Other western European countries have had trouble squarely facing such history; many Belgians were unaware of the atrocities in Congo under Belgian rule until the past generation. But Denmark, with less of a colonial record to confront than some countries, has had more trouble confronting it, according to Mr. Palsson.
“Somehow it annoys them more than others knowing about this background,” he said.
Hans Jonathan was born in 1784 in St. Croix, then a Danish possession and now part of the United States Virgin Islands. His mother was a black house slave owned by the Schimmelmanns, a Danish-German family, and his father was a white man.
When he was about 7, the Schimmelmanns took him to Copenhagen. In 1801, he volunteered to fight with the Danish navy, and emerged unharmed from a fierce battle with British ships.
“It was crazy warfare,” said Mr. Palsson, whose biography of Hans Jonathan was published in Icelandic in 2014, and in English in 2016. “The ship was bombarded heavily.”
Hans Jonathan earned the support of his superior officers, who spoke on his behalf to the royal household. Denmark’s crown prince and de facto ruler, the future King Frederik VI, wrote in a letter that Hans Jonathan “is considered free and enjoys rights.”
The French revolution had unleashed new ideas about equality and liberty. Like several other colonial powers, Denmark still allowed slavery in the Caribbean, but abolition movements at home were gaining ground, and the status of slaves brought to Europe from the colonies was murky.
Henrietta Schimmelmann tried to reclaim Hans Jonathan and take him back to St. Croix, and he went to court to assert his freedom, in a case that was famous in its time. But he could not produce the letter from Prince Frederik, for reasons unknown, and in 1802, the court dismissed his claim and ordered him to return to the Schimmelmanns, who wanted to sell him in St. Croix.
Instead, he fled, sailed to Iceland, settled in the small village of Djupivogur, married a local woman, had children, and lived as a free man until his death in 1827.
Though he was largely forgotten in Denmark, in Iceland he became a well-known figure in local folklore, said Kari Stefansson, head of the company deCode Genetics, whose father grew up in Djupivogur.
“People think it’s a beautiful story,” he said. “It shows that racism isn’t innate. It’s a learned behavior.”
DeCode published a study this year that takes advantage of Iceland’s highly homogeneous gene pool, its remarkably thorough genealogical records, and Hans Jonathan’s unique place in the country’s history.
Researchers identified some 780 living descendants of Hans Jonathan, took DNA samples from 182 of them, and isolated snippets that were characteristically African, and could only have come from him. They said they were able to reconstruct 38 percent of his mother’s genome, and trace it to parts of West Africa.
“It is certainly of interest how bits and pieces of an African genome are found in genomes of current Icelanders,” Mr. Stefansson said.
Some Icelanders knew of Hans Jonathan already, but many of his descendants, who are spread among several countries, lived most of their lives unaware that they had an ancestor who was a black slave.
Among them was Kirsten Pflomm, a communications manager from Connecticut, who did an online search of her own name 15 years ago, and found herself and entire family listed on a website written in Icelandic. She contacted the site’s administrator and learned the dramatic story of her five-times-great-grandfather.
”I clearly look very white,” she said. ”So I can’t claim to ever have experienced anything” resembling racism.
Ms. Pflomm is not seeking an official apology for Hans Jonathan’s time in shackles, or hoping to achieve greater Danish awareness of slavery and colonization. “That’s a bigger conversation that doesn’t include me,” she said.
But she petitioned Prime Minister Lars Lokke Rasmussen, asking him to award Hans Jonathan his freedom. In a letter back to her, the prime minister wrote last month that her ancestor was a “beacon of liberty,” but that he “cannot reverse time or the verdict of the past, no matter how incomprehensible it may seem.”
But Astrid Nonbo Andersen, an expert on colonial history at the Danish Institute for International Studies, said that Danish views may have begun to change last year, during the centennial of the sale of the Denmark’s Caribbean islands to the United States.
“There was an explosion of events like never before,” she said. “But if it was just a news interest or something lasting remains to be seen.”
The wealth of the colonial era is visible in the elegant Rococo and Baroque mansions of Copenhagen. The first tribute to the people who suffered to create that wealth appeared on April 2, when a statue was unveiled of a black woman who led a 19th-century rebellion against Danish rule on St. Croix.
The statue, 23 feet tall, may be hard to ignore.
“It’s really big and right on the waterfront,” Ms. Andersen said. “Tourists will begin to ask, and tour guides will have to add a new chapter to their guiding.”