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Before the arrival of Europeans in the Americas in the late 15th and early 16th centuries, Native Americans had occupied these continents for thousands of years. The oldest human remains and artifacts found in North America are dated to about 11,500 years old. This age roughly coincides with the end of the last ice age. For many years, the similarity in timing was thought to explain the origin of the first Americans. However, human settlements have been discovered in South America that predate access to an inland migration route, suggesting that new hypotheses about the earliest human migration into the Americas must be considered. Between about 65,000 and 10,000 years ago, during Earth's most recent era of global cooling, the Pleistocene, temperatures were on average five degrees Celsius (nine degrees Fahrenheit) cooler than they are today. This decrease in temperature led to greater amounts of precipitation falling as snow, which caused glaciers to become widespread over high latitudes. This, in turn, caused sea levels to drop and exposed relatively shallow continental shelves, including the shelf connecting Asia with Alaska. For decades, the leading hypothesis proposed that Pleistocene hunters crossed the 1,500-km-wide (932-mi) Bering land bridge in pursuit of large game animals, such as mammoths and musk oxen. From there, these ancient hunters were thought to have benefited from the timely retreat of the glaciers and an ice-free corridor through west-central Canada and into the rest of North America. However, recent evidence suggests that this ice-free corridor may have remained closed until about 11,000 years ago, 500 years later than evidence places people much further south. In fact, recent archaeological finds in Chile and Venezuela provide evidence that there were people in South America as early as 12,500 years ago. In coastal areas of Alaska, scientists have found remains of land and sea mammals, as well as birds and fish, dating as far back as 13,000 years ago, suggesting that people paddling small boats along the coast en route to the Americas might have had sufficient resources to survive. In light of these new pieces of evidence, some archaeologists propose that early migration from Asia to the Americas may have taken place along the coasts, rather than further inland. To date, however, scientists have found no evidence of human settlements from this important time period.