C. S. Lewis wrote most of The Chronicles of Narnia without a long-term plan. In most cases, he published each volume with little to no idea what the next one would be about. (The major exception occurs with The Silver Chair and The Horse and His Boy. Before he started on The Silver Chair, Lewis had already written a draft of The Horse and His Boy. When the two books were published in the opposite order from order in which they were composed, Lewis included references to the upcoming The Horse and His Boy in The Silver Chair.)
When reading the books as a child, in the published order, I noticed that the role of humans in the Narnian world becomes more pronounced as the series progresses. In fact, there are some significant retcons over the course of the Chronicles:
In The Lion, the Witch, and the Wardrobe, it is explicitly stated by the talking beavers that there have never been any humans in Narnia before. Mr. Tumnus the faun has a book entitled Is Man a Myth?, and merely being human is considered a qualification to be king or queen of the country.
Prince Caspian contains no outright retcons, but it does introduce other humans in Narnia. The descendants of some mutinous sailors and their Polynesian wives get into a bloody conflict on their new South Pacific home (the whole episode seemingly inspired by the story of the mutiny on the Bounty) and find their way by accident through a portal to the uninhabited land of Telmar, adjacent to Narnia. This occurs long after the events of the first book, and there are no indications that there are any humans in the world other than the Telmarines. The kingdom of Calormen is mentioned, but the race (human or not) of its inhabitants is not.
Things shift further with The Voyage of the Dawn Treader, which contains multiple continuity snarls. In the the Lone Islands, the voyagers meet foreign humans (Calormene slave traders), and it is unclear whether the inhabitants of the islands are Telmarines or originated elsewhere. Moreover, the Lone Islands appear to have somehow maintained a continuous series of governors owing fealty to Narnia since the time of The Lion, the Witch, and the Wardrobe, which makes no sense at all. (A similar continuity error occurs when they find the belongings of Lord Restimar on Deathwater Island. They know he is a Narnian by the lion-headed coins among his things, but there is no reason why a Telmarine lord should have been carrying such currency.)
The next book to be written was The Horse and His Boy, in which it is shown that there are substantial human civilizations outside of Narnia, during the time of The Lion, the Witch, and the Wardrobe. At this point, the retcon is clearly complete; the only element to be added to the retconned history is the presence of human kings and queens in Narnia from its earliest days, as described in The Magician's Nephew.
What I wondering is whether Lewis indicated at what point in the writing process he decide to dispense with the idea that humans had not been a part of Narnia's world prior to The Lion, the Witch, and the Wardrobe? Was it between the first two books, or between the second and third? Moreover, was it a conscious decision Lewis made to include more humans in Narnian history, or did it just happen "by accident," through a sequence of unintentional retcons?