When did the first Americans come across the bering string?

Answer 1

Arizona Lumberjack should check his dates again. There's a lot of debate about this. Now it's known that early man was able to use boats made of hides.
The Pacific Ocean at that time was quite a bit narrower then it is now, in fact the coastline of the Western shoreline of the United States was out as much as 600 miles then it is now.

Early man could have and, artifacts have been found, that he did in fact travel this route rather then a land route, following the shoreline. Many years earlier then he did then a land route.
18,000 years was the earliest date known that can be proven for Early man. Artifacts found in the far North show early man there as early as 30,000 years ago.
Now we know Early man was in South America as early as 24,000 years ago from, most probably the Pacific Islands or even Australia. And, don't scoff at early man landing on the coast of California from the Pacific islands.

As times goes and methods improve we're finding out more and more. We now now that there was no Native Americans but many early Americans from many different places.

Answer 2

A little over 40,000 years ago.