Haber Marc, Mezzavilla Massimo, Bergström Anders, Prado-Martinez Javier, Hallast Pille, Saif-Ali Riyadh, Al-Habori Molham, Dedoussis George, Zeggini Eleftheria, Blue-Smith Jason, Wells R Spencer, Xue Yali, Zalloua Pierre A, Tyler-Smith Chris
Wellcome Trust Sanger Institute, Wellcome Genome Campus, Hinxton, Cambridge CB10 1SA, UK.
Wellcome Trust Sanger Institute, Wellcome Genome Campus, Hinxton, Cambridge CB10 1SA, UK; Institute for Maternal and Child Health, IRCCS Burlo Garofolo, University of Trieste, 34137 Trieste, Italy.
Am J Hum Genet. 2016 Dec 1;99(6):1316-1324. doi: 10.1016/j.ajhg.2016.10.012. Epub 2016 Nov 23.
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.
了解非洲的人类遗传多样性对于解读全人类的进化至关重要，然而非洲的广大地区，如乍得，在遗传学上仍未得到充分研究。在这里，我们使用了来自乍得、近东和南欧的480个样本的基因型数据，以及其中19个样本的全基因组测序数据，来表明如今许多人群的基因组源自古代非洲 - 欧亚大陆的混合。我们在乍得南部说未分类孤立语言拉尔语的人群中发现了早期欧亚大陆回流到非洲的证据，并通过连锁不平衡衰减估计这发生在4750 - 7200年前。它给非洲带来了一个Y染色体谱系（R1b - V88），其最亲近的亲属在当今欧亚大陆广泛分布；我们从序列数据估计乍得的R1b - V88 Y染色体在5700 - 7300年前发生了聚合。因此，这次迁移可能起源于非洲湿润期的近东农民。我们还发现，先前记录的约3000年前发生的欧亚大陆回流到非洲的事件，原本被认为主要局限于东非，但其影响更向西扩展，波及乍得北部的人群，如图布人，他们如今有20% - 30%的欧亚血统。我们观察到混血非洲人的杂合度下降，并发现欧亚大陆的混合会使对他们聚合历史的推断产生偏差，并混淆来自适应性和古老基因渗入的遗传信号。
