When Cassius Clay joined Islam in 1964 and changed his name to Muhammad Ali he had a straightforward explanation. “Why should I keep my white slavemaster’s name visible and my black ancestors’ invisible, unknown, unhonoured?” Ali asked.

Then, it was more of an abstract concept, a statement against white oppression; Ali did not know much, if anything, about his ancestors or his own family tree.

Decades later, though, Ali’s family has made a discovery that appears to shed new light on the boxer’s lineage – where he came from, and also his place in American history.

Ali, according to his family’s research, is the great-great-great grandson of Archer Alexander, a slave who heroically fought both for his own freedom and against slavery.

Alexander escaped from bondage and surreptitiously fed information to the Union Army during the American Civil War. He was later the model for the slave depicted in the Emancipation Memorial, a statue in Lincoln Park, about east of the US Capitol.

“The beautiful thing about Ali is that he acted all along as if he were royalty, that he had a claim to greatness,” said Jonathan Eig, the author of Ali: A Life.

“Ali spent much of his life attacking racist ideas,” Eig said. “If he had known that his great-great-great grandfather was such a brave and intelligent man, it surely would have strengthened his argument.”

After being alerted to the family’s discovery in recent weeks, Eig investigated the claim and then included it in the paperback edition of the biography, which comes out this week.

“To the best of my ability to confirm this, it checks out,” he said.

Ali’s daughter, Maryum, said that her father would have been proud to call Alexander family. “He would have loved knowing he was connected to someone like that,” she said. “He was ahead of people in understanding that there was a connection that went back through slavery to the kings and queens in Africa.”

The discovery was made by Ali’s third cousin, Keith Winstead, who is retired from a career in computer manufacturing and is something of an amateur genealogist. Winstead discovered the connection between Ali and Alexander while conducting research on the website 23andMe. The finding is supported by DNA evidence, which, according to Maryum Ali, was collected when Ali and his wife, Lonnie, participated in a study with 23andMe to raise awareness for Parkinson’s disease, from which Ali suffered.

The lineage, according to Winstead, goes like this: Ali’s father, Cassius Clay snr, was the son of Edith Greathouse, who was Alexander’s great-granddaughter.

“I didn’t know who Archer Alexander was when I traced the family tree,” said Winstead, 67. “I Googled him, and I just said, ‘Wow.'”

Ali was born in segregated Louisville in 1942, and his role as a pioneer and activist would become as much a part of his legacy as his boxing career (he passed away in 2016). In the 1960s, he joined the Nation of Islam, an African American political and religious movement led by Elijah Muhammad, who advocated for racial separation, believing White America had stripped African Americans of their family histories. Indeed, descendants of slaves can have a difficult time tracing their ancestry because their forebears had their identities purposely and systematically stripped.

“The fact that Ali didn’t even know of his heroic ancestry lends proof to at least part of the Nation of Islam’s claim,” Eig said. “Had Ali been a white man with a courageous and a celebrated ancestor, his family might have enjoyed wealth, fame, and political power. Instead, his ancestors struggled to survive.”

Amidst the hardship, Alexander led a remarkable life.

He was born into slavery in Virginia in 1813 before he was sold and taken to Missouri. Though Missouri remained neutral during the Civil War, Alexander was owned by a Confederate sympathiser, and in 1863 he learnt that Confederate troops had sawed a train bridge that Union soldiers were planning to cross. He walked 8 kilometres to warn the Union Army, potentially saving hundreds of lives. He also passed along information about hidden arms.

Accused of feeding information to the enemy and with his safety in danger, Alexander ran away, evading slave catchers by climbing out of a tavern window before he reached St Louis. He later arranged for the escape of his wife and children.

“Go for your freedom ef [sic] you dies for it,” Alexander once said.

In St Louis, Alexander found work as a gardener for William Greenleaf Eliot, Washington University’s co-founder and the grandfather of the poet T.S. Eliot. Eliot obtained an order of protection for his employee, though slave catchers came for Alexander again, beating and capturing him before Eliot produced the protective order to secure his release.

Eliot published a biography of Alexander and had his photograph taken and sent to Italy, where it was used by the sculptor Thomas Ball when he constructed the Emancipation Memorial. The statue was dedicated in 1876.

The sculpture depicts a freed slave with Alexander’s likeness kneeling with broken chains at the feet of Abraham Lincoln. The statue was one of the first monuments to be funded by former slaves, but has been criticised more recently for its patriarchal message: a glorious depiction of Lincoln as a saviour and the freed slave crouched below him.

In 1980, The New York Times published a story that traced some of Ali’s heritage on his mother’s side. “I never knew much about my ancestors until now,” Ali said then. “When I’m gone I want my grandchildren and great-grandchildren to give me credit for what I did – and in the same way, I’m happy to know about my ancestors so I can give them credit. ”

Ali added: “Someday I’d like to dig up everything that can be found about all the people I’m descended from.”