After the engineers fucked off or went extinct, the bio-mechanical cyborg penis machines they used to breed living tools made in their image kept creating life without purpose or direction, until eventually the creatures they created were a crude mockery of their makers. And then for a trillion years or so, all those bio-mechanical inbred abominations kept fighting against each other until the xenomorphs reigned triumphant and the space jockeys came second. Or something along these lines