Most archeologists believe that, apart from their original Ice Age migrations across Beringia and the Bering Strait, the Native American cultures developed in complete isolation from the rest of humanity, until the voyages of Columbus. They interpret the archaeological record to show in situ, original cultural development through that period, with people interacting across local regions but not with other continents (or other planets). The sole generally accepted exception are the visits by the Vikings to Newfoundland, at the L'Anse aux Meadows site — which apparently did not have any lasting effect on the native cultures.
The opposite theory, that American cultural development was either derived or affected by trans-oceanic contacts, was widely believed until the early 19th century, but today it is only a minority opinion. While some advocates of these diffusionist theories are compelled by their religious beliefs or influenced by ethnocentrism, there is a significant minority of scholars who see enough cultural parallels to justify them.
The reversal of the consensus was so complete that, for about a century, any suggestion of pre-Columbian trans-oceanic contacts was automatically dismissed by most mainstream historians. It became a dogma that no trans-oceanic voyages to the Americas could have occurred before the age of European exploration, which culminated in Christopher Columbus's voyage of 1492. The presumed technical impossibility of such trips was supposedly confirmed by the lack of any solid evidence of cultural influences.
Some ancient Viking chronicles talked about a land called Vinland to the west of Greenland. Historians debated the meaning of these chronicles, and whether the Vikings had ever visited the New World in Pre-Columbian times. These debates were settled by archeological evidence in 1961. In that year Helge Ingstad and Anne Stine Ingstad discovered the remains of a Viking settlement at L'Anse aux Meadows in Newfoundland. This was clear proof that the Vikings had crossed the Atlantic by way of Iceland and Greenland around 1000 CE.
The realization that Polynesians had been able to spread as far as Easter Island by boat led to theories of trans-Pacific contacts with Oceania, an hypothesis that Thor Heyerdahl proved possible by experiment.
Only recently have the Western scholars become aware of the navigational exploits of the Chinese, such as the voyage of Zheng He's fleet. This awareness has led to proposals of Chinese-American contacts, e.g. by off-course Shang Dynasty ships. The possibility of Muslim trips from Asia (see Sung Document) were also tabled.
The presence of Basquecod fishermen and whalers in North America, just a few years after Columbus, has been well established. It has therefore been conjectured that they may have made such trips earlier.
Others have conjectured that Columbus was able to convince the kings of Castilla to back up his proposal only because they were aware of some earlier trip.
In the 20th century, extra-terrestrial civilizations have been added to the long list of conjectural visitors to the Americas. According to popular writers like Erich von Däniken, these celestial visitors were the real builders of the ancient monuments of the Americas, or at least the masters who taught the natives how to build them.