Muhammad Ali's recent 72nd birthday was occasion for celebration and sadness.

Celebration because it allows us to reflect on the huge impact he has had on the world both in and out of the ring - sadness because his poor health means the chances of him being around for his 73rd appear slim.

The sombre truth is that the days of the fighter known as The Greatest are drawing to a close.

But what days they have been.

Who could have predicted when that young, gangly, loose-limbed boxer from Louisville, Kentucky, took the light-heavyweight gold medal at the 1960 Rome Olympics - dismissed by most sportswriters as lacking the technical know-how and power to make a significant impact as a professional - that he was about to explode onto the world stage like a hurricane unleashed.

He became not only the fastest and most beautifully aesthetic heavyweight in the history of the sport, but a thorn in the side of an Establishment under which US blacks suffered apartheid, racism and structural injustice.

Who could have guessed that this young man would become a beacon of hope to millions throughout the globe, and a cultural icon whose very name would remain an inspiration over five decades on.

At that first Liston fight in Miami in 1964 only a few insiders were aware of the anger, defiance and political and religious consciousness bubbling away under the surface of Cassius Clay's playful braggadocio and exuberance.

It was just after the astonishing victory where he "shook up the world" that the 22-year-old newly crowned heavyweight champion revealed that he was a member of the Nation of Islam, renamed the Black Muslims by reporters and TV broadcasters looking to court controversy.

This was followed by the name change from Cassius Clay to Cassius X and finally Muhammad Ali. All of a sudden a tiny, fundamentalist religious sect became the bete noire of the US Establishment.

But where the Nation of Islam connected with Ali, and for a time its most famous representative, Malcolm X, was in its assertion of defiance in the face of the undoubted history of oppression suffered by black people in the US, along with its promotion of black pride.

In Ali's possession the heavyweight championship of the world transcended boxing, even sports. With him it assumed the mantle of political and social banner, behind which a new generation of black men and women could assemble to declare their pride in being black in defiance of a system in which their parents and grandparents had been left in no doubt they were considered inferior.

Ali paid a terrible price for his apostasy. He quickly became the most hated man in America, excoriated by sportswriters, commentators, politicians, and even black leaders of the day.

People lined up to attack both him and his beliefs, and ticket sales for his fights plummeted. And this was before his stance on the war in Vietnam, when after being reclassified he told a reporter that had had no quarrel with "them Vietcong."

It was a quote that unleashed the forces of hell, with Ali openly accused of treason in newspapers across the country.

Most men would have buckled under this kind of public animus but Ali seemed to grow in stature, finding new purpose as a torchbearer of resistance to the war and the contradictions it exposed regarding the suppurating sore of racist injustice in his country.

For refusing the draft he was stripped of his title and faced prison. Exile from the ring followed and he spent the next three years struggling to make ends meet.

But Ali's shadow continued to loom large over the heavyweight championship, which became cheapened in his absence.

At the beginning of his exile he was hated, but with the civil rights movement building to become the social phenomenon it did, and with the anti-Vietnam war movement doing likewise, three years later Ali was a folk hero, lauded where before he'd been vilified - and respected for sticking to his principles no matter the cost.

His return to the ring in 1970 against Jerry Quarry in Atlanta was a seminal moment in US sporting and cultural history. Celebrities packed the ringside seats as Ali received the adulation of the thousands in attendance and the millions watching the fight on TV or listening to it on radio across the world.

The legend from then on has been well recorded. The three epic fights against Joe Frazier, the unbelievable victory over George Foreman, fighting most of his first fight against Ken Norton with a broken jaw, and of course the sad decline and slide into Parkinson's.

Now, just turned 72, Muhammad Ali remains one of the few survivors from the great era of heavyweights whose names resonate even more as legends when compared to the current crop.

Even more important than his achievements in the ring, however, Ali stood tall outside it at a time when blacks were still being lynched and racism was as American as apple pie.