G. San / Khoisan Evolution, World Population Family Tree, DNA Migration of Modern Human Origin

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The application of molecular genetic approaches to the study of human evolution       - By L. Luca Cavalli-Sforza, Department of Genetics, Stanford Medical School, Stanford University, Stanford, California, USA, and Marcus W. Feldman, Department of Biological Sciences, Stanford University, Stanford, California, USA

Summary Phylogenetic Tree of world populations

Figure 1. 

Phylogenetic tree based on polymorphisms of 120 protein genes in 1,915 populations, grouped by    continental sub-areas, statistics of genetic diversity amongst populations and genetic distances.

Co-evolution of genes with language and some slowly evolving cultural traits, together with the genetic evolution of commensals and parasites that have accompanied modern humans in their expansion from Africa to the other continents, supports and supplements the standard model of genetic evolution.

Reconstructing human evolution requires both historical and statistical research of various disciplines such as physical and social anthropology, archeology, demography and linguistics.

Migration is another important factor in human evolution that can profoundly affect genomic variation within a population.  Most populations are relatively isolated, however, exchange of marriage partners between groups does occur.  Sometimes a whole population, or a fraction of it, migrates and settles elsewhere.  The migrant group is initially small but subsequently expands. 

Relationship between genetic and geographic distance

Figure 2.

Genetic distance of population pairs measured, by statistics of genetic diversity among populations, as a function of geographic distance between members of the pairs.  Only samples of indigenous people were included.  Continents where primitive economies predominate, hunting-gathering or tropical gardening, show highest asymptotes.   

Studies showed that genetic differences between populations are relatively small as compared with those within populations.  Subsequent analysis, including molecular polymorphisims of 14 populations, representing all continents, confirmed that the within - population variants was about 85 % of the total.  A recent analysis of 377 autosomal microsatelite markers in 1,065 individuals from 52 worldwide populations found that only 5 - 7 % of the variation was between populations.  It is the remaining 5 - 15 %, the between-population component, that can be used to reconstruct the evolutionary history of populations.

From the analysis of several mitochondrial DNA polymorphisms, derived two important conclusions: The first major separation in the evolutionary tree of modern humans, was between Africans and non-Africans; and the time back to the most recent common ancestor of modern human DNA was 190,000 years ago (however with a large error). 

Tracking migrations of our species using DNA  

Figure. 3

The scheme outlined hereunder begins with a radiation from East Africa to the rest of Africa about 100,000 years ago, and is followed by an expansion from the same area to Asia, probably by two routes, southern and northern between 60-40,000 years ago.  Oceania, Europe and America were settled from Asia in that order.

 

A recent synthesis of Y chromosome phylogeography, paleoanthropological and paleaclimatological evidence suggests a possible hypothesis for the evolution of human diversity.  Around 100,000 years or shortly after, a small population of about 1,000 individuals, most probably from East Africa, expanded throughout much of Africa.  Genetic data indicates that the spread of humans into Asia occurred through two routes.  The first was a southern route, along the coast to South and South-East Asia, and from there north and south.  In the south, these modern humans reached Oceania between 60,000-40,000 years ago, where as the northern expansion later reached China, Japan and eventually America.  This was a central route through the middle East, Arabia of Persia to central Asia, from where migration occurred in all directions reaching Europe, East and North-East Asia, about 40,000 years ago, after which the first and principle migration to America occurred +- 15,000 years ago.

Language families of the world

Figure. 4

The oldest family is the Khoisan, that includes Bushmen and Hottentots, many of whom also belong genetically to the oldest haplogroups of both mtDNA and NRY.  Australian and Indopacific are also old families.  Other African languages are Niger-Kordofanian, mostly West Africa, Nito-Saharan and Afroasiatic, that includes Semetic languages like Arab and Hebrew.

American languages belong to three families, Amerinds were the first to migrate from Asia +- 15,000 years ago, and Merind shows afinities with Eurosiatic.  One of the other two American families is Na-dene, belonging to Dene-Caucasion, a family that spread to Eurasia and includes Sinotibetan, spoken in almost all of China, as well as some isolated, probably relic, languages, Basque, a few Caucasion languages and Burushasjki, spoken in North Pakistan, that all survived the latest spread of Eurasiatic languages. 

The third American family is Eskimo-Aluet, the last to spread to America, from North-East Siberia.  The Austric family is very large and is spoken in South-East Asia, Indonesia, all of Polynesia, to the east and Madagascar to the west.

For geneticists, it seems natural to think that modern languages derive mostly or completely from a single language spoken in Africa, around 100,000 years ago, given that today's genes also derive from that population.

Evidence supporting the existence of a common single language, include the shared lexicon, sounds and grammar of present-day languages.  Langauge, like many other forms of co-operation, must have originated as intrafamilial communication.

Parasites and organisms

Humans carry many parasites of commensal organisms, some of which began their relationship with humans more than 100,000 years.  If their transmission is even partly vertical- as it is for hepatitis B virus- then their evolution is similar to that of humans, with origins in Africa, and a spread first to Asia and then, independently, from Asia to the other three continents.  It has been suggested that this is true of the other viruses, such as polyomavirus, and also of the bacterium Helicobacter pylori, which was recently found to be the causative agent of gastric ulcer.  It is likely that the same evolutionary properties will be detected for other commensals and parasites, indicating that at least part of their transmission is vertical. 

 

   

 

 

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