It's not really known who arrived first in the Americas. The most common theory is that people of the northern area of modern Russia arrived about 30,000 years ago via the Bering land bridge (Beringia). However, there is evidence of people settled in the Western Hemisphere long before these people could have arrived. Some theories support ocean going people from the northern Japanese islands. Other theories support the arrival of early Polynesians. No matter which theory you support, it is at best a guess with some evidence. The fact is that modern dating techniques prove that the Beringia theory is not perhaps as accurate as once believed.
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There is no way to know. Ancient explorers migrated from Asia and Africa to North America the route from Asia probably by what is now the Bering Strait that separates Asia from Alaska and the route from Africa probably from the trading posts on the Niger, probably 40,000 years or more ago. The North American continent was populated sparsely at the time of the Younger Dryas, a sudden cooling of the Earth that may have been caused by a comet or asteroid impact in what is now Canada. The impact and climate change caused the extinction of many large mammals including the Saber-Tooth Tiger and the Wooly Mammoth, and the demise of the "Clovis People", one of the earliest-known human settlers in the Americas until the arival of the "Olmecs".
However, by the time that Leif Erickson and Columbus arrived in the north American continent less than 1000 years ago, the continent had been pretty well re-populated.