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THE GREAT HUMAN ROAD TRIP

Let’s say you went on a road trip, but you were blind folded the whole way through and instead of it being a few days it took a few million years. How would you trace your way back?


Scientists have been studying human evolutionary migration patterns for years now. We commonly hear how DNA can be used to identify criminals. In a similar way by taking DNA samples, scientist can track back to where groups of different people on the earth first migrated from.


How is this done?


It is done by analysing DNA but not from the same place that it is extracted when identifying criminals. When identifying criminals, nuclear DNA is analysed. For human migration, scientist most commonly use mitochondrial DNA(mtDNA) The mitochondria is an organelle found inside cells and is passed down directly from mother to offspring with no recombination. This means all mitochondrial DNA is the same and comes directly from the mother . Non- recombinant Y-chromosomal DNA can also be used for the same reasons as mtDNA. This is passed down from father to each male offspring .

There are two main theories of human evolutionary migration. The multiregional theory and the ‘out of Africa ‘theory. The former being based of archaeological findings and the latter heavily backed by genetic evidence . This post is going to focus on the latter as mtDNA evidence backs this theory.


OUT OF AFRICA THEORY


When we look at humans from various continents we see many clear visual differences, but genetically we are more similar than you would imagine. Scientist analyse the mutational differences in mtDNA to track migration routes. All differences in the mtDNA are caused by mutations. mtDNA has a high mutation rate which creates more sites for analysis. These sites are known as polymorphic sites. A polymorphic site refers to major differences in the mtDNA.


A sample of blood, saliva etc is taken, purified and the mtDNA is extracted. The area of interest in the mtDNA is then amplified and analysed to observe the sequence. mtDNA profiles are created by amplifying the non-coding regions in DNA. These non-coding regions show differences between non-biological relatives and are usually the same for biological relatives . After amplification, a sequence is deciphered using computer analysis programs. The sequences are then compared between individuals for similarities. Similarities in mutations can be the same for people with even very distant ancestral linkage and therefore it can track back where humans migrated from. Mismatch analysis of mtDNA sequences is analysed to note differences between individuals. Analysing the differences allows for identification of which mutations changed or differ between individuals of the same and different groups. .


Fig.1 (Wallace, 2013)– shows Africa highlighted in red where the L haplotype originates. Within Africa, the figure shows the M and N haplotypes stemming from the L3 haplotype. It also shows that the M and N haplotypes are what all non-African populations originate from. The numbers in the figure indicate the approximate timing of when the specific mtDNA haplotypes first appeared.


From the many experiments that have been carried out using similar methods to that explained above it is concluded that there are different mtDNA lineages. The different lineages are referred to as haplogroups. These haplogroups are illustrated above(the different letters). The haplogroups represent related groups of sequences that are defined by shared mutations. When you put all the findings together of mtDNA research into a phylogenetic (ancestral linkage) tree; based on either non-recombinant Y-chromosomal DNA or mitochondrial DNA, it shows that Non- African populations emerged from a small group of individuals that left Africa. Haplotypes L1, L2 and L3 are restricted to African populations and M and N are found in all non-African populations. The M, N are subtypes of haplotype L. They are believed to have dispersed into Eurasia from east Africa .This is illustrated in fig 1, The arrow shows haplotype M and N originating from L3. Groups M and N migrated out of Africa and from there other haplotypes were created.


In a rough nut shell, your road trip started in Africa and ended wherever you find yourself right now reading this.




PAPERS USED


Cavalli-Sforza, L. L. & Feldman, M. N., 2003. The Application of Molecular Genetics


Approaches to Study Human Evolution. Nature Genetics , Volume 33, pp. 266-277.

Klug, W. S., Cumminngs, M. R., Spencer, C. A. & Palladino, M. A., 2016. DNA Forensics. In: Concepts of Genetics. Essex: Pearson, pp. 703-743.


Oppenheimer, S., 2012. A single southern exit of modern humans from Africa : Before and after Toba. Quatenary International, Volume 258, pp. 88-99.


Pakendorf, B. & Stoneking, M., 2002. Mitochondrial DNA:human evolution. Annual Review of Genetics, Volume 3, pp. 611-621.

Underhill, P. A. & Toomas, K., 2007. Use of Y-chromosome and Mitochondrial DNA


Population Structure in Tracing human migrations. Annual Review of Genetics , Volume 41, pp. 539-564.


Wallace, D. C., 2013. Bioenergetics in human evolution and disease: implications for the origins of biological complexity and the missing genetic variation of common disease. Philosophical Transactions B, Volume 368, p. 1622.

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