What historical event in the 15th century permanently linked the old and new worlds?

The event that permanently linked the old and new worlds in the 15th century is European exploration. This period marked significant exploration and expansion by European nations, driven by a desire for new trade routes, wealth, and the spread of Christian faith. One of the most notable outcomes of this exploration was Christopher Columbus's voyage in 1492, which led to the European awareness of the Americas and initiated a series of interactions between Europe and the New World.

This exploration resulted in profound changes, including the establishment of trade networks, the transfer of plants, animals, cultures, and populations, which is often referred to as the Columbian Exchange. The ramifications of this exchange were vast, leading to the colonization of the Americas and significant demographic and cultural shifts on both sides of the Atlantic, thus creating a permanent link between the old and new worlds.