The Cardassians were a humanoid species from Star Trek. They were native to the planet Cardassia Prime, capital world of the Cardassian Union. Known throughout the Alpha Quadrant for being extremely ruthless, the Cardassians became one of the greatest enemies of the United Federation of Planets and the Klingon Empire, when the Cardassian Union joined the Dominion in 2373. Cardassians had a xenophobic attitude towards other species, which was well established throughout the quadrant after the Setlik III massacre during the Cardassian Border Wars, as well as when Cardassian atrocities during the Occupation of Bajor were revealed after they withdrew from the planet Bajor in 2369.
Cardassians and many of the Milky Way's species all shared a common ancestry back to the Ancient Humanoids and the DNA code they produced and seeded across the galaxy. Though each evolved under separate conditions from each other, their shared DNA allowed these species to remain roughly compatible and viable with each other.