I remember reading publicity material from 1980 which took great pains to point this out. Tegan Jovanka was very much sold to the audience as being a 'back to basics' companion: the first Earth human companion in nearly five years, and the first contemporary Earth human since (arguably) the 1960s (it's up for debate, but the intention seems to have been that Liz Shaw, Jo Grant and Sarah Jane Smith were all conceived to be from the 'near future', from the viewers' stand-point).At one point wasn't he running around with an Alien, an Australian and an American?
Not at the same time. Actually the Fourth Doctor had no Earth-human companions from Sarah Jane's departure until Tegan's arrival four and a half years later, instead hanging around with human colonist-descendant Leela, far-future robot dog K9, Time Lady Romana, Alzarian Adric, and Nyssa of Traken (though she technically joined the same time Tegan did).
I think that was his longest span without an Earthling companion, but there were other times when he had no present-day companions. After Ian and Barbara left, the companions were Vicki and Steven, both from the future; Katarina from Ancient Troy; and Sara Kingdom from the future. But there was only about a 10-month gap between Ian and Barbara's departure and the brief tenure of Dodo, a present-day English girl, followed by her contemporaries Ben and Polly. But after Ben and Polly left, the Second Doctor's companions for his remaining two seasons were all from past or future Earth.
And on the 'back to basics' thing, she even ended up an unwilling passenger aboard the TARDIS who ultimately grows to accept time/space travel, not unlike Ian and Barbara. So that was kind of where Producer John Nathan-Turner was going with her.

(The more I think about it, the more I think Graham Williams' era was possibly the most 'exotic', space-opera-esque era of the show. It's clearly got an eye on that kind of vibe. The concept of the Doctor having non-human companions was quite in vogue in the 1970s. It was quite different to the thinking that the companion had to be contemporary in order to be relatable for the viewers...)