Founded in 1985, SETI's goal was to locate evidence of extraterestrial intellegence. In 1995 they recieved a message in the form of a genetic code. This alien DNA was found to be easily synthesized, and was compatable with human DNA. An experiment to hybridize the two species was started, and resulted in three successful embryos.
SIL was the principal character of Species. SIL was one of the three successful hybridizations of human and alien DNA, and her embryo was allowed to develop while the other two were put in cold storage. Her "name" was a unique three letter code assigned to all the embryos in the experiment, and her sex was selected as female because the project leaders thought a female would be more docile. After only a month she had grown to roughly a tween. Unfortunately for her, the project was cancelled and she was marked for termination.
Utilising super-human strength she broke out of her enclosure and the compound, killing several guards with her bare hands. She eventually escaped on a train, and learned about humanity and how to speak English from television. The train also marked the last stage of her growth, as her body suddenly formed a crysalis which she would emerge from as a fully grown adult. As an adult she also developed a new need: to mate.
Her first forays into the LA club scene were unsuccessful, and she also had to deal with a government task force getting closer to tracking her down. She concoted a plan to fake her death by leaving a cut-off finger (which regrew within seconds) in an exploding vehicle, faking her death and allowing herself to slip under the noses of the task force and mate with one of its members. Now pregnant, she escaped the task force in an underground cave network long enough to give birth, then set about killing them to defend herself and her baby. The baby rapidly grew to adolescense, but neither it nor SIL were strong enough to kill the task force, and both were killed.
EVE was the second sythesized alien-human hybrid, genetically identical to SIL, and a main character in Species II. She grew to adulthood in a lab and was a willing participant in tests designed to find a way of killing the alien half of her DNA. When the astronauts on a mission ot Mars come back infected by alien DNA, EVE's telepathic link to other aliens is used to track down the male infected astronaut, Patrick Ross. Ross eventually becomes aware of EVE, and allows himself to be captured to get closer to her.
Meeting another hybrid overwhelms EVE with a need to breed, and the two of them escape the laboratry complex. They meet in the farmhouse where Ross has been storing his many offspring, but are inturrupted mid-coitus by the alien hunting task force. EVE momentaryily regains herself and stabs Ross with her spines, but he is mostly unharmed and responds by assaulting and raping her. Both hybrids are then sprayed by an aerosol of diabetic cells designed by the task force, which kills Ross, EVE, and all of Ross's cocooned offspring. Later, after EVE's body is placed in an ambulance, her belly begins to expand with a rapidly growing newborn.