It would not be surprising to discover that the trade routes between China and the West were opened many centuries, even millennia, earlier than 200 B.C., contrary to what is currently believed.
Conclusion (opinion = not surprising)
It would not be surprising if the trade routes between China and the West were open way earlier than is currently thought (~2200 years ago).
Evidence (after all, and)
These trade routes were attractive because of their easily traversable terrain. This trait would have likely made it an attractive option for migrations that began one million years ago.
Evaluation
The conclusion is the first sentence, as is very common on Role and Main Conclusion questions. The "after all" tells us that what follows will be Support for that first sentence. The final sentence makes two claims, joined by an "and". 99% of the time "and" is connecting to supporting ideas.
Goal
The claim they're asking about is one of our two premises, for the conclusion that is found in the first sentence.