Reptilians are a supposed alien species that came to Earth during the time of the ancient Sumerians. In fact, much of what we know of them is documented through Sumerian writings. What we do know is that at some point during the Sumerian reign, the Anunna (which translates to gods) came from the sky and settled on the earth.
These gods were then referred to as Anunnaki (gods who live on Earth). Many people associate the Reptilians with the strange arrival of the Anunna because of the sudden emergence of Reptile worship after the Anunnaki were settled.