He first observed them amid the rural poverty of his birthplace, Biran.
Tucked away in the eastern province of Oriente, the main attraction in the town is his former home.
Now a museum, it has been painstakingly preserved for tourists and left-wing pilgrims keen to see Castro's roots.
Fidel Castro's father, Angel, was an immigrant from the Galicia region in northern Spain.
Over time, he became a wealthy landowner.
Official and unofficial Castro biographers alike tend to agree that it was witnessing first-hand the exploitation of Haitian sugarcane workers on the farms that first influenced the young Fidel's fledging worldview.
His half-brother, Martin Castro, still lives in Biran and told the BBC a few stories of their youth.
"He had a little horse called 'Careto' and would go hunting with a small rifle, that's what he enjoyed most," the 87-year-old Martin Castro recalls.
A picture of a youthful Fidel, rifle on his hip, which hangs in the family home backs up his recollection.
"He'd go over to the Haitians' homes and give them vouchers for credit he'd taken from our father's store," he remembers with a chuckle.
That rebellious streak eventually saw Fidel Castro sent to study under the Jesuits, first in the city of Santiago, and then in the Jesuit School of Belen in the capital, Havana.
The Jesuit teachings also had a significant bearing on the young Fidel Castro.
"The Jesuit brothers didn't even earn a wage for teaching classes, they tried to be examples of modesty and honesty," says Dolores Guerra, a researcher at the government-run Cuban History Institute.
Under the influence of a priest called Father Llorente, Fidel Castro excelled in certain areas, including sports and outdoor expeditions.
At the end of his course in 1944, he was awarded a prize for discipline.
"He liked humanities and languages; Spanish, English and history, Ms Guerra says.
"But by his own admission, he wasn't a model student. He would leave everything to the last minute.
"Still, he did recognise the importance of discipline, punctuality and obtaining good grades."
Coming of age
Once he left the safe environment of the Jesuit boarding school, Fidel Castro studied law at Havana University.
It was as he entered the febrile, often violent world of student politics in 1940s Havana that he came of political age.
In a speech he delivered in late 1947 he railed against "the country's wealth in foreign hands" and urged his fellow students to militancy saying: "a young nation can never say 'we surrender'."
It was during this period that Fidel Castro had one of his most formative political experiences not in his native Cuba but in Colombia, where he travelled in April 1948 for a Latin American youth congress.
In the few days that Fidel Castro was in the Colombian capital, Jorge Gaitan, the leader of the country's Liberal Party, was assassinated in the street outside his office.
As word spread of the murder of a leader who was emerging as a hero to many poor and disaffected Colombians, huge riots broke out on the streets of Bogota.
Fidel Castro was in the middle of them.
Tad Szulc takes up the story in his 1986 biography, Fidel - a Critical Portrait: "The Bogotazo, as that urban revolt is now known, became Fidel Castro's real baptism as a revolutionary, and had an enormous impact on him, his thinking, and his future planning."
"It was the most important single event of his life until then, and unquestionably one of his major experiences, providing him with a unique opportunity to observe an unfolding revolution, and to learn from it," argues Szulc.
Today that young student is the great survivor.
At 90 years old, Fidel Castro is one of the world's last Cold War leaders.
Recently the elder statesman of Cuban politics appeared to bid the Communist Party congress goodbye.
"Soon, I'll be turning 90 years old," he told the delegates, his voice weaker but still defiant.
"It's something I'd never imagined would happen. It wasn't the fruit of any labour, but rather a whim of destiny.
"Soon I'll be like all the others; to all of us, our time must come. But the ideas of the Cuban communists will live on."
Every July, Cuba holds its annual celebration of Fidel Castro's most audacious moment: the 1953 attack on the Moncada barracks, the start of the Cuban revolution.
Though he is now retired from public life, Fidel remains the watchword for socialist hardliners, especially those who oppose recent changes on the island.
When in doubt, they turn to the man who has shaped modern Cuba more than any other and quote from his most-famous speech: "History will absolve me."