by Jane Harris Zsovan
Mitochondria - the energy batteries in our cells - may yield clues not only to diseases and aging, but to our ancestral past. However, much of what we read about mitochondria in popular media is either overstated or just plain wrong. That's too bad, because the truth is as fascinating as the myth. And human health sometimes depends upon separating fact from fiction.
What are mitochondria?
Mitochondria provide energy for cells.
They do this by cellular respiration - turning oxygen and the glucose from our food into energy.
Were mitochondria once independent life forms?
Lynn Margulis, Distinguished Professor in the Department of Geosciences at the University of Massachusetts, thinks she knows how mitochondria (in animals) and chloroplasts (in plants) got started. She contends that they descend from bacteria that were once independent life forms, but later became incorporated into more complex organisms.
Her theory, called symbiogenesis, challenges a central tenet of neo-Darwinism, that most key changes occur through competition between organisms (that is, natural selection or survival of the fittest). Her theory assigns a leading role to co-operation. A detailed discussion of Margulis's theory is found in her book, Symbiosis in Cell Evolution: Microbial communities in the Archean and Proterozoic eons (second edition, 1993).
Mitochondrial DNA (MtDNA)
DNA (deoxyribonucleic acid), which provides the genetic instructions for making living organisms, is found in both the cell nucleus and in its mitochondria. The DNA of the mitochondria, called MtDNA, has a separate genome from the DNA in the nucleus. This DNA inside the mitochondria looks promising to researchers for two reasons. Its unique attributes may shed light on some medical problems and, according to some researchers, on human ancestry as well. These unique attributes include: Most mammals show a tendency to inherit MtDNA from their mothers.
MtDNA is easily damaged, which may trigger chronic illness and premature aging in humans.
The smaller genome contained in MtDNA, plus the fact that life forms tend to inherit MtDNA primarily from the mother may help protect offspring from MtDNA damage.
MtDNA damage and health
MtDNA damage is related to chronic disease and premature aging.
Late life Prenatal Programming of Depression and Schizophrenia?, (published in Neuroembryology) Manuel Dafotakis1, Jochen Vehoff1, Hubert Korr, and Christoph Schmitz of the Department of Anatomy and Cell Biology, RWTH University of Aachen, Aachen, Germany, examine prenatal MtDna damage as a cause for some mental illnesses.
Can MtDNA be used to trace human ancestry?
Many evolutionary biologists consider MtDNA a valuable tool in tracing human evolution. If MtDna offers an accurate view of our innheritance from our mothers (maternal line), it supports the "African Eve" theory, according to which, all of humanity shares a genetic link that scientists can trace back to one woman living in Africa over 150,000 years ago.
African Eve is popular with a number of groups in society, for reasons that are easy to understand and sympathize with. She underscores our common humanity, and supports the Biblical account of a single mother of humankind. At minimum, she offers the hope of shedding light on our distant past. However, as we shall see, there are difficulties with tracing our ancestry through MtDNA.
The path of MtDNA Inheritance
It is widely believed that during fertilization, the sperm tail is excluded from the egg, thus excluding mitochondrial genetic material from the male parent (paternal line). But that is not correct. In every known mammal species except the Chinese hamster, tail and midpiece mitochondrial sheath do enter the egg at fertilization and can be traced for several cell divisions. So, while most MtDNA may come through the maternal line, it is inherited both through the maternal life and the paternal line in most mammals.
For example, according to Shagli (1994) and Ankel-Simmons (1996), sperm contribute mitochondria to the egg. Gyllensten et. al found that "Paternal inheritance of MtDNA also means that MtDNA phylogenies are not exclusively matriarchal." We do not yet know, however, what role sperm MtDna plays in the embryo. For now, let's look at the role is has played in helping researchers understand our origins.
Part Two: What Does Our Mitochondrial DNA Say About Human Ancestry?
Part Three: African Eve - when pop culture falls in love with science