There's a character in FFX-2 named Shinra, who's descendants go on to develop space travel, and then find the planet of FFVII. The 'humans' in FFVII are really descendants of this Al-Bhed child. The Cetra were the original inhabitants on the planet before the Al-Bhed came. Jenova arrives on the planet only a while before the Al-Bhed, so the Jenova crisis was avoided. That's why the Cetra died out, but the Al-Bhed still lived. Also, the Al-Bhed then go on to found Midgar and the Shinra corp. That's why it's named "Shinra".