Genetic Consequences of the Transatlantic Slave Trade in the Americas

Abstract

According to historical records of transatlantic slavery, traders forcibly deported an estimated 12.5 million people from ports along the Atlantic coastline of Africa between the 16th and 19th centuries, with global impacts reaching to the present day, more than a century and a half after slavery's abolition. Such records have fueled a broad understanding of the forced migration from Africa to the Americas yet remain underexplored in concert with genetic data. Here, we analyzed genotype array data from 50,281 research participants, which-combined with historical shipping documents-illustrate that the current genetic landscape of the Americas is largely concordant with expectations derived from documentation of slave voyages. For instance, genetic connections between people in slave trading regions of Africa and disembarkation regions of the Americas generally mirror the proportion of individuals forcibly moved between those regions. While some discordances can be explained by additional records of deportations within the Americas, other discordances yield insights into variable survival rates and timing of arrival of enslaved people from specific regions of Africa. Furthermore, the greater contribution of African women to the gene pool compared to African men varies across the Americas, consistent with literature documenting regional differences in slavery practices. This investigation of the transatlantic slave trade, which is broad in scope in terms of both datasets and analyses, establishes genetic links between individuals in the Americas and populations across Atlantic Africa, yielding a more comprehensive understanding of the African roots of peoples of the Americas.

Keywords: Africa; Americas; admixture; ancestry; genetics; history; identity by descent; migration; population genetics; slave trade.

Figure 1

Location of Individuals and Cohorts Arrows highlight the general direction of the triangular trade routes between continents during the transatlantic slave trade. Pie charts indicate the documented number of enslaved people embarking out of regions of Africa (∼12.5 million total) and disembarking in regions of the Americas (∼10.5 million total) between 1515 and 1865. Representatives of regions of the Americas and Europe indicated that they each have four grandparents born within the same country or US state. Representatives of Atlantic Africa either indicated four grandparents born within or historical ties to a country. Points indicate the ∼16,000 unique grandparental geo-coordinates provided by participants. ^∗Cape Verde is an Atlantic African island country that, in the 15th century, was colonized by the Portuguese and inhabited primarily by enslaved people from Senegambia.

Figure 2

IBD Sharing between the Americas and Atlantic Africa versus Documented Counts of Deportation (A) Comparison between the number of enslaved people embarking out of each region of Atlantic Africa to the mean shared IBD (_msIBD; cM) between each Atlantic Africa region and all individuals in the Americas. (B) _msIBD between regions of Africa and of the Americas (top) and the estimated proportions of enslaved disembarkation into each region of the Americas (bottom). p values indicate significant correlations between _msIBD and disembarkation based on a permutation test using Spearman’s coefficient of correlation. Numbers right of the region labels indicate the mean number (±STD) of _mcIBD connections (≥5 cM) an individual representing that region of the Americas has to the seven primary slave trading regions. (C) Posterior distributions of the estimated TMRCA of IBD segments during the transatlantic slave trade between each Africa region and all of the Americas. (D) The proportion of disembarkation of each Atlantic Africa region into the Americas over time, according to historical documents. All Atlantic Africa regions correspond to the colors in key in (A). _msIBD is the total, cumulative IBD shared between two populations (cM), divided by the total pairwise comparisons between two populations.

Figure 3

Estimated Proportions of African Ancestry across Atlantic Africa, the Americas, and Europe Fill: Mean proportion of continent-level African Ancestry in each region. Bars: Mean local ancestry proportions of the four Atlantic African populations 23andMe identifies. Proportions are derived from local ancestry inference, which employs support vector machines (SVM) to assign one of 33 ancestries to each window along the genome and an autoregressive pair hidden Markov model to combine information across window-level SVM calls. Anchor symbols indicate major shipping port locations used during the slave trade.