In surveys conducted in 2005, people in the United States and 32 European countries were asked to respond … to this statement: “Human beings, as we know them, developed from earlier species of animals.” The United States had the second-highest percentage of adults who said the statement was false and the second-lowest percentage who said the statement was true, researchers reported in the current issue of Science. Only adults in Turkey expressed more doubts on evolution. In Iceland, 85 percent agreed with the statement. (Graph)
via Slashdot, Science Magazine is pointed about the reasons:
The acceptance of evolution is lower in the United States than in Japan or Europe, largely because of widespread fundamentalism and the politicization of science in the United States.