Development and writing
Dennis Feldman had the idea for Species in 1987, as he worked on another film about an alien invasion, Real Men. Having read an article by Arthur C. Clarke about the insurmountable odds against an extraterrestrial craft ever locating and visiting Earth, given that stellar distances are great, and faster-than-light travel is unlikely, Feldman started to think that it was "unsophisticated for any alien culture to come here in what [he]'d describe as a big tin can". Thus in turn he considered that the possibility of extraterrestrial contact was through information. Then he detailed that a message would contain instructions from across the void to build something that would talk to men. Instead of a mechanical device, Feldman imagined wetware. The visitor would adapt to Earth's environment through DNA belonging to Earth's organisms. Mankind has sent to space transmissions "giving out directions" such as the Arecibo message, which Feldman considered unwary, as they relay information to potential predators from outer space. He pointed out that "in nature, one species would not want a predator to know where it hides".
Therefrom emerged a film treatment called The Message. The original script had more of a police procedural approach, with the alien being created by a "bathtub geneticist" who had just had his project aborted by the government, and a biologist who had worked on the project getting together with a police officer to search for the creature. Eventually Feldman came to believe this concept had some credibility issues, and instead changed the protagonists to a government team. After coining the name "Sil", Feldman initially thought of forming an acronym, but in the end chose only the three-letter name after learning about the codons of the genetic code, which can be represented in groups of three letters. Sil would originally emerge from a DNA sequence manipulating human DNA, and constantly mutate as she used the human junk DNA to access "all the defenses of the entire animal kingdom that [humans] evolved through – including ones that had never developed, plus ones [Earth's scientists] don't know about that have become extinct". Research that Feldman did for the script included going to sessions of UCLA's Center for the Study of Evolution and the Origin of Life (CSEOL), talking to SETI scientists, and visiting the Salk Institute for Biological Studies to talk with researchers working on the Human Genome Project. The Message was offered to several studios, but was passed up.
In 1993, Feldman reworked his ideas into a spec script. This was sent to producer Frank Mancuso, Jr., who had hired Feldman to adapt Sidney D. Kirkpatrick's A Cast of Killers. The producer became attracted to the creative possibilities as the film offered "the challenge of walking that fine line between believability and pushing something as far as it can go". Metro-Goldwyn-Mayer got interested in the project, and despite some initial disagreements on the budget, after considering other studios Feldman signed with MGM. In turn, the now retitled Species attracted director Roger Donaldson, who was attracted to its blend of science fiction and thriller. The script underwent eight different drafts, written over an eight-month period, before Donaldson was content that flaws in the story's logic had been corrected. At one point another writer, Larry Gross, tried his hand with the script, but ultimately all the work was done by Feldman. Feldman would remain as a co-producer. While the initial Species script suggested a love triangle between Sil and two government team members, Preston Lennox and Laura Baker, dissatisfaction among the crew eventually led to changes to the ending, which ended up featuring Sil having a baby that would immediately prove dangerous.
When Donaldson was announced as the director in 1994, Mancuso stated that most of the $35 million budget would be spent on effects, as "it's not a movie that calls for stars. We're going to try and put as much money as we can below the line and allow the effects and the creature to be the highlights of the film". The lesser emphasis on actors included a newcomer, former model Natasha Henstridge, as the human version of the creature Sil.
