WILLIAM BRANGHAM: So, for this next story, we need to warn you.

If you have small kids in the room, you may want to mute this and come back a bit later, because we're talking about that visitor from the North Pole, OK?

This Christmas Day, many good boys and girls welcomed that familiar visitor last night, the jolly man in the red suit with a sleigh full of gifts.

But that white bearded figure that we all recognize as Santa Claus, he is a relatively modern creation shaped over centuries.

Stephanie Sy recently talked with an author who unwraps the surprising history of Old Saint Nick.

STEPHANIE SY: That author is Gerry Bowler.

And his book "Santa Claus: A Biography" traces how the legend of our favorite bearded gift-giver evolved over centuries.

Gerry, it's such a pleasure to have you on the "News Hour."

So I want to hop right into it.

Is Santa Claus -- and I quote from the book -- a figure of mythology or a creature of literature or a tool of a clever capitalist?

GERRY BOWLER, Author, "Santa Claus: A Biography": He is a wonderful myth, about 1,700 years old, American in renovation and largely a conspiracy by families.

So it changes over time.

STEPHANIE SY: Saint Nicholas was an actual fourth century bishop.

What was he most known for?

GERRY BOWLER: At the time of his life, he was known for generosity.

But when he died, a cult grew up around him inside Christianity that made him the most influential, popular male saint on the Christian calendar.

He was the patron saint of so many things, but probably his most famous miracle in the Middle Ages was his resurrection of three murdered boys who had been chopped up and put in a pickle barrel.

He discovered this and put them all together again.

So he becomes the patron saint of children.

And thus around maybe the 12th century, he was someone who parents and the church said came on December 6 to bring presents for good little girls and boys to leave something in their shoe.

STEPHANIE SY: So, Gerry, it sounds like there's this darker side of the Santa Claus legend to talk about here that a lot of people are unaware of.

GERRY BOWLER: Well, in the 1500s, when Protestants abolished the cult of saints, parents had to have some kind of magical gift bringer.

They still wanted that aspect.

In many places, they turned to the Christ child.

In French, you call it Le petit Jesus.

In German, it'd be Das Christkind.

The Christ child is certainly a great Christian symbol, but he lacked two things that Saint Nicholas had had.

One, the baby is obviously not going to carry a big sack.

And, two, he's not scary.

And Saint Nicholas could scare kids into good behavior.

So what happened in Germany and in Northern Europe was that the Christ child started becoming accompanied by scary helpers.

They carried a whip or switches or a chain.

In Austria, of course, we have Krampus, which looks exactly like the devil.

So he's one of those scary helpers.

STEPHANIE SY: So this goes back to your first answer, which is there was this conspiracy of families.

Are they basically at the root of the Santa Claus that we know today?

GERRY BOWLER: Well, a number of New York poets and thinkers and rich landowners wanted to make Saint Nicholas the bearer of good things and also a bit of a threat to bad kids.

The first poem that takes Saint Nicholas out of his Catholic bishop's uniform and puts him in a fur-trimmed red robe is called "A Children's Friend" in the 1820s.

It's a poem that describes this Christmas Eve midnight gift-bringer who comes equipped with a reindeer-powered sleigh.

The next year, Clement Clarke Moore that sleigh, multiplies the reindeer, and writes a poem for his family.

STEPHANIE SY: And the poem you're referring to there is "'Twas the night before Christmas."

GERRY BOWLER: "'Twas the night before Christmas, a Visit From Saint Nicholas."

And it goes viral, as it were.

It's adopted by families first in the Northeastern United States.

Then it spreads to Canada and throughout the rest of America.

STEPHANIE SY: So, in other words, there's sort of this amalgamation of traditions that are folded in and layered on.

GERRY BOWLER: That's the nature of Christmas.

Christmas is very adaptive.

By 1900, Santa is pretty much set, though, except with the addition of Rudolph in 1939.

(SINGING) GERRY BOWLER: And despite all kinds of efforts by Hollywood and commerce to make him in their image, he's remained pretty stable since then.

STEPHANIE SY: Why do you think the legend of Santa Claus has endured for centuries?

GERRY BOWLER: Because it is so valuable to families.

Saint Nicholas is this embodiment of generosity, of unmerited favor, to which you add a fantasy, a midnight gift-bringer from some place enormously exotic, powered by reindeer, for crying out loud.

It serves to give kids an idea of fantasy, of generosity.

So, as long as families continue to love Santa Claus, it doesn't matter what Wall Street or any particular denomination happens to be for or against him.

STEPHANIE SY: That is Gerry Bowler, the author of "Santa Claus: A Biography."

Thank you so much for joining us.

Happy holidays.

GERRY BOWLER: My pleasure, and merry Christmas.