The Chitauri started out as a super soldier experiment conducted by the Skrulls to combat the Kree. Using normal Skrull soldiers as test subjects, the scientists altered their genetic structure with cybernetic implants, giving them a muscular gray-like appearance, exposed jaws, three-toed feet, and three fingers and two thumbs on each hand.
A battle between the Skrulls and the Kree that took place in the Skrulls' homeworld resulted in the destruction of their main laboratory, allowing the test subjects to escape and savagely killing the scientists, among other obstacles. They proved to be too strong to handle and were tricked into boarding a ship trasporting them to a planet similar to Saturn. Eventually, the Chitauri grew as a distinct Skrull species with extreme intelligence, allowing them to create weapons and other technology left over by their creators.