The interesting thing is that if you were only to read the opening chapters of Lion, you'd get the impression that humans were utterly alien to Narnia. The later volumes make clear that humans were not; indeed, they not only were part of Narnia from the beginning, but entrusted with a special role - i.e. only a human could be king or queen of Narnia.
Here's my speculative read on that: Jadis knew from the beginning that only humans could rule Narnia, though it was not a land of humans - it belonged to the creatures themselves. She was not herself human, but when she usurped (or conquered, or whatever) the throne of Narnia, she justified her claim by contending that she was human. The Beavers point this out, though given that they knew the real story, we can assume that other loyal Narnians did as well, so it was a hollow claim.
However, to head off any legitimate human from claiming the throne, she killed or ran off all men in Narnia, and even engaged in some revisionist history to try to obliterate any mention or memory of men in the land. This was why the appearance of the Pevensies was so alarming to her: she had the borders secure, but there was a back door into Narnia from the world of men.
That's one way it could have played out. There might be other hypotheses.
Here's my speculative read on that: Jadis knew from the beginning that only humans could rule Narnia, though it was not a land of humans - it belonged to the creatures themselves. She was not herself human, but when she usurped (or conquered, or whatever) the throne of Narnia, she justified her claim by contending that she was human. The Beavers point this out, though given that they knew the real story, we can assume that other loyal Narnians did as well, so it was a hollow claim.
However, to head off any legitimate human from claiming the throne, she killed or ran off all men in Narnia, and even engaged in some revisionist history to try to obliterate any mention or memory of men in the land. This was why the appearance of the Pevensies was so alarming to her: she had the borders secure, but there was a back door into Narnia from the world of men.
That's one way it could have played out. There might be other hypotheses.