Native Americans were the first explorers in America regardless of controversies surrounding origins. Native Americans out of necessity and sheer curiosity explored the continent. Even the Hopi note that rock art in various areas outside of their current area indicates their clans coming from different areas and people more than 1000 years ago to settle in today's NE Arizona location. We know that other small groups of 'Old World' peoples occasionally reached American shores before Columbus (AD 1492). Chinese, Japanese, Viking, and Portuguese came before Columbus but the evidence is minimal and not well documented in written records. Columbus didn't discover anything and was not even sure where he was (go to: Truths and Lies). Subsequently, Columbus' name is not used by Europeans to label the continent. Instead, the Italian navigator, Amerigo Vespucci gets his first name used; probably because he is the first to label America as "New World" on a map.
A more interesting question is why do Europeans begin leaving their homeland and invade the world. The Chinese started before Europeans but withdrew about 40 years before Columbus. The compass (from China) or 'wind rose' was certainly one of the most important tools to help Europeans out of the Mediterranean Sea. One stimulus, ultimately driven by greed, was to find a Northwest Passage. This was certainly the motivator for the Spanish and their competitors the English and the French, since the Pope in Rome gave the Portuguese the Eastern route around Africa, while the rest had to find their way west.
The arrival of Columbus in AD 1492 was significant in many ways to the last 500 years here in America and the rest of the world. To Native Americans Columbus is simply one of many invaders to exploit and later displace Native American people and culture.
To explore the significance of those changes go to my interest area: Columbian Exchange