A Second, Closer Look at DNA – Part II

Continuing from the last post, one aspect of DNA that has been a huge stir among scientists, anthropologists and historians since 1987, when a “scientific discovery” seized the attention of the popular press, with the headline:

“Mitochondrial DNA and Human Evolution.” A paper by Rebecca Cann, Mark Stoneking, and Allan C. Wilson, which appeared in the January 1, 1987 issue of Nature, and in the January 28, 1988, issue of Newsweek magazine, under the heading of “Everyone’s Genealogical Mother: Biologists Speculate that Eve Lived in Sub-Saharan Africa,” behind a provocative front cover presenting a snake, tree, and a nude African couple in a “Garden of Eden” type setting, with Eve offering an apple to Adam.

In a 2001 book by Oxford geneticist Bryan Sykes, The Seven Daughters of Eve (W.W. Norton), the theory is presented of human mitochondrial genetics being traced back to seven “clan mothers” who lived about 45,000 years ago. He also has a list of 27 other “clan mothers” of other than Caucasian descent (Fufei, Ina, Aiyana/Ai, Yumi, Nene, Naomi, Una, Uta, Ulrike, Uma, Ulla, Ulaana, Lara, Lamia, Lalamika, Latasha, Malaxshmi, Emiko, Gaia, Chochmingwu/Chie, Djigonasee/Sachi, Makeda, Lingaire, Lubaya, Limber, Lila, Lungile, Latifa and Layla), and his seven daughters of Eve trace back to a single Mother Eve (Mitochondrial Eve) living in Africa between 140,000 and 290,000 years, which science has averaged out to a round 200,000 years ago. While presented as science, Sykes’ works is thought to be more fictional than fact.

The map of Sykes’ seven daughters of Eve and where they originated some 45,000 years ago. His mother Eve, is said to have originated in Africa 200,000 years ago

The theory is based on the idea that theoretically, if scientists could look farther and farther into the past, they would find that the number of women who contributed the modern varieties of mitochondrial DNA gets less and less until, finally, we arrive at one “original” mother.  She, then, would be the only woman out of all the women living in her day to have a daughter in every generation till the present.  Coming forward in time, we would see that the MtDNA varieties found within her female contemporaries were gradually eliminated as their daughters did not have children, had only sons, or had daughters who did not have daughters.  This does not mean, of course, that we would look like this supposed ancestral mother; rather, it means only that we would have gotten our mitochondrial DNA from her.

While the idea of a single Mitochondrial “Mother Eve” strikes at the imagination of people, and is applauded by the evolutionary community, and pretty much accepted as fact by many scientists—especially the idea of MtDNA traceable back through the mother (they claim men do not have MtDNA)—there is less proof of this than has been presented.

First of all, the theory is that mitochondria have their own DNA, and are tiny organelles that live in the cytoplasm of cells, the fluid-filled space between the cell nucleus and the outer membrane, with thousands of mitochondria in each cell—each having its own small circle of DNA, a reminder of their distant bacterial ancestry. What they claim makes mitochondrial DNA (MtmDNA) so special and so useful is its unique inheritance pattern.

The theory is that human eggs are full of mitochondria, while sperm have only a hundred or so, just enough to power it while it swims towards the egg. After fertilization, when the sperm penetrates the egg, these few male mitochondria are immediately destroyed. This means that, while we all receive our nuclear DNA, with the exception of the X and Y sex chromosomes, from both parents, we get all of our MtDNA from our mothers. She got it from her mother, who got it from hers – and so on back in time. To some, this makes MtDNA useful in connecting the maternal lines of living people in different parts of the world.

The other handy thing about MtDNA is that it changes about 20 times faster than nuclear DNA, because mitochondria lack an efficient proof-reading system to check for errors when DNA is copied. The high mutation rate means that there is plenty of variation in the sequence of MtDNA between people, and variation is the lifeblood of genetics.

Secondly, to find a distant, original ancestor, researchers compared the different varieties of MtDNA in the human family.  Since MtDNA occurs in fairly small quantities, and since the researchers wanted as large a sample as possible from each person, they decided to use human placentas as their source of the MtDNA.  So, Rebecca Cann and her colleagues selected 145 pregnant women and two cell lines representing the five major geographic regions: 20 Africans, 34 Asians, 46 Caucasians, 21 aboriginal Australians, and 26 aboriginal New Guineans, with all placentas from the first three groups coming from babies born in American hospitals—only two of the 20 Africans were born in Africa. In theory, then, mtDNA should be the same as our mother’s mtDNA with small changes (or mutations) in the genetic code, which can and do arise, which have no effect on the proper functioning of either the DNA or the mitochondria. These mutations are then preserved and carried on to succeeding generations.

To figure how far back the generational line went, the scientists developed what they called a “molecular clock” that was based on mutations in the MtDNA. The rate at which the clock ticked was determined from the accumulation of changes over a given period of time. That is, if the assumption was made that there was one mutation every 1,000 years, and if scientists found a difference of 10 mutations between us and our ancient hypothetical ancestor, they then could infer that that ancestor lived 10,000 years ago. So the researchers compared MtDNA from humans with that from chimpanzees, and then used paleontology and additional molecular data to determine the age of the supposed common ancestor.  This (and similar calculations on other species) revealed a mutation rate in the range of 2% to 4% per million years. 

Then they compared the groups in their study that were close geographically, and took the age of the common ancestor from estimated times of settlement as indicated by anthropology and archaeology. Again, 2% to 4% every million years seemed reasonable to them, and a time frame back to the original ancestor was adjudged to be between 140,000 and 290,000 years ago, with 40,000 years to the seven daughters, and 290,000 years to the original Mother Eve chosen as suitable round numbers. The results obtained from this analysis of mitochondrial DNA eventually led to what is known in evolutionary circles as the “Out of Africa” theory, which is the theory that the descendants of mitochondrial Eve were the only ones to colonize Africa and the rest of the world, supplanting all other hominid populations in the process. 

Many evolutionists claim that such an interpretation is in accord with archaeological, paleontological, and other genetic data, and while they have accepted the mitochondrial DNA tree, they differ widely in their views regarding both the source of the nuclear DNA and the “humanity” of Eve.  Some believe that Eve contributed all the nuclear DNA, in addition to the mitochondrial DNA.  Some believe she was an “archaic” Homo sapiens, while others believe she was fully human.  The exact interpretation is hotly debated because mitochondrial DNA is “something of a passenger in the genetic processes that lead to the formation of new species: it therefore neither contributes to the formation of a new species nor reveals anything about what actually happened.”

However, things change rapidly in science.  What is popular one day, is not the next.  Theories come, and theories go.  And so it is with mitochondrial Eve.  She once was in vogue as “the woman of the moment,” so to speak.  Now, she has become virtually the “crazy aunt in the attic” that no one wants to admit even exists.

(See the next post, “A Second, Closer Look at DNA – Part III, “ to see how the two critical assertions about MtDNA are now under critique and coming up short of fact)