Frans de Waal, who used his study of the inner lives of animals to build a powerful case that apes think, feel, strategize, pass down culture and act on moral sentiments — and that humans are not quite as special as many like to think — died on Thursday at his home in Stone Mountain, Ga. He was 75.

The cause was stomach cancer, his wife, Catherine Marin, said.

A psychologist at Emory University in Atlanta and a research scientist at the school’s Yerkes National Primate Research Center, Professor de Waal objected to the common usage of the word “instinct.” He saw the behavior of all sentient creatures, from crows to persons, existing on the same broad continuum of evolutionary adaptation.

“Uniquely human emotions don’t exist,” he argued in a 2019 New York Times guest essay. “Like organs, the emotions evolved over millions of years to serve essential functions.”

The ambition and clarity of his thought, his skills as a storyteller and his prolific output made him an exceptionally popular figure for a primatologist — or for a serious scientist of any kind. Two of his books, “Are We Smart Enough to Know How Smart Animals Are?” (2016) and “Mama’s Last Hug: Animal Emotions and What They Tell Us About Ourselves” (2019), were best sellers. In the mid-1990s, Newt Gingrich, when he was speaker of the House, put Professor de Waal’s first book, “Chimpanzee Politics” (1982), on a reading list for Republican House freshmen.

The novelists Claire Messud and Sigrid Nunez both told The Times that they admired his writing. The actress Isabella Rossellini hosted a talk with him in Brooklyn last year. Major philosophers like Christine Korsgaard and Peter Singer wrote long, considered responses to his ideas.

Professor de Waal’s influence was such that The Times credited his work with unleashing “a torrent of discussion about animal sexuality” and helping to popularize the term “alpha male,” though neither of those accomplishments had much to do with the core of his thought.

His interest in what is shared across species, emotionally and morally, was kick-started in the mid-1970s, at the beginning of his career, when he saw one male chimpanzee raucously confront another, then calm down and extend his hand, palm up, in a peace offering. The apes then embraced and groomed each other. After further research, he concluded that the episode showed a desire and ability to reconcile after fights.

He found further striking evidence that animals other than humans have empathy and a sense of fair play while working with the psychologist Sarah Brosnan in the early 2000s. The scholars designed an experiment in which two monkeys were awarded cucumbers for completing a task. Then one monkey was given a grape and the other was given a less tasty cucumber. The one that got the cucumber began refusing to cooperate, even hurling the vegetable back at the researcher. Some animals that got the better end of the deal declined their grapes.

Many of Professor de Waal’s animal anecdotes were moving. He wrote about a bonobo named Kuni who once picked up an injured starling, climbed a tree, spread the bird’s wings and then released it, enabling it to fly. “She tailored her assistance to the specific situation of an animal totally different from herself,” Professor de Waal wrote in his 2005 book, “Our Inner Ape: A Leading Primatologist Explains Why We Are Who We Are.”

These sorts of episodes indicated that primates had cognition, he said. Other ape behaviors — young females getting maternal training, for example — indicated something even more impressive: that apes were capable of learning, remembering and passing down new skills across generations, meaning that different communities had their own cultures.

All this language was unusual among scientists, and some objected. Donna Haraway, a scholar not of primates but of primatologists, argued that Professor de Waal tended to imagine a world in which “primates became model yuppies” — that he was, in other words, engaging in a kind of projection. A common argument against his work was that he anthropomorphized nonhuman animals.

Professor de Waal replied that the real problem was not anthropomorphism — apes and humans have many commonalities justifying comparison, with similar brains and psychological makeups — but instead a human exceptionalism that rejected even the possibility of humanlike behaviors in other animals, as well as animal-like traits in humans. He called this tendency “anthropodenial.”

For Professor de Waal, his critics were missing out on good news: Morality turned out to be deeply rooted in our evolutionary past.

Franciscus Bernardus Maria de Waal was born on Oct. 29, 1948, in ’s-Hertogenbosch, a city in the southern Netherlands, and grew up in nearby Waalwijk. His father, Jo, was a banker, and his mother, Cis (van Dongen) de Waal, ran the home, raising six sons.

Frans kept pet fish as a child, and by his college years he had a kitten named Plexie, which he said he took on regular interspecies play dates with a puppy.

When he was 22, Frans attended the wedding of his brother Wim, who was close friends with a young Frenchwoman he had met after they were randomly assigned as pen pals in school. Upon meeting, Frans and the Frenchwoman, Ms. Marin, fell in love instantly. A year later, they moved in together.

During Frans’s early years in academia, a job studying macaques led him to develop a specialty in apes. He began working as a researcher of chimpanzees at Arnhem Zoo, in the east of the Netherlands, in 1975. He earned his Ph.D. in biology from Utrecht University in 1977.

He and Ms. Marin married in 1980 to make it easier for them to move to the United States as a couple. The next year, Professor de Waal took a job at the Wisconsin Primate Center at the University of Wisconsin-Madison.

He published 13 books, and at his death he was writing another, about how people’s thinking about animals has evolved over time. John Glusman, the vice president and executive editor of W.W. Norton & Company, Professor de Waal’s publisher, said in an email that the company planned to release it next year.

In addition to Ms. Marin, Professor de Waal is survived by his brothers, Ferd, Wim, Hans, Vincent and Steven.

Professor de Waal’s sympathy for apes was not lost on the animals themselves.

At Arnhem Zoo, one female chimp, Kuif, was unable to lactate sufficiently, leading each of her babies to die. Each time one died, she would rock back and forth, clutch herself, refuse food and scream. Not long after, another female chimp with even more intractable health problems gave birth at the zoo.

Professor de Waal had an idea. He began training Kuif to handle a bottle.

It was hard to teach Kuif not to drink the milk herself. When the baby chimp, Roosje, was first placed on a straw bed in her living area, Kuif almost performatively looked away from her.

Then Kuif approached the bars, where a caretaker and Professor de Waal were watching her. She kissed them and glanced up at them, as if asking permission. The two humans waved their arms and said to pick up Roosje. She did — and became the most caring mother Professor de Waal could imagine.

“After this adoption, Kuif showered me with the utmost affection,” he recalled in his book “Mama’s Last Hug.” “She reacted to me as if I were a long-lost family member, wanting to hold both my hands, and whimpering in despair if I tried to leave. No other ape in the world did that.”