To uncover the genetic link between nine-year-old Meiramgul, the blond child of the mountains of western Mongolia, and the long-dead women warriors of the Eurasian steppes, researchers examined snippets of a particular type of genetic information called mitochondrial DNA.Each cell in a plant or animal carries two varieties of deoxyribonucleic acid, or DNA. Within the nucleus of the cell is the full genetic complement, or genome, representing all of the genes that give an organism its particular characteristics and allow it to function. In humans this nuclear DNA is represented by two sets of 23 chromosomes, one set each passed on by the mother and father. One pair of these chromosomes determines the sex of the individual; a nuclear DNA analysis was used to determine the female gender of the warriors unearthed by archaeologist Jeannine Davis-Kimball and her colleagues during the excavations featured in SECRETS OF THE DEAD: “Amazon Warrior Women.”
Mitochondrial DNA, however, is separate from these 46 nuclear chromosomes. It is found within the cytoplasm of cells — the matrix of proteins, chemicals, fluid, and other structures located outside of the nucleus — and, in particular, within tiny, pill-shaped compartments called the mitochondria. Within mitochondria, a cell’s energy-producing reactions take place. (Some lethal poisons, such as cyanide, work by disrupting the biochemical reactions that mitochondria use to power up cells). Each of the 500 to 1,000 mitochondria inside a cell contain several circular pieces of DNA. There are approximately 16,000 base pairs, the chemical building blocks of DNA, along each strand; human nuclear DNA has about 300,000 times as many base pairs. The mitochondrial strand contains thirteen genes, each used to produce proteins that are involved in energy production. Other bits of the mitochondrial, or mtDNA, strand code for molecules called ribosomal and transfer RNA, which are intermediaries in the chemical process that translates the genes into their respective proteins.
A crucial feature that sets mitochondrial DNA apart from nuclear DNA — and that allows genetic studies like the comparison of the DNA from ancient bones to that of young Meiramgul — is that the strands are only passed from mothers to their children. This is because the mitochondria present in a fertilized embryo come almost entirely from the egg and rarely from the sperm. The mtDNA strands would normally be passed from mother to child in perfect form, but occasionally one of the individual base pairs will change, or mutate, producing a slight deviation in the sequence. Molecular biologists have mapped the entire sequence of human mitochondrial DNA and also have measured the average rate at which these mutations occur, which allows them to use slight differences in the sequence as a way of determining how closely related individuals are through their maternal ancestors. For this reason, mitochondrial DNA has frequently been used to examine the genetic relationships of families, population groups, and even the entire human species.
Mitochondrial DNA studies revealed, for example, that Neanderthals were not direct ancestors of modern humans. Mitochondrial DNA analysis also showed that all groups of humans on Earth could be mapped back to a single woman, a “mitochondrial Eve” who lived in Africa 200,000 years ago. (That is not to say that every human on Earth descended from the same woman living 200,000 years ago; rather, she was the most recent common ancestor of all humans, through maternal lines of inheritance). Because an individual cell possesses a thousand or so copies of the mtDNA strand, it is useful in the analysis of ancient or damaged tissue, teeth, hair, and bones, for the simple reason that there is much more of the material to locate and extract than there would be of nuclear DNA. Of course, the examination of any ancient DNA samples is fraught with difficulty, and contamination is a constant risk so the analyses are only done in the world’s best molecular biology laboratories.