Today marks the anniversary of former heavyweight champion boxer Sonny Liston’s 1964 defeat at the hands of Cassius Clay, a young boxer who boasted that he would “float like a butterfly, sting like a bee.” Later changing his name to Muhammad Ali, the new heavyweight champion would go on to secure himself a permanent place in the annals of history, not just in sports but in worldwide popular culture. He became a voice in his native United States for social justice and civil rights, particularly in relation to African Americans, and his celebrity status led him to rub shoulders with other icons of his generation, including Malcolm X. In fact, Malcolm X was in attendance the night Clay defeated Liston, his presence indicative of the bond between the two. As a mentor, Malcolm X played a significant role in Clay’s eventual conversion to Islam, transition into Cassius X and then Muhammad Ali. But there existed more than just a student-teacher dynamic between the two, and a new book explores the friendship that developed in their relationship.

Blood Brothers: The Fatal Friendship Between Muhammad Ali and Malcolm X is co-written by Randy Roberts and Johnny Smith and according to NPR, the book examines the intricacies of the interactions between the two giants. In an audio package for the Code Switch program, Karen Grigsby Bates uses recordings of Malcolm X’s speeches, audio clips of Ali’s bombastic words, and interviews with the authors to illustrate a brief history of their friendship, and it’s pretty incredible to imagine the two spending time with one another. What did they talk about? How did they address one another? How did they meet? The latter is answered in the segment, with a 1962 Nation of Islam meeting in Detroit credited as the place where the two would first interact. After hearing Malcolm X speak, Ali – at the time still Cassius Clay – was impressed, having never heard a Black man speak that way. As co-author Smith describes, “there was this confidence, this authority that Malcolm offered when he spoke at the podium that day, and it made a profound impression on young Cassius Clay.” Malcolm was similarly drawn to the young fighter, and as Bates describes, “he thought people could be drawn to the Nation by Clay’s swagger.”

What ensued after their initial meeting is a complex and multi-layered piece of American history, with elements of politics, racism, religion, and death intertwined. As Bates describes, “The two began to spend time together whenever possible. Malcolm was part father figure, part big brother to Clay. His faith was also a draw, because Clay had rejected the faith he’d grown up with as empty. He was looking for a spiritual home, and perhaps the Nation could be that place.” However, Ali was reticent to share his spiritual development openly with the American public, at least at first. Ali feared that announcing his conversion to Islam would invite the end to his athletic career. “As Roberts explains to bates, “If a rising sportsman aligned himself with [the Nation of Islam], [t]he boxing establishment – sportswriters, promoters, backers – wouldn’t tolerate that.” But that all changed when he beat Sonny Liston, and the effects of his announcement reverberated throughout American society. “Here you have the most famous mainstream civil rights leaders, heroes — King, (Jackie) Robinson, Floyd Patterson — who were criticizing Clay for joining the Nation of Islam,” says Smith. Civil Rights leaders who disagreed with Malcolm X’s separatist style began to criticize Ali, adding that complexity to their friendship.

Soon enough, the friendship would be tested when the Nation’s founder Elijah Muhammad forced Malcolm X out. As the story goes, Malcolm was certain Ali would join him, but as Gates writes, that was not the case.

Related: 5 Decades Later, Muhammad Ali Remains The People’s Champ. He is Celebrated in a New Documentary (Video)