Portland, Ore. ? Twenty-five years ago, Jean Auel sat down to write the first threads of an epic, prehistoric tale and realized she didn’t know anything about her subject.

She wanted one of her main characters, a young girl, to help a crippled Neanderthal man get a drink of water. The problem was, Auel didn’t know if Neanderthals had cups or if they did, how they used them.

“How do you get a drink of water? I mean, did she have to take him to the water where he lay down on his belly and lapped it up like an animal or something?” Auel said.

“I didn’t know what I was writing about. I didn’t know who these people really were. I didn’t know what they looked like, I didn’t know what they wore and I didn’t know where they lived.”

A lot has changed since then.

Auel’s first fumbling attempts to write a short story “about a girl living with people who were truly different” have evolved into the best-selling Earth’s Children series. The volumes are an intricately researched tale set 35,000 years ago, when many believe early human and Neanderthal societies overlapped.

The six-book series began with the best-selling “The Clan of the Cave Bear,” which tells the story of a 5-year-old abandoned girl taken in by a group of Neanderthals. The latest book, “The Shelters of Stone,” goes on sale Tuesday with an international release of 2 million copies.

“I sincerely do not know what I’m doing right,” the 66-year-old author said. “I’m writing the story that I always wanted to read. It’s important to do the detail of it, because that’s how you make the story come alive.”

Developing that detail has been a tremendous challenge and joy for Auel.

Whenever possible, she draws on fact for her latest novel, using details from tools, skeletons and cave paintings found at prehistoric archaeological excavations throughout the world. But artifacts of stone and bone don’t reveal the intricacies of daily Cro-Magnon life in the harsh climate of the last Ice Age. How did people interact? How did they worship? What did they wear?

Auel had to piece together those details, melding hints from the past with archaeological theories and her imagination.

“It’s hints, it’s suggestions, but that’s enough for me to write about,” she said. “There are pieces missing and I have to color them in. I’m not going to put palm trees on top of a mountain. On the other hand, so long as it is within the constraints, I can color that any way I want.”

Compelling characters

Auel’s coloring has paid off handsomely. Her first four books, beginning with “The Clan of the Cave Bear” in 1980, have sold a total of nearly 35 million copies.

Fans are already ensuring that “The Shelters of Stone” will be just as popular. On April 15, it was the No. 3 book on Amazon.com.

“I think the following that she has now is incredible,” said Barbara Marks of Crown, Auel’s publisher. “There’s something about the characters, and it’s not only stepping into a different world. These characters are just so compelling.”

Auel’s success is perhaps most surprising to the author herself.

Born in Chicago, Auel married her husband, Ray, just after high school. They moved to Roswell, N.M., and she had five children by 25. She struggled to raise a family, work and go to night school, first in math and physics and then for an MBA at Portland State University.

Until her first novel, which she completed in her mid-40s, Auel’s only published work was “So You Want to Build a Circuit Board,” an instruction manual she wrote while building circuit boards at an electronic testing equipment manufacturer. She also wrote poetry in her spare time but didn’t publish it.

“It was never a goal, never a dream for me to be a writer,” she said.

Character choices

Auel emphasizes equality between men and women in “The Shelters of Stone,” often making women powerful spiritual leaders, healers and community leaders. Women participate in almost every daily activity, from hunting to cooking to cave-painting.

The author drew on a study showing that scientists have found an equal number of burial sites for Cro-Magnon women and men, with a slight preponderance for women.

It’s shaky evidence, but it’s enough for Auel to go on.

“Scientists can say, ‘Well, Dr. So-and-So thinks this and Dr. So-and-So thinks that,’ but if you’re writing fiction, characters either do or they don’t,” she said. “You have to make a choice you can’t have it all.”

Auel worked hard to make her characters and their daily lives seem real. She learned how to shape flint tools, weave baskets and mats, tan hides and prepare medicine from wild plants in the high-desert steppe of Central Oregon.

“I have to live the lives of my characters; I have to make them move in this world,” she said. “You had to be darned intelligent to survive in those days. You had to be intelligent, you had to figure things out, you had to know.”

For the most part, Auel said, archeologists have reacted favorably to her books, even if they don’t agree with every idea.

Doug Kennett, an archaeology professor at the University of Oregon, said “The Clan of the Cave Bear” was required reading when he was a graduate student. The goal was to help students understand the importance of telling a story based on their research, he said.

“Of course, it’s sort of fanciful. But in certain cases, she captures aspects of what the Pleistocene might have been like,” Kennett said. “As long as it’s grounded in some kind of scientific reality, that’s fine.”

Auel said some archeologists had asked her to write about their excavation sites, because even the scientists enjoy thinking about the “what ifs” of prehistory.

“I think that whole period is so much fun,” she said. “I wanted to show that early humans were not stupid that they were not just savage, but that they had some sophistication.”