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Scientists explore humankind's past with mtDNA.

Tracing Ancestry with MtDNA
by Rick GroleauIn 1987, three scientists announced in the journal Nature that they hadfound a common ancestor to us all, a woman who lived in Africa 200,000 yearsago. She was given the name "Eve," which was great for capturingattention, though somewhat misleading, as the name at once brought to mindthe biblical Eve, and with it the mistaken notion that the ancestor was thefirst of our species—the woman from whom all humankind descended.

The "Eve" in question was actually the most recent common ancestor throughmatrilineal descent of all humans living today. That is, all people alive todaycan trace some of their genetic heritage through their mothers back to this one woman.The scientists hypothesized this ancient woman's existence by looking withinthe cells of living people and analyzing short loops of genetic code known asmitochondrial DNA, or mtDNA for short. In recent years, scientists have usedmtDNA to trace the evolution and migration of human species, including when thecommon ancestor to modern humans and Neanderthals lived—though there hasbeen considerable debate over the validity and value of the findings.

In reproduction, the nuclear DNA of one parent mixes with the nuclear DNA ofthe other. MtDNA, on the other hand, passes on from mother to offspringunaltered.

Nuclear DNA vs mitochondrial DNAWhen someone mentions human DNA, what do you think of? If you know a littleabout the topic, perhaps you think of the 46 chromosomes that inhabit thenucleus of almost every cell that comprises your body. These chromosomes holdthe vast bulk of genetic information that you've inherited from your parents.

Outside the nucleus, but still within the cell, lie mitochondria. Mitochondriaare tiny structures that help cells in a number of ways, including producingthe energy that cells need. Each mitochondrion—there are about 1,700 inevery human cell—includes an identical loop of DNA about 16,000 base pairslong containing 37 genes. In contrast, nuclear DNA consists of three billionbase pairs and an estimated 70,000 genes. (This estimate has been revised upward several timessince the announcement that the human genome had been decoded, and likelywill be again.)

Inheriting mtDNA

Whenever an egg cell is fertilized, nuclear chromosomes from a sperm cell enterthe egg and combine with the egg's nuclear DNA, producing a mixture of bothparents' genetic code. The mtDNA from the sperm cell, however, is left behind,outside of the egg cell.

So the fertilized egg contains a mixture of the father and mother's nuclear DNAand an exact copy of the mother's mtDNA, but none of the father's mtDNA. Theresult is that mtDNA is passed on only along the maternal line. This means thatall of the mtDNA in the cells of a person's body are copies of his or hermother's mtDNA, and all of the mother's mtDNA is a copy of her mother's, and soon. No matter how far back you go, mtDNA is always inherited only from themother.

If you went back six generations in your own family tree, you'd see that yournuclear DNA is inherited from 32 men and 32 women^[1]. Your mtDNA, on the otherhand, would have come from only one of those 32 women.

See the difference between mtDNA and nuclear DNA ancestry with this interactivefamily tree.

Go to "Tracing Ancestry with MtDNA"

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Defining mitochondrial ancestorsLet's get back to "Eve." The ancestor referred to in the 1987 Naturearticle can be more precisely stated as "the most recent common ancestorthrough matrilineal descent of all humans living today." In other words, she isthe most recent person from whom everyone now living on Earth has inherited hisor her mtDNA. This certainly does not mean that she is the ancestral mother ofall who came after her; during her time and even before her time there weremany women and men who contributed to the nuclear genes we now carry. (To seehow this can be, check out Tracing Ancestry.) It also does notmean that the mtDNA originated with this "Eve"; she and her contemporaries alsohad their own "most recent common ancestor though matrilineal descent," a womanwho lived even further into the past who passed on her mtDNA to everyone livingduring "Eve's" time. (We get our mtDNA from that same, older ancestor. She'sjust not, to us, the most recent common ancestor.)

So what about all of the mtDNA of the other women who lived during "Eve's" time?What happened to it? Simply this: Somewhere between now and then, they hadfemale descendants who had only sons (or no children). When this happened, thepassing on of their mtDNA halted.

