Suggarfoot J. David Attenborough. I don't know that he ever gained a postgradute degree, which I presume is what you mean by postdoc. He did though study geology and zoology at Cambridge and graduated with a degree in natural sciences.

But, as you go on to say, it's all down to his knowledge, understanding and ability to communicate, rather than qualifications. And I would add to that list, enthusiasm, and sheer love of his subject matter. Ditto for Bert Lloyd, whom I mentioned in my first posting on this thread. Bert had even fewer formal qualifications than DA, but he combined a phenomenal knowledge of folklore, and quite a few other things besides, with all the qualities possessed by Attenborough. Could both Unthanks combined hold a candle to either one of them? Not on present showings.

Joanna Lumley. Worryingly gullible will do just fine. I thought she did an excellent job of securing citizenship rights for the Ghurkas though. Which probably just goes to highlight the old adage; stick to what you're good at. In the case of Lumley and the Unthanks, that seems not to include the presentation of interesting documentaries.