MITOCHONDRIAL DNA AND ITS ROLE IN PALEO ANTHROPOLOGY

Human mitochondrial DNA (mtDNA) is a circular double-stranded molecule, 16,569 base pairs (bp) in length that codes for 13 subunits of the oxidative phosphorylation system, 2 ribosomal RNAs (rRNAs), and 22 transfer RNAs (tRNAs)

It is present in hundreds to thousands of copies in each cell, not within the nucleus, but within the cell’s energy-generating organelles, the mitochondria.

MtDNA consists predominantly of coding DNA, with the exception of a ∼1100-bp long fragment that has mainly regulatory functions and is therefore termed the control region. It is useful in Studies of human evolution, migration, and population histories

Unique properties of human mitochondrial DNA (mtDNA)

• lack of recombination – The process of recombination in nuclear DNA (except the Y chromosome) mixes sections of DNA from the mother and the father creating a garbled genetic history. 
• High copy number – They are present in large numbers in each cell, so fewer samples is required. 
• High mutation rate- They have a higher rate of substitution (mutations where one nucleotide is replaced with another) than nuclear DNA making it easier to resolve differences between closely related individuals. 
• Maternal inheritance – They are inherited only from the mother, which allows tracing of a direct genetic line. 
• It can be possible to extract mtDNA from sources when the nuclear DNA is degraded beyond use. This is partly because many more copies of mtDNA exist and also because the circular configuration of the genome makes it more stable in nature. In fact mtDNA has been extracted amplified and sequenced from the fossilised remains of Neanderthal bones at least 30,000 years old.

The above property have made it the molecule of choice for studies of human population history and evolution.

The science:

Mitochondrial DNA (mtDNA) is inherited only from the mother. Every few generations, a random mutation creeps into this familial signature. So comparison of two samples of mtDNA will show degrees of kinship and ancestral origin. Conversely, the Y chromosome is inherited by males from the father. Random infrequent changes once again provide a way of estimating the number of generations back to a shared ancestor. 

Thus mtDNA studies are (and should be) supplemented with analyses of the Y-chromosome.

The story:

DNA research suggests that all surviving humans are descended from one woman who lived perhaps 200,000 years ago. Research also shows that the story begins in Africa, home to the greatest variation in human DNA, and therefore the oldest location. Accordingly the woman was promptly dubbed “the African Eve”.

Not surprisingly, people of the same ethnic and linguistic group turn out to be genetically more closely related to each other than to the rest of the planet, but the same research shows a great deal of mixing of populations as well.

Studies of telltale markers in the DNA sequence have been used to reconstruct the journeys of ancient human groups around the globe, and not just ancient humans. Along the way from East Africa to Easter Island, early human voyagers picked up fellow travellers such as the stomach ulcer bug Helicobacter pylori. This bacterium also carries a DNA signature of its origins.

In 2009, it delivered an answer to one of the great mysteries of the human migration: all the settlers on the islands of Polynesia and Melanesia carried stomach bugs that their ancestors could only have picked up in Taiwan. So that became the jumping-off point for the colonisation of the Pacific.

Significance of mDNA in anthropology:

1. In examining socio-cultural influences on human genetic variation that might have influenced human evolution, such as polygyny, the effects of matrilocality versus patrilocality, or the social stratification induced by the caste system.
2. Study of ancient DNA: Because of the high copy number of mtDNA versus the diploid autosomes and haploid Y-chromosome, mtDNA is crucial in studies of ancient DNA. Depending on the age of the fossil sample, often only mtDNA will still be present, and therefore this is the only insight one can get into the genetic affinities of ancient populations
3. Forensic DNA applications: Using mtDNA enables identification of otherwise unidentifiable victims, if maternal relatives are alive for comparison. This is becoming increasingly relevant for victims of war or terror.
4. In tracing personal genetic history: mtDNA is increasingly used in so-called personalized genetic histories. This is the use of genetic testing to investigate individual genealogies, including tracing the origins of immigrant/slave ancestors.

Example: Using mtDNA gathered from a number of different populations, scientists at the University of California, Barkley, constructed “trees” that, they claimed, demonstrated that the entire population of the world today descended from a single African lineage.

Issues: A major problem with ancient DNA studies is contamination with modern DNA. This is particularly a problem when ancient samples of modern humans are analyzed, because the contaminating mtDNA will not differ greatly from the authentic mtDNA. Therefore, caution should be exercised when evaluating studies of ancient DNA in modern humans. This holds especially true for very old fossils, such as Neandertals or ancient modern humans.

Examine the significance of Mitochondrial DNA in paleoanthropology. Give examples.