It was a time of disintegrating government control of the countryside, of historic FARC victories over a dispirited military, when private far-right militias known as paramilitaries massacred suspected rebel sympathizers.
De la Calle, who is married with three children, took refuge from his agitated public life by being "in love with literature, with poetry ... a lover of good music, both classical and Colombian," said Carlos Holmes, a friend and former education minister.
Little is known of Marquez's private life except that he has eight siblings and three children from a marriage that pre-dated his life as a fugitive, according to government intelligence officials who spoke on condition of anonymity because they were not authorized to discuss the matter. They said Marquez operated as a rebel commander in the Caribbean banana-growing region of Uraba, where the FARC took a beating from paramilitaries and murders of union organizers by right-wing death squads were rampant.
In 1991, Marquez was named to the FARC delegation for peace talks in the Venezuelan capital of Caracas after being promoted to the rebels' seven-man ruling Secretariat.
It was in Caracas that he and De la Calle met for the first time.
The two "carefully measured their words in public and in private. They committed no imprudences," said Hector Riveros, a government negotiator at the time.
He remembers Marquez as being opposed to the negotiations.
Ironically, so apparently was De la Calle. Or so he was ordered.
De la Calle only showed up for the talks' inauguration, and was ordered home by then-President Cesar Gaviria because the latter wasn't interested in serious talks, said Alvaro Leyva, a Colombian politician from the Conservative Party trusted by the rebels who would later help arrange 1999-2002 peace talks.
Gaviria thought the FARC could be beaten on the battlefield, Leyva said.
At any rate, conditions were not good for peace. A coup attempt led by Hugo Chavez convulsed Venezuela in 1992, and Gaviria's government was battling cocaine kingpin Pablo Escobar, who was killing civilians indiscriminately and would be tracked down and killed in December 1993 with U.S. help. The talks moved to Tlaxcala, Mexico, where they died.
Marquez and De la Calle didn't meet again until Oslo.
Because he is viewed as having no presidential ambitions, De la Calle was a good choice to lead Santos' negotiating team to the current talks, even though he didn't participate in the secret talks leading up to them, Riveros said.
The role of chief FARC negotiator fell to Marquez pretty much by default.
The rebels have lost their four most senior commanders since 2008 as a U.S.-supported military buildup depleted their ranks, triggering record desertions. Three members of the ruling Secretariat were killed in military attacks, including Alfonso Cano, with whom it had kick-started the current peace process. A fourth, founding leader Manuel Marulanda, died in a jungle camp, apparently from a heart attack.
During much of that time, Marquez is believed to have been living in Venezuela.
Associated Press writers Cesar Garcia and Libardo Cardona in Bogota and Frank Bajak in Lima, Peru, contributed to this report.