Mitochondrial Eve: Notes

Slide1- Mitochondrial Eve Theory states that the mitochondrial DNA in all humans is inherited from one common female ancestor in Africa ~200,000 years ago. This is just one of several theories about human evolution that are being debated and this is the only theory that will be discussed in this presentation.

Slide2- Mitochondria are cytoplasmic organelles that contatins an outer membrane and an inner membrane that invaginates to form membrane folds called cristae. Cristae contain enzymes and other compounds involved in oxidative phosphorylation and electron transport. Surrounding the cristae is matrix. Matrix contains proteins, some DNA and RNA, granules, as well as enzymes for the citric acid cycle and fatty acid beta oxidation. Mitochondria have many functions such as the generation of energy, cellular respiration, and heat production. It is thought that mitochrondria result from a symbiotic relationship between primitive bacteria and eukaryotic cells. The bacteria provided the cell with a highly efficient energy source and the cell synthesized many components needed by the bacteria.

Slide3- Mitochondria have double-stranded DNA which containes ~16,569 base pairs that code for ~37 proteins. DNA replication in the mitochondria is independent of DNA replication in the nucleus. Neither DNA repair or recombination are involved in mitochondria DNA processes, which may explain why mitochondrial DNA has a 10 times higher mutation rate than nuclear DNA.

Slide 4- The number of mitochondria varies between male and female gametes. Oocytes have 100,000 mitochondria and sperm have less than 100. Sperm mitochondria are destroyed shortly after fertilization. It is thought that the sperm is tagged for destruction by the oocyte because it is seen as a foreign substance. So this may explain why sperm is inherited only from the mother. This information initiated the use of mitochondrial DNA to reconstruct family trees and eventually led to the Mitochondrial Eve Theory.

Slide 5- Again, the Mitochondrial Eve Theory states that the mitochondrial DNA in all humans is inherited from one common female ancestor in Africa ~200,000 years ago.

Slide 6 - There are several possible events that may have resulted in Mitochondrial Eve. The most popular explanation for the Mitochondrial Eve Theory is a bottleneck. This occurs when a severe population constriction takes place probably caused by disease or natural disaster such as climate fluctuations. Bottlenecks may correlate with major evolutionary changes. The bottleneck could have occurred in several ways. One way is that a large population of modern humans may have greatly dwindled in size, therefore losing mitochondrial DNA. Some of the remaining mating partners may not produce daughters where additional mitochondrial DNA is lost until after so many generations only one mitochondrial DNA exists among them. Another form of bottleneck would be in a small isolated population of archaic humans where a highly restricted speciation event took place that caused some of the individuals to begin to evolve into modern humans.

Random Genetic Drift is similar to bottleneck, but it is the accidental loss of mitochondrial DNA, where some women do not produce daughters and after many generations there is a high probability that all the mitochondrial DNA is from one female ancestor.

The name parthenogenesis came from incidents involving the Parthenon in Greece.

Slide 7 - This is from a post card of a full-scale replica of the Parthenon in Nashville, Tenn. The Parthenon was built as a dwelling place for the Greek gods, especially the goddess Athena. There were vestals, young female virgins, who looked after the place and left food and wine offerings for the gods. The interesting thing is that many of the vestals kept getting pregnant. The people thought these virgin births were seeded by the gods and it was called parthenogenesis, but the priests were probably responsible for the pregnancies. Parthenogenesis is common in many lower animals such as ants, honeybees, plant lice, some ticks, wasps, certain lizards and snakes. This phenomenon was discovered in the 18th century and in 1900 artificial parthenogenesis was 1st conducted by Jacques Loeb when he stimulated unfertilized frog eggs with a needle. Some eggs began normal embryonic development and these offspring were usually identical to the mother in all inherited aspects.

Of the three possible causes of Mitochondrial Eve, a bottleneck is the most popular explanation. Now that we have a theory and possible causes, you might want to know about supporting evidence.

Slide 8- During the 1980ís, several groups at UC Berkely used DNA restriction analysis to trace maternal lineages of people sampled from around the world. Non-coding regions within mitochondrial DNA accumulate neutral mutations rapidly and steadily. The number of individuals sampled varied with each group, where 21, 147, and 241 individuals were involved. They found many types of mitochondrial DNA that were separated into 2 groups, a group of sub-Saharan Africans and a group consisting of N. Africans and all other population groups. The fact that Africans are present in both groups suggest an African origin for modern humans. They also showed tht within the African population, there was much more variation in mitochondrial DNA that any other group, indicating that the African population has been established longer. The information generated from these studies was used to calculate a mutation rate using geographical groups of known colonization or by comparing the mutations among humans with the divergence of humans and chimpanzees. It has been estimated that Mitochondrial Eve lived 150,000-200,000 years ago and that mitochondrial DNA mutates 2-4% per million years.

In 1991, Vigilant and colleagues sequenced polymorphic regions of the mitochondrial DNA of 189 individuals. The results were the same as those from DNA restriction analysis. They separated their findings into 2 groups (Africans and all other populations + some Africans). As in the other studies, there were more mutations in the Africans, again suggesting that they are an older population.

Slide 9- All of these groups independently constructed an evolutionary tree to present their results and compare genetic differences among and within various racial and geographical groups..

Slide 10- There is some supporting anthropological evidence for the theory of Mitochondrial Eve. The oldest modern human remains were 1st found in Africa, dated ~100,000 years ago, while younger remains were found in the Middle East, then in Europe and Asia. This supports an African origin of modern humans. In addition, an advanced blade tool was found in Africa long before it replaced the more primitive flake tool ~90,000 years ago in Europe and Asia.

Slide 11- Based on the evidence, it is highly probable that the Mitochondrial Eve Theory is true.

Slide 12- References