Over this New Year period I've been continuing to show my girlfriend a selection of old, meaning proper, Doctor Who stories. Previously, it had been some Tom Baker and a couple of Troughtons, but a solid introduction into Pertwee was embarked with Terror of the Autons, The Dæmons, The Three Doctors and Carnival of Monsters. Together, while not all the best stories, these give a good feel for what the era was like, the UNIT family, introductions of the Master and Jo Grant, the restoration of the Tardis dematerialisation circuit and finally a return to normal travels in space and time. If you're interested, her favourite was the Dæmons followed by Carnival.
I don't watch Doctor Who alone very often, I prefer it much more as a communal experience now. As I'm already familiar with most existing stories, the joy is often seeing others watch it as it is for myself, and being able to discuss things afterwards. It's from this that I've come to realise just how much the character of Jo Grant has provided the shape for Doctor Who in the decades to come.
It makes sense that during the 60s they had larger Tardis crews of 2-3 companions supporting the Doctor. The stories were longer with more screen time dedicated to dialogue, and given the gruelling schedules the work load had to be spread out. And allowing for those occasions when a cast member had a week off leaving the character in stasis sleeping under a rug or locked in a cell. Also, characters all had their niche, the strong younger male (Ian, Steven, Jamie) could do the physically demanding work for the Doctor, the older female (Barbara) was a guide for the naive younger (Susan, Vicki). As the Doctor became more physically able the requirement for a 'strongman' figure wasn't needed. You don't need Jamie to fight anyone when the 3rd Doctor can thrash anyone with Venusian Akido. Also they seems to tighten up stories with four episodes being the norm and the occasional six parter. Reducing the crew to a single companion and UNIT serving as non-companion semi-regulars is the natural progression. In fact the six parters throughout the 70s are often a little bloated and padded leaving main characters with little to do other than go in circles.
During the 80s the show struggled with a 'crowded Tardis' as it attempted to merge both a large Tardis crew with shorter story lengths. They didn't appear to have any reason for a larger Tardis crew either, unlike the 60s where characters had more defined roles, the early Davison era had a crew with two child geniuses and a slightly older but female character that was headstrong but not wise. They just don't mesh like Ian, Barbara and Susan. Furthermore, the changes in TV meant they didn't have the luxury of screen time being largely dialogue, given greater expectation on action and effects, even if it's just a Myrkra lumbering around. Eventually they drastically slimmed the crew back by killing one of them off. But while overcrowding the Tardis seemed an obvious problem, the show didn't actually create the 'single companion model' of the show until the 70s. The idea of the Doctor travelling alone with a single female companion didn't come about until Jon Pertwee took on the role and Jo Grant came on board. The UNIT regulars are semi-companions to flesh things out for much of this period and while Liz Shaw is the Doctor's only companion, they don't travel in the Tardis and there's little to distinguish her from the UNIT team. Prior to this the only occasion in which the Doctor was previously left with a single companion was after The Savages where the Doctor leaves only with Dodo, until the War Machines immediately brought in Ben and Polly. Yes, Dodo was, for all of a couple of episodes, the first companion to travel alone with the Doctor.
When Jo Grant was introduced, she became the single female companion approach that has been used many times since and probably considered the norm. After her came Sarah Jane, Leela, Romana, Peri, Ace and every New Series companion. Leela and Romana were quite different as companions go but both were of the single-female approach. The New Series has been very unimaginative with companions, every one is a solitary female companion from contemporary Earth. The New Series quickly established this as the standard even going as far as to introduce failed-companion Adam just to show how much it wouldn't work having a man on board long term. Rory is the only exception, and they even killed him off for half a series to keep the Doctor and Amy alone. The New Series women have greater issues though, in that each one superficially appears 'feisty' and 'independent' but ends up fancying the Doctor and being somewhat shallow.
But before all them Jo loved the Doctor, in her own way of course. It was much more clearly a parental relationship, she loved the Doctor and he loved her, as a father would. The New Series finds it much easier to have the Doctor obviously flirting with his companion in a way which would have been unthinkable when 'sex in the Tardis' was actually an issue. But that's where the character of Jo Grant would eventually lead. She was the first female character to have adventures alone with the Doctor and provide a somewhat vacuous blend of sexiness with a little action, and plenty of "what's that Doctor?"