There is a deeply appropriate resonance in the opening of “Earth Matters: Land as Material and Metaphor in the Arts of Africa” on Monday, Earth Day, at the National Museum of African Art. It is a large-scale, ambitious joining of the conversation by artists singularly positioned to weigh in. African people have humanity’s longest-standing relationship to the planet, and the fact that they have been having artistic conversations about that relationship for centuries is one of the points the exhibition seeks to make clear.