Starbucks had 100 ideas for its first fall beverage. These 3 flavors almost beat the Pumpkin Spice Latte.

• Starbucks is celebrating the 20th anniversary of its beloved Pumpkin Spice Latte.
• Peter Dukes — known as the father of the PSL — told Insider the drink's origin story.

Twenty years ago, Starbucks released a new beverage that instantly captured the nation's attention.

That drink was the Pumpkin Spice Latte.

In honor of Starbucks' milestone anniversary, Insider recently sat down with Peter Dukes, the "Father of the PSL," to learn more about the drink's origin story — including the fall beverages that almost took its place on the chain's menu.

Dukes told Insider that Starbucks got the idea to create a special fall drink following the successful launch of its peppermint mocha during the 2002 holiday season.

Early in 2003, Dukes and his team got together at the "Liquid Lab" — the former name of Starbucks' research and development space at its Seattle headquarters — and began brainstorming and came up with 100 different ideas for the company's first-ever fall drink.

Dukes said they narrowed that list down to 10 beverage concepts, which they polled their customers on.

"We asked our customers two questions," Dukes recalled. "How likely are you to buy this? And how unique is it?"

"We got back the results and, not too surprisingly, the flavors that had chocolate or caramel in them scored quite high in terms of 'how likely to buy,' but the unique levels were pretty flat," he said.

Also on the list was a pumpkin latte, which Dukes said scored "really high" in uniqueness but really low for likelihood to purchase.

Dukes said the team had approval to develop four of the 10 concepts into drinks with the help of Starbucks' food scientists. The team automatically knew what three of the four concepts would be: a maple pecan flavor, a honey nut drink, or a cranberry white chocolate mocha.

"There was a lot of debate about what the fourth flavor was going to be, and pumpkin was in that debate," Dukes said. "Go back 20 years. If you walked down the grocery aisle, there were only two things that were pumpkin — pumpkin purée and pumpkin-pie topping. Nothing else existed out there. There were no pumpkin beverages that existed."

It was a huge risk, but the team agreed it was one worth taking, Dukes said.

"We knew we had something that was pretty unique," he added. "And the rest from there is history."

Dukes, who still works at Starbucks, said it's been incredible to watch the Pumpkin Spice Latte continue to grow in popularity.

"Fall is my favorite time of year. I love how the leaves are changing colors. It's that time you're getting back into your rhythm and routine," he said. "And the Pumpkin Spice Latte helps capture that in a cup a little bit — it brings a smile to my face."