By Helen Murphy and Rosa Tania Valdés BOGOTA/HAVANA (Reuters) - Colombia's government and Marxist FARC rebels reached a "fundamental agreement" on the guerrillas' future in politics, one of the thorniest issues addressed in peace talks in Cuba, according to a joint statement on Wednesday. The FARC, or Revolutionary Armed Forces of Colombia, has been fighting the government in a jungle and urban conflict that has killed more than 200,000 people in the five decades since it began as a peasant movement seeking land reform. The partial accord may pave the way for FARC to enter Colombian politics, which chief government negotiator Humberto de la Calle said would provide a "new democratic opening" and cement peace after an end of conflict. Like other Latin American guerrilla groups, the FARC aspires to become a political party if a peace deal is signed.