With respect to the dominant culture, the ' cultural ' continent that is called North America is over 500 years of age, some 300 years before the inception of what is now called the United States.
The Spanish's and Portugese's influences and arrivals in this hemisphere commenced the historic beginning. Columbus was one, but there were many trekkers with respect to the entire western hemisphere, which includes North and South America. Columbus benefited by a joint collaboration and funding from the Spanish and Portugese monarchies and aristocracies.
Now, when you speak to the allegorical Pangea, however, you are addressing geologic events, not cultural ones.
In strictest terms, U.S. History began when the U.S. was born in July 1776. Before that time, no formal, or official, nation existed. This takes in Nordic traces evidenced in the centuries preceding the founding of the U.S. as well, because no nation was founded as result of those earlier European settlements.
As to why -- it is this. Little is known that Columbus had a sage Persian teacher and counsel, of whom Columbus was a dedicated student, who advised Columbus to undertake the great journey and founding of the New World because of the poor state of health that was then so prevalent in Europe. Well known by certain enlightened ones and sages was the New World already in terms of its natural wealth and resources that could feed the teaming masses of Europe. Therefore, Columbus abided his conseling and advice and teachings and did set out to fulfill the mandate so as to pull Europe out of the morass of hunger and ill health. And yes, as well was known of its monetary and material good that would prove beneficial to the then European coffers.