Are we all descended from a common female ancestor?

Biologists have been aware of mitochondria since the 19th century. But it wasn't until the late 1970s that the value of using the DNA within mitochondria to track ancient human history became clear. Mitochondrial DNA differs in a few key ways from nuclear DNA -- the variety of DNA located within the nucleus of each of your cells determines your eye color, racial features, susceptibility to certain diseases and other defining characteristics. mtDNA, on the other hand, contains codes for making proteins and carrying out the other processes mitochondria undertake.

The genes you carry in the form of nuclear DNA are the result of a merger between your mother's and father's DNA -- this merger is called recombination. mtDNA, however, is derived almost exclusively from your mother. This is because the egg of a female human contains lots of mtDNA, while male sperm contains just a bit of mitochondria. One of the functions of a single mitochondrion is generating power for the cell that contains it, and sperm use a few mitochondria in the tail to power their race towards the egg for fertilization. These mitochondria are destroyed after the sperm fertilizes the egg, and thus any mtDNA that could be passed on from the father's side is lost.

This means that mtDNA is matrilineal -- only the mother's side survives from generation to generation. A mother who gives birth only to sons will see her mtDNA lineage lost. Examination of mtDNA so far has yielded only rare and unusual cases where paternal mtDNA survives and is passed onto the child.

Mitochondria are also valuable to evolutionists because copies of the exact same mtDNA you have can be found in cells throughout your body. Within each cell, too, there may be thousands of copies of mtDNA. Conversely, the nuclear DNA in a cell usually contains just two copies. It's also easier to extract mtDNA than nuclear DNA, since it's found outside the fragile and more rapidly decaying nucleus of the cell.

What all this adds up to is that your mtDNA is the same as your mother's, since there is no recombination to form a third version, distinct from both your mother's and father's but a combination of both. This makes mtDNA much easier to track from an anthropological standpoint.

Humans have been around for a long time. In the hundreds of thousands of years we've been walking the planet, our numbers have grown. How is it that only about 200,000 years ago a single woman became the great-grandmother of us all? Shouldn't human history go further back than that?

Read the next page to find out about how humanity may have come close to extinction, setting the stage for Mitochondrial Eve to leave her enduring legacy.

Cann and her fellow researchers estimated that Mitochondrial Eve lived about 200,000 years ago. With their margin of error included, she would have been alive between 500,000 and 50,000 years ago. Given that Eve is thought to have lived duri­ng a time when there were other women alive, how is it that all of us alive today descended from her alone? There are a couple explanations for how only Eve’s mtDNA alone could have survived, and most likely a combination of converging factors is responsible.

The likeliest possibility is that an evolutionary bottleneck occurred among humankind while Eve was alive. This is a situation where a large majority of the members of species suddenly die out, bringing the species to the verge of extinction. This sudden decrease in numbers isn’t due to any kind of failure to adapt. Instead, it's more likely the result of a catastrophe of some sort, for example, the result of a comet hitting the Earth. Afterward, just a few members remain to repopulate the group and continue to evolve. Bottlenecks are suspected to have taken place at different times in humanity’s history, so it’s not a farfetched notion that an event like this could have taken place during Eve’s lifetime.

A report issued in 1998 concluded that about 70,000 years ago, humanity was reduced to only about 15,000 people on the whole planet [source: Whitehouse]. With very few people spread out across the planet, humankind was indeed on the verge of extinction. The event that caused the near-loss of our species was an eruption of Mount Toba in Sumatra. This volcanic eruption was so immense that it lowered global temperatures, killed off the animals and plants that nourished humans and spurred the coldest ice age the planet has seen, lasting 1,000 years.

­The Mitochondrial Eve theory evokes similar scenarios. If the human population was reduced dramatically, and there weren’t many women around to have kids, the stage is set for one “Lucky Mother,” as Cann puts it, to emerge as a most recent common ancestor. It’s possible that after a few generations, the mtDNA of the other women died out. If a woman produces only male offspring, her mtDNA won't be passed along, since children don’t receive mtDNA from their father. This means that while the woman’s sons will have her mtDNA, her grandchildren won’t, and her line will be lost.

It’s possible that this was the cause of Eve emerging as the sole “Lucky Mother” who in essence gave birth to us all.

Having a bit of trouble understanding? Not to worry. Read the next page for an illustration of what mtDNA is theoretically capable of on the next page. It’ll clear things up a bit.

