Eurasian backflow

The term Eurasian backflow, or Eurasian back-migrations, has been used to describe several pre-Neolithic and Neolithic migration events of humans from western Eurasia back to Africa.^[1] Although preceded by many waves of earlier human dispersals into Eurasia, all present-day humans outside Africa descend primarily from a single expansion that occurred between 70,000–50,000 years ago.^[2][3][4]

In addition to intrinsic diversity within the continent borne by population structure and isolation, influxes of Eurasian populations into Africa has been seen as a critical contributor to the existing genetic diversity.^[5]

Migration history

Pre-Neolithic and Neolithic migration events in Africa.^[6]

Beginning around 30,000-15,000 years ago, a population closely related to Paleolithic individuals from Dzudzuana Cave in Georgia expanded from the Near East back into Northern Africa, spreading in connection with microlithic-backed bladelet technologies that would later become ubiquitous across North Africa. This diffusion contributed up to 55% of the genetic ancestry of the Iberomaurusianculture, and introduced the mitochondrial haplogroups M1 and U6 to the region, which along with the aforementioned genetic ancestry, define the "autochthonous/typical North African" or "Maghrebi" component, as it is referred to in genetic studies.^[7]

Approximately 8,000 years ago, populations of the Neolithic Levant expanded into North Africa via the Sinai Peninsula. This diffusion was rooted in wider displacements of people in the wake of the 6200 BC climatic event, and coincided with the introduction of Southwest Asian domesticates such as sheep, goats, and cattle into Africa, leading to the rise of Saharan cattle pastoralism and the appearance of Ashakar Ware pottery in the Maghreb.^[8] This expanding population interacted and admixed with local ancestries, as seen in both Skhirat-Rouazi individuals of Morocco and eastern African pastoralist groups of the Pastoral Neolithic.^[9] This diffusion may have also provided the demographic and cultural conditions for the early dispersal of Afro-Asiatic languages across North Africa, whose known branches of Cushitic, Chadic, Egyptian, and Berber correspond strikingly with the geography of these Neolithic expansions.^[8]

Outside of North Africa, about 3,000 years ago,^[10][11] or already earlier between 6,000-5,000 years ago,^[12] farmers from Anatolia and the Near East migrated into other regions. Signs of this migration can be found in the genomes of contemporary peoples from all over East Africa. Moreover, analysis has also recognized that some of the Eurasian ancestry in Northeast Africa could possibly pre-date agriculture, from around ~12-23 ka.^[13] Significant Eurasian ancestry is found among populations of Eastern Africa, and among specific ethnic groups of the Horn of Africa, Northern Sudan, the Sahel region, as well as among the Malagasy people of Madagascar. West Eurasian ancestry thus arrived in Africa first during the Palaeolithic, then followed by several later Neolithic, Chalcolithic, and Bronze Age migration events.^[10] Although Medieval events, such as the Arab expansions, also brought gene flow into various African populations,^[14] these left a lower mark on North African genetic structure, relative to Neolithization.^[15][16]

Map of major prefarming population stratification across the African continent.^[17]

The Neolithic populations migrating back-to-Africa were said to be similar to the Neolithic farmers who had brought agriculture from Anatolia to Europe about 7,000 years ago, and whose demographic legacy is most closely preserved in present-day Sardinians, their contemporary descendants.^[1] Although, other studies have revealed some genetic distinctions between the populations that brought farming into Europe and the Maghreb, and the Levantine related groups that spread southward into East Africa.^[18][8] A study from 2020 inferred two sources for the spread of Eurasian admixture in Northeastern Africa, with one being associated with pastoralism, involving groups originating from the Levant and North Africa.^[19] Further research has shown that the back-migration into the region was a complex process, identifying multiple origins for the Eurasian component in Northeast African groups today.^[20][21]

A report in November 2015 on a 4,500-year-old Ethiopian genome^[22][23] had originally overestimated the genetic influence of the Eurasian backflow, claiming that signs of the migration could be found in genomes all over Africa. This mistaken claim was based on a data-processing error and was corrected in February 2016. The West Asian admixture was only predominant in the populations of the Horn of Africa, in particular Ethiopian highlanders, and less relevant or absent in the genetic makeup of West and Central Africans.^[24]

Neanderthal admixture and Eurasian ancestry

An investigation in 2012 discovered that unlike most sub-Saharan Africans, North Africans have similar levels of Neanderthal DNA to South Europeans and West Asians, which is pre-Neolithic in origin, rather than via any recent admixture, as the Neanderthal's genetic signals were higher in populations with an autochthonous 'back-to-Africa' genomic component that arrived 12,000 years ago. These Neanderthal genomic traces do not mark a division between Africans and non-Africans, but rather a division between sub-Saharan Africans and the rest of the modern human groups, including those from North Africa.^[25]

In 2016, researchers recognized that the Neanderthal ancestry in African populations, strongly corresponds with the levels of Western Eurasian ancestry. The geneticists elaborated that: "Neanderthal ancestry is not expected in Africa, yet today many Africans carry Neanderthal-derived alleles. The plot shows that the Neanderthal ancestry proportion in Africans is correlated with gene flow from Eurasians. For example, knowing that today Eurasians carry ~2% of Neanderthal ancestry, we observed that East Africans (Ethiopians) had ~1% Neanderthal ancestry and ~50% Eurasian ancestry. Correspondingly, Near Easterners showed a decline in Neanderthal ancestry proportional to their levels of African ancestry."^[26]

