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All "brown" pigmented Caucasoids are admixed with non-Caucasoid groups. There is no such thing as a brown, or black, skinned pure Caucasoid.
Darker Mediterranean, Arabian, Middle-Eastern, North African, and South-Asian Caucasoids are all racially-mixed. Caucasoids are a white race.
Many will argue that the original Caucasoids were dark-skinned, but this has yet to be 100% determined. Even if this was so, light-skinned Caucasoids were geographically isolated due to climatic events such as Ice Ages (or geographic barriers such as the Sahara Desert), before coming into contact with other races.
The two main examples of "brown" Caucasoids are various Near Eastern populations, especially Arabians, who have Sub-Saharan African, East African, South Asian and even sometimes East Asian (Mongol/Turkic) and Austraoid/Denisovan genetic admixture.
For example:
Another major populations sometimes considered to be "brown" Caucasoids, South-Asians, are in reality mixed-Caucasoid-Australoid (and a distinct Dravidian race) reflected in the division of of genetic material associated with the Aryan Invasion of South-Asia:Another ASHG 2010 abstract. Using CEU and Maasai reference populations, the authors estimate Yemenis average about 14.8% African admixture.
India has been underrepresented in genome-wide surveys of human variation. We analyse 25 diverse groups in India to provide strong evidence for two ancient populations, genetically divergent, that are ancestral to most Indians today. One, the "Ancestral North Indians" (ANI), is genetically close to Middle Easterners, Central Asians, and Europeans, whereas the other, the "Ancestral South Indians" (ASI), is as distinct from ANI and East Asians as they are from each other. By introducing methods that can estimate ancestry without accurate ancestral populations, we show that ANI ancestry ranges from 39-71% in most Indian groups, and is higher in traditionally upper caste and Indo-European speakers...Our analyses suggest that major ANI-ASI mixture occurred in the ancestors of both northern and southern Indians 1,200-3,500 years ago, overlapping the time when Indo-European languages first began to be spoken in the subcontinentIn otherwords white Caucasoids invaded a dark Australoid-Dravidian population leading to todays South-Asians with:Because of admixture between Australoid, Caucasoid, and Mongoloid racial groups, one cannot speak of a biologically separate "Dravidian race" distinct from non-Dravidians on the Indian subcontinent. In a 2009 study of 132 individuals, 560,000 single-nucleotide polymorphisms in 25 different Indian groups were analysed, providing strong evidence in support of the notion that modern Indians (both Indo-Aryan and Dravidian groups) are a hybrid population descending from two post-Neolithic, genetically divergent populations referred to as the 'Ancestral North Indians' and the 'Ancestral South Indians'. According to the study, Andamanese are an ASI-related group without ANI ancestry, showing that the peopling of the islands must have occurred before ANI-ASI gene flow on the mainland
Basu et al. (2006) emphasize that the combined results from mtDNA, Y-chromosome and autosomal markers suggest that " there is an underlying unity of female lineages in India, indicating that the initial number of female settlers may have been small; the tribal and the caste populations are highly differentiated; the Austroasiatic tribals are the earliest settlers in India, providing support to one anthropological hypothesis while refuting some others; a major wave of humans entered India through the northwest; the Tibeto-Burman tribals share considerable genetic commonalities with the Austroasiatic tribals, supporting the hypothesis that they may have shared a common habitat in southern China, but the two groups of tribals can be differentiated on the basis of Y-chromosomal haplotypes; the Dravidian tribals were possibly widespread throughout India before the arrival of the Indo-European-speaking nomads, but retreated to southern India to avoid dominance; formation of populations by fission that resulted in founder and drift effects have left their imprints on the genetic structures of contemporary populations; the upper castes show closer genetic affinities with Central Asian populations, although those of southern India are more distant than those of northern India; historical gene flow into India has contributed to a considerable obliteration of genetic histories of contemporary populations so that there is at present no clear congruence of genetic and geographical or sociocultural affinities."
http://hms.harvard.edu/news/genetics...mixture-8-8-13
So we see that those groups sometimes considered "brown" Caucasoids are in fact mixed-racial groups, only phenotypically Caucasoid in skeletal structure, physical features and so forth. Caucasoids are all fair-skinned in their pure form.
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