Although talk of genetic mutations and DNA sequences makes it seem complex, at its core, tracking mtDNA is based on a deceptively simple notion: People whose ancestors were once closely related should have almost identical mtDNA. mtDNA can undergo mutations over time, but it takes time for these mutations to occur. Logically, the fewer there are, the less time has gone by since two families' ancestors diverged. Those people who have just a few differences in their mtDNA sequences would be more recently related than those sequences which bear many differences.

Think about it this way. Say your great-great-grandmother on your mom's side -- whom we'll call Mildred -- had a sister, whom we'll call Tillie. Both shared identical mtDNA which they received from their mother. But imagine that Tillie and Mildred had a terrible argument, and Tillie moved across the country, while Mildred's descendants -- including you -- stayed put.

Tillie and Millie never spoke again. Both women gave birth to girls, and so their matrilineal mtDNA was passed on. But as the generations continued, the families of the two grew less and less aware of the existence of the other branch, until neither line was aware of the other. But the two lines are about to be inadvertently reunited. Researchers placed a national advertisement asking for test subjects for a study of recent human population trends using mtDNA for mapping. By coincidence, you and a distant cousin of yours on Tillie's side of the family both decide to volunteer.

After they collect a DNA sample from you, the researchers compare your mtDNA to the sequences from the other candidates. Lo and behold -- they find that two volunteers are cousins. Comparing your mtDNA to your cousin's, the geneticists should be able to tell about how long ago Tillie and Mildred had their argument. If they checked the local populations of your area and your cousin's area, they should also be able to tell whether it was Tillie or Millie who migrated, by finding which population shared more of the mtDNA present in your family line -- more people with the same mtDNA means that that sequence has been around longer. What's more, they can also conclude that since you and your cousin share similar mtDNA, you have a most common recent ancestor, the woman who is mother to Tillie and Mildred.

Since it takes a while for mtDNA mutations to occur, it would be pretty difficult for these imagined geneticists to pin down you and your cousin with accuracy, but when this technique is extrapolated over a period spanning tens or hundreds of thousands of years, it becomes much more viable.

Not everyone buys the Mitochondrial Eve theory, however. Read the next page to learn about criticism of the study.

Evolutionary mapping through the use of mtDNA is inexact. As mtDNA study continued after the late 1970s, scientists discovered a property known as heteroplasmy -- the presence of more than one sequence of mtDNA found in the same­ person. Even within a single person, there are differences between mtDNA that make comparing one person or group to another tricky.

The 1987 study that introduced the concept of Mitochondrial Eve to the world came under attack when it was pointed out that the "African" ]population the researchers sampled was actually made up almost entirely of African-Americans. Is it possible that in the few hundred years since Africans had been imported to the Americas against their will that African-Americans' mtDNA had mutated enough so as to render the sample useless? In the face of the criticism, Cann and her colleagues took an additional sample of Africans living in Africa, but found virtually the same results.

­Another problem with mtDNA study is the differences in the rate of mutation. Think about it this way, if you looked at how long it took for a particular sequence of mtDNA to develop a change -- a mutation -- and concluded that it took 1,000 years, then two strains of mtDNA from the same lineage with two mutations would have diverged about 2,000 years ago, right? This is how Cann and company decided Mitochondrial Eve was living around 200,000 years ago.

The researchers said that in their study they assumed that mtDNA mutates at a consistent rate. The problem is, science isn't exactly sure what the rate of mutation for mtDNA is, if there even is a measurable rate. If you look at the rate of mutation among a whole group of organisms, say, all people alive today -- called the phylogenetic rate -- you might conclude that mtDNA mutates at a consistent rate. But if you look at a single family line within that larger group -- the pedigree rate -- you'll most likely find an entirely different rate of mutation.

Since the "mutational clock" used by Cann and her co-authors was called into question, they expanded the date for Eve's existence to between 500,000 and 50,000 years ago.

Decades after the Mitochondrial Eve study was published, the results are still hotly debated. Are we all descended from a most recent common ancestor who lived 200,000 years ago? Can mtDNA even tell us precisely? These questions remain unanswered and frame the future work of evolutionary geneticists. But the 1987 study was groundbreaking enough that it changed the way we think about ourselves as humans. It pointed out that somewhere down the line of history, we are all related.

For more information on evolution and related topics, see the next page.