By the end of the sixteenth century the trans-Atlantic slave trade was quite active.

But it wasn't only the desire for sweets that triggered it--it was a much more complex set of circumstances that enabled the trade in humans.

How come there was very little sea trade with West Aftican kingdoms before the 16th century?

The winds. It wasn't that difficult to sail from Europe south to W. Africa, but there was a "little" problem with getting back. The prevailing winds didn't blow in that direction. Prior to the 16th century, the vast majority of the trade of goods and slaves of West Africa was via the Sahara. This was not what you would call a pleasant journey, but it was doable.

So what changed? New ways of rigging sails allowed for ships to tack (zig-zag) into the wind. And ships underwent improvements in other areas, such as steering.

But the biggest change was the discovery of northerly trade winds out a ways in the Atlantic. Ships returning from Africa to Europe could sail west and catch these winds, which would take them where they wanted to go.

Why hadn't these been discovered before?

People hadn't (for the most part) sailed west. Why not? Because until the development of the chronometer they couldn't estimate their longitude very well. Latitude was easier--it could be done by star positions. But longitude was trickier, because lines of longitude aren't parallel. (It also requires a precise knowledge of time.)

So, by the end of the fifteenth century just about everything was in place to allow the trans-Atlantic slave trade to explode.

From the sixteenth century to the latter part of the nineteenth century approximately 10 million Africans were enslaved and taken to the so-called New World. Of these, about 300,000 came to what is now the United States. (The United States banned importation of slaves as of about 1820.) The rest went to South America and the Caribbean islands where cane sugar grew most enthusiastically.

What about the trans-Saharan trade? The estimate there is about 7.5 million from the eighth century through the nineteenth.

The middlemen on the trans-Saharan trade were Muslim traders; the middlemen on the trans-Atlantic trade were Europeans--and the profits of the slave trade went a long ways toward getting the Industrial Revolution underway.

So the sixteenth century was, in its own way, pivotal to the development of the highly technical world we live in today.