Human migration is one of the captivating area of research in anthropology and evolution. Migration of Homo sapiens from Africa to different continents lead to evolution of different subspecies, that commonly are referred to as specific races. J. Craig venter, head of Celera genomics, put it during publication of the preliminary draft of the human genome, “Race is a social concept, not a scientific one”. Study of early migration along with using emerging techniques of genetics and genomics will in the end help us to overcome the toxic and controversial thinking of race.
Earlier studies of human migration mainly targeted on fossil records, with emphasis on morphological, continuity of fossil records and use of language, in which linguistic patterns are used for determining connection between people. However, these approaches have limitations as fossil data indicate multiple origins in various continents. Limitation of linguistic approach are, language is also affected by environment, many language connections might have been lost, lack of written or oral practices for most languages. Genome analysis, on other hand, employs genome sequencing and statistics for interpreting information contained in human population genetics. Two of the important markers used for genetic analysis from long time are Mitochondrial DNA and HLA variations (HLA region is made of closely linked highly polymorphic genes, whose product control variety of immunological functions). Various new approaches have been introduced such as 1) diversity of non-recombining region in Y chromosome (NRY), 2) SNP mapping in 565bp region of chromosome 21, 3) Use of bioinformatics approach to make SNP maps to find Linkage disequilibrium and 4) study of evolution history of specific viruses, as migrating humans carried these viruses with them.
Genetic analysis have given great insights into understanding disease susceptibility of particular race along with their pigmentation pattern. We are interested in understanding the role of genetics in evolution of pigmentation patterns and how these make a population predisposed to a disease or disorder. This kind of studies are particularly interesting because this will give insights into determining genetic variations among different populations along with molecular markers, important for the phenotype and how these play into migration pattern.
One of the major limitation with analysis of genetic data from current population is, lot of mixing happened during last 2000 years, by population movement along continents. Mongol invasion causes spread of their DNA along Asia and Eastern Europe. Arabs brought North African and West African DNA into southern Europe and Asia, thus making separation of populations difficult based on their initial patterns.
This review focus on studies of human pigmentation related to disease susceptibility and migration. Various studies were performed to correlate the UV radiation exposure and its effect on pigmentation as it is correlated more strongly in comparison to other factors (Chaplin, 2004; Jablonski, 2000). Pigmentation is important as it determine the amount of light penetration in to body thus it has wide ranging effects on health such as Cancers and Rickets.
Genus Homo and ancestral Homo sapiens evolved skin with dark pigment, which is rich in natural sunscreen eumelanin, which has a protective function against radiation (Jablonski, 2000; Jablonski, 2010). Eumelanin absorbs and scatter UV and other radiations thus preventing generation of reactive oxygen species which will be generated when radiation reacts with cellular contents (Mason, 1960). This phenotype was established by stabilizing selection of gene MC1R (Melanocortin 1 receptor) even in present day population of Africa (Makova, 2005). When dispersed outside Africa, leads to exposure to less intense UVR, thus leading to synthesis of pre-vitamin D3 in skin which can cause loss of permanent constitutive pigmentation (Chaplin, 2009).
Global population plotted in relative to ability to generate cutaneous pre-vitamin D3 for skin with light pigment.
Other studies led by Geneticist Dr Iain Mathieson, from Harvard University in Massachusetts given insights into skin color of European people, this study showed that as early as 8000 years ago most Europeans didn’t look like what they are today. It was hypothesized that migration of Homo into Europe started around 40,000 years ago from Africa. These studies were performed from genome of 83 ancient individuals from archaeological sites throughout Europe. They compared ancient genomes with recent ones. They have narrowed down to five genes which are associated with changes in diet (Ability to digest milk) and skin pigmentation. These studies associated that inability of hunter gatherers of Europe to digest milk 8000 years to a gene LCT. In case of skin color, this group found various patches of evolution in different places. Three separate genes shown to be playing role in light skin, thus telling complex story of European skin pigmentation. Around 8000 years ago, Hunter gatherers in Luxembourg, Spain and Hungary had dark skin. Absence version of two genes, SLC24A5 and SLC45A2 lead to pale skin in Europeans today by depigmentation. In case of northern Europe where sunlight levels are very low different genetic association was seen, Motala archaeological site in Sweden which is 7700 years old given insights where seven people had lighter skins and they contain variants of SLC24A5 and SLC45A2. They also contain a third gene, HERC2/OCA2, which has role in blue eyes and may have led to light skin and blonde hair. This confirms that ancient hunter gatherers from northern Europe are pale and had blue eyes, at the same time skin color is still darker in central and southern Europe (Mathieson, 2015).
On the other hand, genetic relation of disorders and migration can be seen in case of Leprosy, where leprosy did not exist in Americas until Europeans arrived (Ashmead, 1894). Along with Spanish, Portuguese and African slave labor leprosy was spread across Americas and Caribbean Islands. SNP analysis by Monot et al. showed the (Single Nucleotide Polymorphisms) SNP subtypes correlation between strains of Americas and European populations (Monot, 2005).
This review explains importance of migration of species Homo and how various environmental genetic factors affect a specific population and how individual characters such as blue eyes, blonde hair are associated with specific genes. Study also correlates melanin variations in European population and how these differences are correlated with genetic variations.
Human migration is one of the fascinating feats achieved by any Land dwelling species. Understanding of human migration is important for various reasons ranging from social reasons such as persecution or inequality towards other humans due to skin color which can be explained now with scientific support as we understand different pigmentation pattern among various species which in turn coded by various genetic determinants. It is important for understanding spread of various diseases among population groups and determining genetic determinants for such susceptibility or resistance to particular disorders. Origin of various diseases and their spread can be explained by understanding migration patterns.
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Chaplin, G., & Jablonski, N. G. (2009). Vitamin D and the evolution of human depigmentation. American journal of physical anthropology, 139(4), 451-461.
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