Supplemental Data
Document S1. Figures S1–S18 and Tables S1–S8
Document S2. Article plus Supplemental Data

Web Resources
The URLs for data presented herein are as follows:1000 Genomes, http://browser.1000genomes.org
1860 Census, https://www.census.gov/history/www/through_the_decades/overview/1860.html
2010 Census, http://www.census.gov/2010census/
American Community Survey Summary File 2007–2010, http://www2.census.gov/acs2011_5yr/summaryfile/
American FactFinder, http://factfinder2.census.gov/
Free Zipcode Database, http://federalgovernmentzipcodes.us/
How Many Genetic Ancestors Do I Have?, http://gcbias.org/2013/11/11/how-does-your-number-of-genetic-ancestors-grow-back-over-time/
International HapMap Project, http://hapmap.ncbi.nlm.nih.gov/
Python, https://www.python.org/
R statistical software, http://www.r-project.org/
Roots into the Future, https://www.23andme.com/roots/
The Black Population: 2010, http://www.census.gov/prod/cen2010/briefs/c2010br-06.pdf
Trans-Atlantic Slave Trade Database, Introductory Maps, http://www.slavevoyages.org/tast/assessment/intro-maps.faces

Acknowledgments
We thank the customers of 23andMe who answered surveys and participated in this research. We are grateful to Dr. Jeffrey C. Long at the University of New Mexico, Dr. Claudio Saunt at the University of Georgia, and Sarah Abel at Centre International des Recherches sur les Esclavages, CNRS, Paris for invaluable discussions and comments on a manuscript draft. We thank Nick Patterson and Priya Moorjani for helpful statistical discussions on f statistics. Of course, all mistakes and inaccuracies are our own. Thanks to all the employees of 23andMe, who together have made this research possible, especially Emma Pierson, Robin Smith, Youna Hu, and Scott Hadly. K.B., E.Y.D., and J.L.M. are current employees (and J.M.M. is a former employee) of 23andMe, Inc., and have private equity interest. This work was supported by NIH award 2R44HG006981-02. D.R. was supported by NSF HOMINID award BCS-1032255 and NIH grant GM100233, and D.R. is a Howard Hughes Medical Institute investigator. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).