Human dispersals within Africa

Chen, Lu's publication found back-migrations contributed to the signal of Neanderthal ancestry in Africans. Data indicated that back-migrations giving Neanderthal sequences came after the split of Europeans and East Asians, from populations related to the European lineage. The overlap of this ancestral European ancestry and Neanderthal segments was highly significant.^[27]

Average Eurasian frequencies in African populations

Estimates in Modern Africans

African Population Group	Linguistic Affiliation	Proportions of Ancestry
Afrikaaner	Indo-European	97.9%[28]
Rashaida Arab	Semitic	97.92%[29]
Canarian	Indo-European	97.4%[30]
Tunisian Jew	Semitic	96.49%[31]
Moroccan Jew	Semitic	96.05%[31]
Libyan Jew	Semitic	95.76%[31]
Chaoui	Berber	85.12 - 87.51%[32]
Moroccan	Semitic	80%[33]
Tunisian	Semitic	79.37 - 81%[31][34]
Egyptian	Semitic	79 - 86%[33][35][36]
Figuig Berber	Berber	78.98%[29]
Bouhria Berber	Berber	78.68%[29]
Mozabite	Berber	77.33 - 82.39%[31][33][32]
Asni Berber	Berber	76.99%[29]
Algerian	Semitic	76.86 - 82.45%[31][32]
Libyan	Semitic	76%[37]
Saharawi	Semitic	74.32 - 84.9%[31][37]
Tigrayan	Ethio-Semitic	50 - 50.4%[38][39]
Beja	Cushitic	50%[40]
Amhara	Ethio-Semitic	49.2 - 54%[39][35][41]
Ethiopian Jew	Semitic	46.4 - 55.12%[42][31]
Afar	Cushitic	46.0%[39]
Kababish Arab	Semitic	45.99%[29]
Oromo	Cushitic	41.6 - 51%[39][41]
Agaw	Cushitic	41 - 50%[43]
Sudanese Arab	Semitic	40 - 52.73%[14][29]
Nubian	Nilo-Saharan	39.41 - 54.74%[14][29]
Somali	Cushitic	38.4 - 44%[39][44][31][41]
Cape Verdean	Indo-European	36 - 54%[45][46]
Wolayta	Omotic	34.1 - 43%[39][41]
Samburu	Nilo-Saharan	32.19%[29]
Datooga	Nilo-Saharan	32.17%[31]
Malagasy	Austronesian	30 - 68%[47][48]
Baggara Arab	Semitic	25.57 - 33.6%[29]
Fulani	Niger-Congo	21.4 - 29%[49][50]
Toubou	Nilo-Saharan	20 - 31.4%[51][37]
Maasai	Nilo-Saharan	18.9 - 26.06%[39][31]
Turkana	Nilo-Saharan	18.33%[29]
Aari	Omotic	15.7 - 18.2%[39]
Sandawe	Khoisan	15.4%[39]
Nama	Khoisan	14%[39]
Kikuyu	Niger-Congo	13.89 - 18.33%[42][31]
Zaghawa	Nilo-Saharan	12.58%[29]
Kalenjin	Nilo-Saharan	10.99 - 12.86%[42][29]
Kanembu	Nilo-Saharan	10.97%[29]
Hadza	Khoisan	6.4 - 12.71%[39][31]

Estimates in Ancient African Samples

African Population	Time Period	Number	Proportions of Ancestry
Egypt, Nuerat Egyptian	2,868 - 2,492 BC	N=1	90%[52]
Tunisia, Carthaginian	450 - 170 BCE	N=17	[53]
Canary Islands, Guanche	3rd - 16th Century CE	N=40	89.4%[30]
Morocco, SKH (Skhirat-Rouazi)	6733 - 6121 BP	N=3	87%[54][55]
Morocco, KTG (Kaf Taht el-Ghar)	7429 - 6945 BP	N=4	86%[54][55]
Algeria, Numidian	1 - 700 CE	N=1	85.87%[56]
Egypt, Abusir-el-Meleq Egyptian	1388 BCE - 426 CE	N=3	85 - 94%[36]
Morocco, KEB (Khef el-Baroud)	3000 BCE	N=8	81.75%[57]
Tunisia, Hergla	5900 BP	N=1	[58]
Tunisia, Kerkouane Carthaginian	900 - 360 BCE	N=27	[53]
Tunisia, Kerkouane Carthaginian	650 - 250 BCE	N=12	77.33 - 97.3%[59][60]
Tunisia, DEK (Doukanet el Khoutifa)	7000 - 6600 BP	N=5	[58]
Sudan, Kulubnarti Nubian	650 -1000 CE	N=66	46 - 64%[61]
Morocco, IAM (Ifri n'Amar)	5000 BCE	N=7	[57]
Tunisia, Djebba	8000 BP	N=2	[58]
Morocco, OUB (Ifri Ouberrid)	7660 - 7506 BP	N=1	[54]
Algeria, ABR (Afalou Bou Rhummel)	10,000 BP	N=1	[58]
Morocco, Iberomaurusian	15,100 - 13,900 YBP	N=7	46 - 63.5%[43][62]
Sudan, Kadruka Nubian	4033 BP	N=1	45.2%[63]
Tanzania, Luxmanda	3141 - 2890 BP	N=1	37.2 - 39%[64]
Kenya, Pastoral Neolithic	3500 - 1500 BP	N=41	30 - 40%[65][9]
Tanzania, Swahili	1250 - 1800 AD	N=80	26 - 68%[66]
Libya, Takarkori	7,000 YA	N=2	20 - 30%[67]

See also

• African admixture in Europe
• Genetic history of Africa
• Genetic history of North Africa
• Genetic history of the African diaspora
• Neolithic Revolution