DNA in mitochondria, the power producers
of the cell, is providing clues to human
(Image: National Geographic)
Research carried out by the National Geographic Genographic project has revealed that the DNA of a small population of people living in the Karoo region of South Africa can be traced directly back to human ancestors of 100 000 years ago.
The Karretjie people of South Africa’s Great Karoo region are so named because they are nomadic and carry all their worldly possessions with them in “karretjies” or donkey carts. They rely largely on seasonal sheep-shearing to earn a living.
Yet in spite of their humble circumstances and the fact that they are one of the most marginalised of peoples in South Africa, this particular group of people is of immense scientific interest because they are related to the Khoisan and, uniquely, the DNA that they carry in every cell in their bodies can be traced back for more than 100 000 years.
This gives researchers a more accurate glimpse of the movements of the human population at that time, and the ability to track the way that branches of that population split off and moved out into other areas.
The Karretjie people are descended from both the Khoekhoen, the aboriginal herders of the Cape, and the San or Bushmen, who were hunters instead of herders. These are the First People, or earliest inhabitants of the region, and scientists have not yet established where they came from before they settled in Southern Africa.
South Africa helping to map the movements of humankind
The National Geographic Genographic project was set up to map human migratory patterns going back many thousands of years. The project was launched in April 2005 in partnership with IBM and the Waitt Family Foundation, an organisation that provides funding for projects related to human ancestry. It will run over five years.
Using the techniques of mitochondrial DNA (mtDNA) and Y-chromosome DNA analysis, scientists can reconstruct the hereditary lineage of individuals and their families. This is leading to a better understanding of humankind’s migratory history and how the human race spread out from its suggested origins in Africa, embarking on a journey 60 000 years ago to eventually cover the world.
The public at large is encouraged to voluntarily contribute samples of their DNA for analysis and inclusion in the database, which will provide valuable information about specific genetic markers of descent. These are variations or natural mutations in DNA that are passed down in families and can be used to track migration and ancestry.
Dr Spencer Wells, the director of the project, and a global team of scientists from ten prominent international institutions are in charge of studies in the field in their regions. The ten centres are located in Australia, Brazil, China, France, India, Lebanon, Russia, South Africa, Spain, and the US.
Professor Himladevi Soodyall, director of the South African Medical Research Council’s (MRC) Human Genomic Diversity and Disease Research unit, is at the helm of the African section of the Genographic Project. Soodyall received a South African national order in 2005 for her outstanding work in the science of human origins. As a respected academic in the field she was invited to participate in the Genographic Project as the principal investigator for sub-Saharan Africa.
Soodyall hopes to address several anthropological and genetic puzzles while working on the project, among them the question of how Indian Ocean trade activity influenced the gene pool in Africa, and the extent to which females have contributed towards shaping the gene pool, using mtDNA.
Research at a local level
The function of the Human Genomic Diversity and Disease Research unit is to integrate regional population history with the process of mapping and modelling human genetic variation.
In collaboration with Professor Mike de Jongh of the University of South Africa’s Department of Anthropology and Archaeology, the unit is studying the Karretjie people of Colesberg.
“There is still overwhelming evidence from genetic data to support the theory that modern humans evolved in Africa,” said Soodyall. “Our own research has shown that some of the oldest mtDNA lineages and Y-chromosomes haplotypes found in living humans are found in Khoisan populations. Thus, there is stronger evidence from genetic data to claim that the origins of our species lies here in Southern Africa, and not East Africa, as is usually claimed.”
Tracing our ancestry
The MRC has found that in recent years genetic ancestry testing using the mtDNA and Y-chromosome techniques has grabbed the interest of South Africans and, says the council, more than 70% of people who call the Human Genomic Diversity and Disease Research unit for information decide to follow their call up with a test.
Since February 2006 the unit has been offering ancestry tests to the public and, it says, several hundred people have already been tested. Interested parties can go to branches of the National Health Laboratory Service. The process starts with a 30-minute consultation with the subject; during which the whole procedure is explained.
The subject, if still willing, signs a consent form, after which a cheek swab or blood sample is drawn and submitted for testing and comparison with data already held in databases. The process takes no longer than an hour and the subject receives a full explanation of their results. Since 2004, says the unit, over 600 people have been tested.
Mitochondrial DNA a powerful tool for identification
Most of the genetic material in a living organism is found in the nucleus. The nucleus contains the chromosomes, which are made up of proteins and DNA. However, mitochondria, the structures in the cell that are responsible for generating power for the body through the burning of energy-rich molecules found in food, also contain a small amount of their own DNA. This is known as mitochondrial DNA or mtDNA and is thought to have a different evolutionary origin to that of nuclear DNA.
First sequenced in 1981, the human mitochondrial genome is a small ring of DNA that contains 37 genes. At a very distant stage of human evolution mitochondria were once independent living cells, almost like bacteria, but over millions of years they invaded primitive amoeboid cells and gradually became dependent on their hosts, losing the ability to exist independently.
Mitochondrial DNA is passed down from mother to daughter without recombining. Sons receive mtDNA but don’t pass it on. Diseases caused as a result of mtDNA mutation are also passed down exclusively through the maternal line.
Nuclear DNA is inherited from both parents and is a wholly individual characteristic because genes are rearranged through the process of genetic recombination, which takes place during cell division, resulting in the formation of sperm and egg cells.
No such change occurs when mtDNA is passed from parent to child. This feature allows mtDNA to be used as a tool to track ancestry through the female line, going back hundreds of generations. It can also be used in forensics to identify human remains or to exclude matches between missing persons and unidentified remains. Because mtDNA remains the same across a span of many maternal generations it is better suited for the identification of older remains.
The mtDNA technique was used to identify the woman known as Mitochondrial Eve, the most recent common ancestor through the mitochondrial pathway that connects mother to daughter. She lived about 140 000 years ago in East Africa and while she was not the only woman living at the time, she is the woman whose mitochondrial DNA is found today in every human alive.
Y-chromosomal Adam is her male counterpart, a man who lived about 60 000 years ago in Africa. Traces of his DNA are today found in the Y-chromosomes of all living men.
- National Geographic Genographic Project
- MRC Human Genome project
- Waitt Family Foundation
- National Health Laboratory Services