 6 issue mini. This mini-series revealed that Darwin, along with a few other mutants, were part of an interim X-Men team created in-between the scenes of Giant-Size X-Men #1 (1975). The mini-series was a retcon of previously established events.
In original continuity as published in Giant-Size X-Men #1, in spring 1975, the original X-Men (Cyclops, Beast, Angel, Iceman, Marvel Girl/Jean Grey, along with Polaris and Havok) were sent on a mission to the island Krakoa, only to be attacked and defeated by the island itself (which turned out to be sentient). Cyclops was able to escape and informed Professor X of the rest of his team's capture. Professor X then recruited a new team of mutants to join Cyclops on a rescue mission. Those mutants were Wolverine, Storm, Nightcrawler, Colossus, Banshee, Sunfire and Thunderbird. This story marked the first appearance of Storm, Nightcrawler, Colossus and Thunderbird, and is also when Wolverine joined the X-Men.
The big reveal of the 2006 mini-series, which retroactively changed continuity, was that after Cyclops' arrival, Professor X went to a group of young mutants, expedited their training by cramming it into their noggins with his telepathy, and then shipped them out in a short time span. This lineup included Darwin, Vulcan, Petra and Sway. All of them were killed by Krakoa, leading Professor X to then recruit Wolverine, Storm, etc.
Darwin, with his adaptation ability, was discovered as alive during the events of Deadly Genesis that were set in modern day continuity. Darwin then joined the X-Men for a mission to space, helped in their fight against the Hulk in World War Hulk, and eventually found a home with X-Factor Investigations in the comic book, X-Factor (Issue #31, 2008).
Darwin was written out of X-Factor in 2011, but in an open-ended manner that implies he will return as part of a forthcoming storyline.
And a note about X-Men: First Class and all other comic book cartoons and movies: Cartoons and movies based on superhero comics are almost never meant to depict strict adaptations of the source material, nor do they reflect chunks of continuity. They always pick and choose sometimes disparate elements to form a new whole. The only exceptions to this that I can think of are the Watchmen and Sin City films. In the case of X-Men: First Class, almost nothing presented in that film is the same as what happened in the comics.
- Professor X and Magneto met in a hospital overseas
- Professor X was crippled in a battle with the alien Lucifer
- Moira MacTaggert was a Nobel Prize winning scientist, not a CIA agent
- Banshee is Irish and also noticeably older than most other X-Men
- Havok is Cyclops' younger brother
- Beast is the only member of First Class to be part of the comics' first class
- Mystique and Professor X have no history together
- Riptide, Angel and Azazel are not members of the Hellfire Club
- Emma Frost did not gain the ability to turn into diamond until 2001
I could go on and on. None of this changes the fact that First Class is an awesome movie.