Scientists presume that natural selection is the prime moving cause of evolution. How can you prove that if you do not prove how life originated. And if you cannot prove a natural cause for life, then what is left is a designed cause by a supernatural being. God.
Biologists don't presume NS they conclude it; based on evidence.
What does not knowing about the start conditions of life have to do with observing the change in allele frequency over time in a population?
As has been pointed out many time scientist don't 'prove things'.
There could be a supernatural cause for life but there is no more evidence for a supernatural cause than there is for life (and the universe) being a fantastically intricate computer simulation and we are but sentient AIs existing in a virtual reality.
If you are going to say that not being able to 'prove' that life is natural means that one must entertain a supernatural origin then you must give equal weight to a technological origin.