If I may add my two cents worth ... there is always going to be things unknown and unknowable ... due to the very nature of man. We are finite creatures and very, very small on a galactic scale. To think that it would be possible for humans to understand everything must be the epitome of hubris itself.
This being the case, there has to come a point at a individual and collective level, that assumptions must be made. However has history has shown over and over again, our understanding of the universe is often flawed. And being dogmatic about ideas, just become a new religion, in itself.
Belief in the evolution of all life in the universe, by pure random chance, is a classic example of this kind of assumption making. Why ... because the evolution theory is a philosophical discussion, not a scientific one, no matter how much noise people make to the contrary. Why, the whole driving force of Darwin's On the Origin of Species can be directly traced back to the Enlightenment philosophers of the 1700s and as part of the development of the secular world. Thus saying with 100% confidence that all life on Earth evolved, is a risky assumption, due to the opening premise of the movement, of which this theory has been used to support.