Well like any MD PhD statement you will have to make the case for why your career as a physician will be enhanced by your work in anthropology and why the kind of work you want to pursue as a researcher will be enhanced by your medical training/clinical knowledge/access to patients. The statement is all about showing that you have a vision for a career that is more than the sum of its parts. You don’t have to be hyper specific, I.e know exactly what research problems you will work on. But you should, using your past experiences, make a convincing argument that you understand your field and medicine well enough to see a place for yourself in translation between them.
Unfortunately I don’t know anyone on this site doing something like this or id tag them but if I were you I would contact current MD/PhD students doing this (their e-mails are usually easy to find online) for specific advice.