D But, you are right in the sense that no one was there to witness it. So, faith it is.
Scientists don't much mess with faith when doing science, but they do readily engage in hypothesis formation based on best available evidence, followed by attempts by other scientists to tear down those hypotheses also based on best available evidence. They are far from certain about the formation of the first single cell life forms, since there's very little factual evidence to operate on other than a good guess about the environment in which they first came into existence. By today's standards, that environment was highly toxic, but the earliest organisms just loved it. In fact, when oxygen first came along as a waste product of green plant life, it was a toxic pollutant to most early life forms, and killed most of them off in a mass extinction. The survivors, however, actually made use of this pollutant, and prospered in a high oxygen environment. That's why we like oxygen so much.
