It's not "assuming the conclusion", it's a working assumption to facilitate investigation - if you don't assume there's a material cause for material events, then there's no point in looking, because science is not equipped to investigate the nonmaterial. If it turns out that no material cause can be found, so be it, but if you don't start with that assumption, you can't start looking at all. As a matter of science, of course - theological or philosophical investigations are another matter entirely.
I have no problem with the evolutionists positing their ideas as a working assumption, i.e. as a possible solution. But they move from science to faith when they insist that it must be a necessary solution.
If it turns out that no material cause can be found, so be it ...
This is my point, there may be a point where the natural sciences should simply respond "we do not know."