Lewis & Clark are usually counted as the first, in 1803; as for natives, there's a huge documentation problem - if someone had it is quite likely we wouldn't know about it.
But for nomadic hunters, there seems no reason why it wouldn't be possible in theory provided they didn't have to cross a desert or impassable mountain range. Which are serious problems for walking across all the continents.
But for nomadic hunters, there seems no reason why it wouldn't be possible in theory provided they didn't have to cross a desert or impassable mountain range. Which are serious problems for walking across all the continents.