This is what I could think if:
Tracing the roots and looking for the ancestors of protists will be hard because protists are an animal group that has extended really far, which makes things harder when looking for the roots of these organisms. Most of the corrent living organisms are protists, and there are may, manny different families, genus, species, etc. within this group. Tracing the origins and ancestors of the modern day protist would take years. And that is not the only case. Protists have lived on Earth for millions of years, which means that they haved changed and adapted a lot since the first protists appeared.
Hope it helped,