In comparison to teachers from Argentina and Uruguay, Brazilian teachers face greater difficulties in teaching biological evolution, due to religion. While Argentina has an official religion set in its constitution, Uruguay and Brazil are considered secular countries. Despite being a self-proclaimed secular country, Brazil tolerates and encourages religious teaching in public schools. Acceptance of the natural origin of mankind, of origin of life, and of biological evolution were investigated in all three countries through interviews and a survey of teachers. Barbour's categories of conflict, independence, dialogue, and integration were used in the analysis of responses. Argentinian and Uruguayan teachers' answers pointed to Barbour's category of independence between science and religion, while Brazilian teachers' answers ranged between the categories of conflict and a merging between integration and dialogue, which was reinforced by high religiosity linked to knowledge gaps. The religious beliefs of Brazilian teachers appeared to influence their answers about the origin of the human species. In Brazil, religion was frequently identified as an obstacle to the teaching of biological evolution, while responses varied in Argentina and Uruguay. Conflicts in classrooms were also reported more frequently in Brazil than in Argentina and Uruguay. Combined with poor teacher training, personal internal conflicts towards biological evolution help to explain the answers of the Brazilian teachers. Argentinian and Uruguayan teachers generally reported being able to teach biological evolution as part of the curricula without interference from religion, in contrast to approximately half of the Brazilian teachers reporting the same.