The significance of 1492 for the slave trade is largely the same as its significance for the world as a whole. This year was significant for the slave trade because it was the year in which Columbus “discovered” the New World.
Before Europeans had found the Americas, the slave trade was not a tremendously large industry. There were some slaves needed in Europe, but not that many. Europeans used African slaves as domestic workers and they used them to raise and process sugar cane on some islands off the coast of Africa and Europe. However, they did not need them in large numbers.
After 1492, that changed. It did not change immediately, but it did change. The Caribbean islands, and eventually parts of the mainlands of North and South America came to demand large numbers of slaves. There was much more demand from this area than there ever had been from Europe. This meant the beginning of the trans-Atlantic slave trade. It was the beginning of the time when huge numbers of Africans were to endure the “Middle Passage” and become slaves in the European territories in the Americas.