Mitochondrial Eve is the name given to the idea that the mitochondrial DNA in all modern humans can be traced back to a single genetic lineage, carried by a woman who lived in Africa approximately 250,000 to 150,000 years ago. This idea has been misunderstood by people who incorrectly think that it means that all modern humans can be traced exclusively to one single human ancestor, like the biblical “Eve.” This is incorrect because “mitochondrial Eve” is not the ancestral person from whom we can trace our entire nuclear DNA, the DNA that carries the codes to make humans.
Mitochondrial DNA
Mitochondria are small, energy-producing organelles found in eukaryotic cells. They have their own DNA (called mitochondrial DNA or mtDNA), separate from nuclear DNA, and replicate by themselves. They are thought to have entered into a symbiotic relationship with protoeukaryotic cells earlier than 1.5 billion years ago.
MtDNA is much easier to study than nuclear DNA. There are hundreds of mitochondria in the cytoplasm of each cell, making it easy to extract DNA for study. Human nuclear DNA contains the instructions to make a human being and is 3 billion base pairs long; mtDNA, which primarily contains the information to make the mitochondria, is much shorter, only 16,000 base pairs long (about 0.05% as long). However, there are two major features of mtDNA that make it particularly useful for evolutionary analysis. First, mtDNA accumulates mutations at a faster (but fairly constant) rate than nuclear DNA; these mutations may more often be neutral, instead of advantageous or disadvantageous, so mutations persist instead of being deleted. Second, mtDNA is inherited solely from the mother, so it does not recombine like nuclear DNA.
This latter point requires some explanation. A person inherits nuclear DNA equally through the egg and sperm of the parents. When sperm fertilizes an egg, it contributes almost exclusively nuclear material; any mitochondria-containing cytoplasm from the sperm does not survive entry into the mother’s egg. The two nuclei combine to create a person equally related to each parent. However, the parental mitochondria do not combine; the person inherits mtDNA exclusively from the mother.
Matrilines and “Mitochondrial Eve”
So, while males and females inherit nuclear DNA equally from each parent and pass it on equally, both males and females get their mtDNA only from their mother. Going back one generation, a person is equally related to all four grandparents through nuclear DNA, but only one grandparent through mtDNA (the maternal grandmother). This direct inheritance of the mtDNA lineage makes it possible to reconstruct phylogenetic (or evolutionary) trees using these lineages. This means that the mtDNA of all living people are copies (with some mutations) of the mtDNA from one woman who lived thousands of generations ago.
Sometimes, lineages can be lost. In each generation, some women will not leave mtDNA descendants, as they will have either no children or exclusively sons. This lineage loss is analogous to the family name loss experienced in some cultures where women take their husband’s family name; if she has no children or exclusively daughters, the name does not continue. As mtDNA lineages slowly die out over time, there are fewer lineages; eventually, there will be only one lineage remaining: the originator of this lineage has been dubbed “mitochondrial Eve.” This name is misleading: “Mitochondrial Eve” may be the most recent common ancestor of our mtDNA, but in the grandparents example above, she is only one of our countless other nuclear DNA ancestors who lived alongside her.
Of course, the picture is muddied because new mtDNA lineages can be added over time. When mutations occur in the mtDNA, these mutations are transmitted down the generations as new lineages. Because of these mutations, modern mtDNA has diverged from the original mtDNA from “mitochondrial Eve.” Since these mutations occur at a fairly constant rate, the amount of divergence is roughly proportional to the amount of time that has passed. This helps to calibrate the molecular clock used to determine when “mitochondrial Eve,” and her population, lived.
The Molecular Clock
The hypothesis behind the molecular clock is that genetic change occurs at a relatively constant rate and therefore can be used to measure the time elapsed since two species (or two populations) diverged from a common ancestor. Because mtDNA accumulates mutations at a higher rate than nuclear DNA, it is used to calibrate a molecular clock to track recent evolutionary changes occurring from several hundred thousand to a few million years ago.
Comparisons of the mtDNA variation in many species indicate that humans are less variable than many other species, most notably chimpanzees. This suggests that humans have undergone a population bottleneck sometime in the past, followed by a rapid population explosion. For mtDNA, this would mean a large number of lineages were lost quickly, followed by a slow buildup of new lineages. Since modern African populations are more variable than other populations, containing more mtDNA lineages, it is thought that the population explosion began in Africa. Using the molecular clock concept, the evidence suggests that “mitochondrial Eve” lived between 250,000 and 150,000 years ago in Africa.
There are several issues involved in assessing the meaning and significance of the genetic data. To explore modern human origins, the mtDNA is used to track past population movements and to determine when and where the earliest modern human ancestor lived. To do this, one must model gene flow between past populations in different geographic regions, which has proven difficult. Another issue is that of stochastic lineage loss; it is possible to remove old mtDNA lineages from a population while retaining the nuclear DNA material. A third problem is that the assumption that the mtDNA mutations are selectively neutral might not be correct. If natural selection favors one or more of the mtDNA lineages, that can create inaccuracies in the molecular clock. Finally, it has been challenged that mitochondria carried by sperm sometimes make it into the fertilized egg, making it possible for some male mtDNA to occasionally be passed on; if this is found to happen more than just rarely, it would throw into question the idea that mtDNA is a pure matrilineal marker and invalidate the concept of a “mitochondrial Eve.”
“Mitochondrial Eve” and Modern Human Origins
Was “mitochondrial Eve” a modern human? Some of the oldest fossils that are considered to be anatomically modern humans do date from around this time, in Africa. Most phylogenetic trees constructed using mtDNA show that the earliest branches occur in modern Africans and all other modern human groups branch from them. It is possible, then, that “mitochondrial Eve” lived in the population that later left Africa to colonize the rest of the world. Many researchers cite this evidence as support for a recent African origin model of modern human origins. However, critics of this model cite problems with the calibration of the molecular clock and suggest that “mitochondrial Eve” lived much earlier, during a period that predates anatomically modern humans in Africa. This, they argue, is evidence that modern humans share a com-mon mtDNA ancestor as old as Homo erectus, which refutes a recent African origin of our species.
“Mitochondrial Eve” and “Y-Chromosomal Adam”
Recent research has suggested that there is a male analog to “mitochondrial Eve,” known as “Y-chromosomal Adam.” Humans have 23 pairs of chromosomes; one of these is known as the “sex chromosome,” which has two forms, X and Y. Females have two copies of the X chromosome, while males have one X and one Y. So, sons inherit the Y chromosome exclusively from their fathers.
Studies indicate that the common ancestor of Y-chromosome lineages lived in Africa between 90,000 and 60,000 years ago, much later than “mitochondrial Eve.” If both sets of dates are correct, this may be evidence that the human species experienced another, more recent bottleneck. It also suggests that the creation and loss of some genetic lineages depends on chance. Alternatively, human behavior may play a role; the practice of polygamy may restrict offspring to a smaller set of all males in the group, which might speed up the loss of some Y-chromosome lineages.
The concept of “mitochondrial Eve,” that all modern humans share mtDNA descent with one African woman, is compelling. Rather than imagining our ancestors as fossils, it encourages us to imagine the lives of our great-great-grandmothers, thousands of generations ago.
References:
- Dawkins, R., & Ward, L. (1996). River out of Eden: A Darwinian view of life. New York: HarperCollins.
- Sykes, B. (2001). The seven daughters of Eve: The science that reveals our genetic ancestry. New York: Norton.
- Wells, S. (2003). The journey of man: A genetic odyssey. Princeton, NJ: Princeton University Press.