Finding mitochondrial ancestors

Even though everyone on Earth living today has inherited his or her mtDNA fromone person who lived long ago, our mtDNA is not exactly alike. Random mutationshave altered the genetic code over the millennia. But these mutations areorganized, in a way. For example, let's say that 10,000 years after the mostrecent common ancestor, one of the mtDNA branches experienced a mutation. Fromthat point on, that line of mtDNA would include that alteration. Another branchmight experience a mutation in a different location. This alteration would alsobe passed on. What we would eventually end up with are some descendants whohave mtDNA that is exactly or very much like that of some people's, somewhat like thatof others, and less like that of yet others. By looking at the similaritiesand differences of the mtDNA of all of these individuals, researchers could tryto reconstruct where the branching took place.

This is what some researchers have done. For the original 1987 Naturearticle, the three authors (Rebecca Cann, Mark Stoneking, and Allan Wilson)looked at the mtDNA of 147 people from continents around the world (though forAfricans, they relied on African Americans^[2]). Later, with the help of acomputer program, they put together a sort of family tree, grouping those withthe most similar DNA together, then grouping the groups, and then grouping the groups ofgroups. The tree they ended up with showed that one of the two primary branchesconsisted only of African mtDNA and that the other branch consisted of mtDNA fromall over the world, including Africa. From this, they inferred that the most recent common mtDNA ancestor was anAfrican woman.^[3]

Dating mitochondrial ancestors

The three researchers went even further—they estimated the age of theancestor. To get the estimate, they made the assumption that the randommutations occurred at a steady rate. And since they now had an idea of how muchthe mtDNA had changed from the ancestor's, all they needed was the mutationrate to determine the age of the ancestor. For instance, if they took themutation rate to be one in every 1,000 years and knew that there was adifference of 10 mutations between the mtDNA of people living today and themtDNA of an ancestor who lived long ago, then they could infer that theancestor lived 10,000 years ago.

Cann, Stoneking, and Wilson estimated the mutation rate by looking at the mtDNAof groups of people whose ancestors migrated to areas at known times. One groupwas Australian aborigines, whose ancestors moved to the island-continent athen-calculated 30,000 years ago.^[4] Since the three then knew how long ittook for that group's mtDNA to diverge as well as how much it diverged, theydetermined the mutation rate. Using this rate, they determined that the mostrecent common ancestor lived 140,000 to 290,000 years ago (which they roughlyaveraged to 200,000 years ago). That was back in 1987. Since then, researchershave updated the estimate to 120,000 to 150,000 years ago. However, the marginfor error for this estimate and the previous one are significant—when all ofthe variables are taken into account, the current range is more like 50,000 to500,000.

Mitochondrial DNA is extracted from the bones of Neanderthals and compared to the mtDNA of living hom*o sapiens.

Neanderthals and mtDNAFinding out about our most recent common ancestor relies solely on inferencesfrom the mtDNA of people living today. What if we could actually compare ourmtDNA with mtDNA of a distant ancestor? This, in fact, has been done, withmtDNA from the bones of Neanderthals. Comparing mtDNA of these Neanderthals tomtDNA of living people from various continents, researchers have found that theNeanderthals' mtDNA is not more closely related to that of people from any onecontinent over another. This was an unwelcome finding for anthropologists who believethat there was some interbreeding between Neanderthals and early modern humans living in Europe (which might have helped to explain why modern Europeans possess some Neanderthal-likefeatures); these particular anthropologists instead would have expected the Neanderthals' mtDNAto be more similar to that of modern Europeans than to that of other peoples. Moreover,the researchers determined that the common ancestor to Neanderthals and modernhom*o sapiens lived as long as 500,000 years ago, well before the mostrecent common mtDNA ancestor of modern humans. This suggests (though it doesnot prove) that Neanderthals went extinct without contributing to the gene poolof any modern humans.

Final note

There are many variables that can affect the mutation rate of mtDNA, includingeven the possibility that mtDNA is not always inherited strictly through maternallines. In fact, recent studies show that paternal mtDNA can on rare occasions enter an egg duringfertilization and alter the maternalmtDNA through recombination. Such recombination would drastically affect themutation rate and throw off date estimates.

