Chad's Eurasian genetic input similar to that in Ethiopia

Quickies

Marc Haber et al. Chad Genetic Diversity Reveals an African History Marked by Multiple Holocene Eurasian Migrations. AJHG 2016. Open access → LINK [doi:10.1016/j.ajhg.2016.10.012]

Summary

Understanding human genetic diversity in Africa is important for interpreting the evolution of all humans, yet vast regions in Africa, such as Chad, remain genetically poorly investigated. Here, we use genotype data from 480 samples from Chad, the Near East, and southern Europe, as well as whole-genome sequencing from 19 of them, to show that many populations today derive their genomes from ancient African-Eurasian admixtures. We found evidence of early Eurasian backflow to Africa in people speaking the unclassified isolate Laal language in southern Chad and estimate from linkage-disequilibrium decay that this occurred 4,750–7,200 years ago. It brought to Africa a Y chromosome lineage (R1b-V88) whose closest relatives are widespread in present-day Eurasia; we estimate from sequence data that the Chad R1b-V88 Y chromosomes coalesced 5,700–7,300 years ago. This migration could thus have originated among Near Eastern farmers during the African Humid Period. We also found that the previously documented Eurasian backflow into Africa, which occurred ∼3,000 years ago and was thought to be mostly limited to East Africa, had a more westward impact affecting populations in northern Chad, such as the Toubou, who have 20%–30% Eurasian ancestry today. We observed a decline in heterozygosity in admixed Africans and found that the Eurasian admixture can bias inferences on their coalescent history and confound genetic signals from adaptation and archaic introgression.

Worth a read no doubt but careful, careful, careful with their chronological guesstimates. Their starting point is the assumption (once and again demonstrated all kinds of WRONG) of:

Eurasians and Africans diverged around 60,000–80,000 ya and subsequently had different patterns of population-size changes: in particular, compared with Africans, the Eurasian population experienced a sharp decrease in size ∼60,000 ya.

So add around 65-70% (x1.7) to all dates, else you are bound to fall in the pit of molecular-clock-o-logical self-complacent pseudoscience. So where it reads c. 6-7 Ka for the first migration (R1b-related), it should be 10,000 years ago (which is the actual dating of Afroasiatic expansion by most accounts, with origin not exactly in "Eurasia" but rather in or near Sudan, where those Eurasian lineages, R1b and J1, had since long before most likely), and when they say 3 Ka ago, it's probably 5000 years ago, within the context of Neolithic inflows possibly: 3000 years ago was already well into Ancient Egypt and peoples just did not cross it without proper paperwork anymore, actually 3000 years ago is the Bronze Age collapse and Egypt, Lower Egypt specifically, fell to Africans, to Libyans and other Berbers known as Meshwesh (Amazigh, probably from modern Tunisia) to be precise.

Dr. Haber: time to update your clock, it just doesn't work, and you are confusing people to no avail.