How did human life start on earth?
That is really an ill-defined question, and probably more relevant to biology than astronomy.
It is also a philosophical or religious question more than a scientific one. What do we define as “human life”? Is that even valid? All scientific inquiries lead to theories, but we do not have any way of really knowing a relative “date” of the beginning of human existence.
If you hold with religious texts, at some point, “God” - however defined – simply made humans. If you want to follow evolution theory you run into the problem of defining “humanity” apart from animal species. At best, we can approximate the time range of the presence of different terran species. How any of them really “started” remains speculation and conjecture.