Not surprisingly, there is currently a heated debate over the value of"mitochondrial Eve"—especially between history-hunting geneticists andsome fossil-finding paleoanthropologists. According to theseanthropologists, even if we could accurately gauge the age of the ancestor,that knowledge is meaningless because all she really is is the woman whosemtDNA did not die out due to random lineage extinctions. Furthermore, herstatus as the most recent common ancestor doesn't mean that she and hercontemporaries were any different from their ancestors. (Remember, she and allof her contemporaries had their own mitochondrial Eve.)

Perhaps the most valuable finding regarding the "most recent common ancestor"is that she probably lived in Africa—a finding that supports the mostpopular theories about the worldwide spread of hominids.

Rick Groleau is managing editor of NOVA Online.Notes1. Unless two or more of those 64 married each other and bore children from which you are descended. For example, your great-great-grandfather on your mother's side might have married and had children with your great-great-grandmother on your father's side. In that case, the number of your ancestors in this example would drop to 63.

2. Although the original study was criticized for using African Americansinstead of native Africans, a subsequent study in which the researchers usedmtDNA from native Africans came up with similar results.

3. Other researchers later showed that the computer program could come up withother variations of the tree, some of which did not place an African at theroot of the tree. This study, then, cannot be viewed as definitive proof thatthe ancestor lived in Africa. However, it does still suggest that humansoriginated in Africa, a hypothesis that other, more recent studies support.

4. The date for the migration to Australia is now estimated to be 50,000 to60,000 years ago.

Sources

"Human Evolution." Svante Pääbo. Trends in Genetics. 15(12): M13-M16, 1999.

"Neanderthal DNA Sequences and the Origin of Modern Humans." MatthiasKrings, et al. Cell, July 11, 1997.

"Mitochondrial DNA and Human Evolution." Rebecca L. Cann, Mark Stoneking,Allan C. Wilson. Nature, January 1, 1987.

"The Case of Mitochondrial Eve." Frank R. Zindler. AmericanAtheist, February 1988.

Shreeve, James. The Neanderthal Enigma: Solving the Mystery of ModernHuman Origins. New York: Avon Books, 1995.

Stringer, Christopher; Clive Gamble. In Search of the Neanderthals:Solving the Puzzle of Human Origins. New York: Thames and Hudson, Inc.,1993.

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The article you provided delves into mitochondrial DNA (mtDNA) and its significance in tracing human ancestry. It explains how mtDNA differs from nuclear DNA, highlighting its maternal inheritance and the concept of the most recent common ancestor through matrilineal descent. Here's a breakdown of the key concepts discussed:

Mitochondrial DNA (mtDNA) vs. Nuclear DNA:

Mitochondrial DNA: Found in mitochondria, structures outside the cell nucleus. It's maternally inherited, passing only from mother to offspring.
Nuclear DNA: Resides in the cell nucleus, inherited from both parents, mixing during reproduction.

Ancestral Tracing with mtDNA:

"Eve" as the most recent common ancestor: Refers to the woman from whom all living humans inherited their mtDNA. She isn't the first human but the most recent matrilineal ancestor.
Mutation accumulation in mtDNA: Mutations in mtDNA occur over time, allowing scientists to trace lineage divergence and construct a familial tree.

Dating Mitochondrial Ancestors:

Estimating age: Scientists estimate the age of the most recent common ancestor by analyzing mutation rates in mtDNA. The estimates have varied over time, initially around 200,000 years ago, later revised to 120,000 to 150,000 years ago.

Neanderthals and mtDNA:

Comparative analysis: Researchers compared Neanderthal mtDNA to modern human mtDNA, finding divergence and estimating a common ancestor about 500,000 years ago.
Contributions to the gene pool: There's debate over whether Neanderthals interbred with early humans, but evidence suggests they might not have contributed to the modern human gene pool.

Debates and Varied Perspectives:

Controversies: There's ongoing debate over the accuracy and implications of the "mitochondrial Eve" concept. Some argue its significance, emphasizing geographical origins, while others question its value in understanding human evolution comprehensively.

Recent Updates and Challenges:

Challenges to the mutation rate: Recent studies suggest complexities in mtDNA inheritance, with rare instances of paternal mtDNA entering eggs, potentially altering mutation rates and dating estimations.

The article references scientific studies, such as those by Rebecca Cann, Mark Stoneking, and Allan Wilson in 1987, highlighting the ongoing evolution of our understanding of human ancestry based on mtDNA analysis.