Final answer:
The large number of African slaves was a distinctive demographic feature in the transatlantic slave trade era, deeply affecting the societies involved. Enslaved Africans were central to colonial economics, demographic changes in both Africa and the Americas, and the cultural developments in their new environments.
Step-by-step explanation:
The most distinctive feature of the population in various regions during the era of transatlantic slavery was indeed the large number of African slaves. These enslaved people, abducted from diverse parts of Africa, were integral to the economic foundations of colonies across the Americas. In the United States, for example, South Carolina was one notable colony where Africans formed the majority of the population very early in its history, with a significant portion of the British colonies' African arrivals coming through the Charleston port.
The high mortality rates during the transportation across the Atlantic - with a quarter of all enslaved individuals dying on the brutal journey - had grave demographic impacts both in Africa and the colonies. Entire African communities suffered from the loss of substantial portions of their adult population, which led to, for example, no population growth in certain regions of Africa and dramatic demographic shifts in New World colonies. Despite the oppressive conditions, African slaves managed to influence their new environments, contributing to a rich cultural tapestry and often resisting their enslavement through various forms of rebellion and cultural retention.
Furthermore, these enslaved populations were at the center of colonial wealth, especially for those involved in cash crop production like rice in South Carolina or sugar in the Caribbean, and the interlinked economies of European markets and the Americas. This era's legacy profoundly shaped the regions' demographics, economy, culture, and society.