Dr. George Carter, a human geographer at Johns Hopkins University, studies man's relationship with the physical world and how civilizations developed. He explains the differences between independent inventionists, researchers who believe in indigenous cultures that developed independently, and diffusionists, scholars who maintain that there was early contact between civilizations. Pre-1492 contacts between the old world and the new appear impossible, but evidence shows similarities in games, instruments, tools, math, religion, etc. in both Asia and the Americas. The existence of domestic plants, such as the sweet potato, in both places and with the same name, seems proof that man crossed the oceans during pre-Columbian times. Evidence in art may support the diffusionists too, according to Dr. Gordon Ekholm, curator of archaeology at the American Museum of Natural History. He points to similar Mayan and Cambodian temples and parallel sculptural details such as trefoil arches in Mexico and in Asia.