Life in the cosmos only comes from previous life. No natural concoction of what makes up a living thing even applied in the correct sequence under the optimum circumstances over the right amount of time could lead to the production of a living thing. Aliens could not just develop on a planet. A previous life form would have had to produce them their. If no such thing has taken place beyond earth then there never were nor are there aliens. But the future is still bright! We are here to someday become aliens on another planet beyond earth.