In the novels of CS Lewis, who, chronologically, is the first human to enter Narnia?

The famous series of books, "The Chronicles of Narnia" by CS Lewis (1898-1963) do not follow a straight linear pattern.

The first book published in the series, and most people's introduction to it, "The Lion, the Witch and the Wardrobe," features wartime evacuee Lucy Pevensie and her siblings entering Narnia through the titular wardrobe. But in the 6th book of the series, "The Magician's Nephew" we are transported back (in earth time!) to London in 1900, where 11-year old Polly is living with her aunt, and befriends her young neighbour Digory Kirke. The two of them find their own attic retreat, and Polly is evidently an aspiring author herself, as she is writing a story (to which Digory is not privy!).

Digory's Uncle Andrew is a mysterious and rather forbidding character, but wins Polly's trust, and gives her a magic ring that bestows invisibility. The children enter another world of giant pools, dying suns, and ruined cities, and also meet Jadis, aka the White Witch. They witness the birth of Narnia and are the first to encounter the benign mighty lion (and Christ-figure) Aslan. Aslan gives them a gift of an enchanted apple that heals Digory's ailing mother - an interesting variant on the poisoned apple in the "Sleeping Beauty".

They remain friends their whole lives long, and reappear in the last novel in the series, "The Last Battle" as wise adults.