Why is there life on Earth in the first place?

Of course this not just an accident where one compound meet another right compound and form the very first nucleic acid as the very basic element of life form. I mean,why the Earth need life on itself,does it really serve a purpose for Earth? Earth just like other planets in this galaxy where all of them will still exist no matter what happen and will have nothing to do with complexity of life. So why there are life on Earth?
Please try to support your opinion with hypothesis/theory if you could since there could be no accurate answer for this particular issue. I think the idea about we were created by aliens as their experiment object will sound ridiculous to most ears but that is the most plausible explanation in this issue,beside the accident hypothesis,of course.