When Jonathan Tweet’s daughter was young, he searched for a book that would introduce her to the complicated, almost mysterious, scientific concept of evolution. “I found pretty much nothing,” he recalled. A designer of board games and self-professed science geek, Tweet set out to fill that void.

Tom Sullivan’s search for the same kind of book came about completely by accident. A graphic designer, he had been working on a story about an alien when he got stuck on what it should look like. “I started thinking about how the way different animals are born determines what they are going to look like,” he said. “I was thinking about mammals and reptiles and eggs, when I got to frogs. I started to wonder how a frog who used to be a tadpole would describe that experience, and a title for a different story just popped into my head.”

I Used to Be a Fish, written and illustrated by Sullivan (Balzer + Bray), is one of two debut picture books on the ticklish topic of evolution released this fall. Feiwel and Friends published Tweet’s Grandmother Fish in September, picking it up after he and illustrator Karen Lewis self-published the title a year ago.

“I had never seen a picture book about evolution before,” said Donna Bray, v-p and co-publisher at Balzer + Bray. “And as a picture book creator, Tom had such a good sense of negative space, palette, and page turns. I felt like, if Dr. Seuss had done a book on evolution, this is what it would look like.”

Feiwel was sold on Tweet’s book after giving the self-published version to colleagues with young children. “A lot of people who brought it home reported that their kids really liked it,” she said. “This is not an institutional book. It’s kid-friendly. I think it will endure.”

Tweet was not wrong about the scarcity of books about evolution for the youngest readers: they are few and far between. His decade-long search for comparable titles turned up only two: Lisa Westberg Peters’s Our Family Tree (Harcourt, 2003) and a Let’s Read-and-Find-Out volume, Evolution, by Joanna Cole (Crowell, 1987) that is long out-of-print (a paperback copy is selling on Amazon for $56.68.) “All around, the Let’s Read book was the best one I found,” he said.

Part of the reason is that evolution is a tough concept to simplify. “Billions of years ago” is not easy for a preschooler to grasp, nor is “descended from apes.” There is also an objection for religious reasons because evolution contradicts the Biblical version of how humans came to be. Both editors bought their manuscripts knowing, as Feiwel put it, that the books might face “resistance.”

“At the acquisition meeting we did have sales people who said, ‘We may have trouble with this,’ ” Feiwel recalled.

So far, so good, both editors reported, though they were also quick to caution: it’s early days.

“Nobody’s voiced an objection but not every account is taking it,” said Feiwel. “I think that the most difficult thing is that the people who do object just won’t stock it or review it and you really can’t smoke that out.”

Though the concerns and subject matter were similar, the path to publication for each book could not have been more different.

Tweet’s journey started more than 15 years ago, when his daughter, now a college graduate, was a toddler. “I had a bright inquisitive young mind in the house and I wanted to share with her the scope and grandeur of life and earth. I couldn’t find a book out there that made it accessible to kids,” he recalled. Tweet worked for years on a text that would be scientifically accurate and engaging to young children. He tried to have it published conventionally but admits, “The story was not good enough yet so I kept working on it and working on it.”

Three years ago he had two breakthroughs. The first was rewriting the text in a call and response style, which invites the audience to find things they share with Grandmother Fish, who “lived a long long long long long time ago.”

She could wiggle and swim fast

Can you wiggle?

And she had jaws to chomp.

Can you chomp?

“The interactive element was more powerful than I could have predicted,” Tweet said. “Mimicry is such a big part of who children are. It’s something that kids really respond to.”

The second key was being introduced by a friend of a friend to Lewis, whose day job is resident cartoonist for Cobblestone, an American history magazine for kids.

“She knows children’s books better than I do and she was really helpful in bringing out the cute side of evolution,” said Tweet, who credits her with helping him figure out how to depict mammal lactation.

“Nobody liked the verb ‘lactate.’ I had people tell me not to bring it up. But that’s the defining thing that makes us mammals,” Tweet recalled. He was able to use the verb “cuddle” instead because Lewis’s discreet illustration of a mother possum and her babies curled together in blissful circle suggests the babies are feeding. “It turned out to be the best page in the book,” he said.

By the time Tweet’s rewritten text met Lewis’s illustrations, “Kickstarter was a real thing. We could publish it ourselves and I could die happy,” Tweet said. They asked for $12,000; they raised $36,000. Their donors’ names are listed on the front and back endpapers of both the self-published version and the new version published by Feiwel. “Those are the thousands of people who believed in the project,” Tweet said. “Macmillan was very gracious in allowing those names to stay in.”

Tweet would have been satisfied at that point, but then Barbara King, a professor of anthropology at the College of William & Mary, wrote about the book on NPR’s Cosmos and Culture blog, quoting an educator from the National Center for Science Education who said Grandmother Fish was “heads and shoulders above any evolution book for children” she’d ever seen.

King’s post went viral. The self-published edition sold out and an agent, David Doerrer of Abrams Artists, contacted Tweet about taking the book to a mainstream publisher. Three houses bid before Feiwel and Friends walked away a winner.

Feiwel said she made few editorial changes. “We did redesign the back matter because it was a little too crowded and we kind of punched up the color on the cover and tweaked the typeface, but for the most part they did a masterful job with a topic that is not easy,” she said. “I think this is a book that will have a very long life. It may not be big out the door but it will build.”

Sullivan’s path to publication was far swifter. He contacted Steven Malk of Writers House, who took him on as a client in May 2015. After a five-house auction a few weeks later, Malk sold the manuscript to Bray in a two-book deal.

Bray’s editing challenge was to make sure that Sullivan’s sly humor didn’t get lost as the book was edited to make the science correct. “It’s a very light introduction to evolution but it was scientifically flawed at the beginning and we had to fix that,” she said. Bray put him in touch with Glenn Branch at the National Center for Science Education who was only too happy to help Sullivan get the facts right. He even forwarded him some material to study.

“He sent me a copy of Grandmother Fish,” Sullivan said.

I Used to Be a Fish by Tom Sullivan. Balzer + Bray, $17.99 Oct. 978-0-06-245198-9

Grandmother Fish: A Child’s First Book of Evolution by Jonathan Tweet, illustrated by Karen Lewis. Feiwel and Friends, $17.99 Sept. 978-1250113238