Oh, that is even much easier. You do not even have to know chemistry, math, and other annoying stuff about how things work. Just mix inorganic stuff in big enough container, supply energy (light, heat, motion, sparks) and wait for a few hundreed millions years or so, and the Shroedinger math will do all the work: you'll have not only flies but all kind of other complex chemical systems. Wait longer, and some of them will be complex enough to create gods and discuss evolution.

In the early Earth history, life was obliterated many times by collisions with big chinks of space debrie littering Solar System, and had to started over again and again. As you may see, the probability of having life "from nothing" is extremely high. It was verified experimentally in 50-s that indeed, complex organic molecules can be created out of inorganic stuff in a lab flask.

Well, what would you expect mathematically: if you have a thermal motion of so many atoms and molecules around (each collides with others about 1 000 000 000 000 times each second at room temperature, i.e. in one cubic inch you have about 10^35 mutual interactions each and every second alone). No wonder that from time to time more complex structures (organic stuff) may form.