Genetics & Genealogy

Mitochondrial DNA and Human Evolution

Extracted from "The search for Eve", by Michael H Brown: Nature, January 1, 1987

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"Mitochondrial DNAs from 147 people, drawn from five geographic populations, have been analyzed by restriction mapping. All these mitochondrial DNAs stem from one woman who is postulated to have lived about 200,000 years ago…"

This article explains that the authors - Rebecca L Cann, Mark Stoneking and Allan C Wilson - in analyzing DNA from the energy producing compartments of the human cell known as the mitochondria, have discovered that this genetic material, inherited through the ages, provides

"new perspectives on how, where, and when the human gene pool arose and grew."

Sufficient hereditary material was extracted and purified from samples of placenta to compare the intricate variations between and among peoples of Asian, European, African, Australian and New Guinean descent. The mitochondrial DNA - "mtDNA" - offered a wholly unique attribute that could not be found elsewhere: it was inherited only maternally, from the mother.

Except for occasional minor mutations, this type of DNA was passed intact from great - grandmother to grandmother to daughter with virtually no input from males and thus no mixing - no blending of father's and mother's genes - that would jumble, complicate, and thereby obscure its history.

It survived the generations over millennia without being adulterated by recombination. For that reason mitochondrial DNA - as opposed to nuclear DNA where such blending does occur - is a powerful new tool

"…for relating individuals to one another."

Mitochondrial DNA is a convenient tool because it is much smaller and simpler than the DNA in a cell's nucleus. While the nuclear DNA, comprising some 100,000 genes, has not yet been mapped to reveal the entire message hidden in its chemical codes (currently the subject of a US$3 billion federal grant aided research effort expected to take 15 years to complete), the DNA in a cell's mitochondria has already been fully mapped and found to contain only 37 genes, all of them inherited solely from our mothers (instead of the complicating mix from both parents that occurs in the nucleus)

"… so you are connected by an unbroken chain of mothers - whether you are a son or a daughter - back into the past, and what we are interested in doing is building up a genealogical tree that connects those maternal chains."

per Allan C Wilson

DNA - deoxyribonucleic acid - contains codes which direct the cell to form the proteins that serve as the foundation for our bodies. Certain portions and sequences of the DNA are called genes which, besides maintaining our bodies' daily activities also determine our heredity: height, skin pigmentation, and eye color down the generations, the transmissions of traits that keep the race or species going for thousands or millions of years. DNA is in fact the blueprint for life itself.

Where DNA in the nucleus determines all our inherited traits, the role of DNA in the mitochondria (bean-shaped structures which are the power plants of the cell) is much more limited but nonetheless vital: it codes for key components of our energy production system, i.e., it is a piece of coded information in the mitochondria the purpose of which is to extract energy from food molecules suspended in the cytoplasm outside the nucleus.

While it is the nuclear DNA that determines what the next generation will look and act like, the mtDNA, densely packed with its own group of genes, has two principal characteristics that make it more valuable in studying evolution:

(a) it is simpler and maternally inherited, allowing a geneticist to look straight into the female past, and

(b) it provides a magnified view of genetic diversity because mutations - changes in the tiny biochemical configurations that occur as a matter of course - accumulate much faster in mtDNA than in the nucleus, giving each individual a "signature" that can be compared with others.

Most evolutionary change, it is believed, starts with mutations in genetic molecules, but many mutations have little or no effect on the functioning of our organisms and are "neutral", seem to occur randomly, and accumulate over time making very slight changes to the genetic codes. By focussing on the patterns geneticists, after relating one person's DNA to another's, can determine how close people are and can then represent them as twigs on a genealogical tree.

"If you are good enough a molecular biologist you can reconstruct that branching diagram leading back to that one mother by taking a count of the number of mutational differences among the twigs. We build genealogical trees by comparing mtDNA from the terminal twigs connecting through unbroken chains of mothers back to the mother of us all."

per Allan C Wilson

The geneticists can date backward for the mutational differences (or "divergences") between people that occurred at a set rate of 2 to 4 percent every million years. Small but regular changes in the mtDNA function through the ages as a molecular clock. The key findings were

"that these mutations, the neutral ones, accumulate at much the same rate in all organisms ranging from bacteria through plants to animals, that the same basic rate of ticking, the same rate per year, is observed in all these creatures, so in this vast array of organisms, we have a timepiece ticking away in a statistical fashion (not like a metronome), and allowing us the possibility of being able to put a time scale onto all of evolution, regardless of whether the species have a fossil record."

per Allan C Wilson

So powerful is the new technique that it is

"entirely possible that we can identify Cleopatra's mitochondrial genes in modern people, and, at least theoretically, also specify her entire mitochondrial genotype."

per Rebecca L Cann

Whatever population has accumulated the most mutations - is the most divergent among each other - can be assumed to be the oldest. The longer a population has existed, the more mutations. The geneticists claim that every existing human can be traced back to a very special colony of humans from Africa. The oldest maternal lineage leading to modern day humans had been discovered - everyone has recent African roots.

The age of "Eve" (their name for the one African woman whose mtDNA had been traced down the ages) has been set at between 140,000 and 200,000 years (her median age rounded to 200,000). She was almost certainly a highly evolved descendant of an African erectus, as opposed to the more famous Asian variety of erectus like Peking Man. Her offspring, in turn, helped found a population that was growing increasingly sapient and migratory, shedding old erectus traits and spreading out of Africa perhaps 90,000 to 180,000 years ago to take over the world - although it is possible that the migration may have occurred as recently as 21,000 B.C.

There is little evidence that this advanced African population interbred with existing and more primitive populations in Asia and Europe. Instead, they simply outbred and conquered them. They must have possessed vast superiority and aggression

"Thus we propose that Homo erectus in Asia was replaced without much mixing with the invading Homo sapiens from Africa."

per Cann, Stoneking and Wilson

While one branch of the tree leads back solely to Africans, the second branch includes Africans, Asians, Australians, New Guineans and Europeans. This is a fairly strong suggestion that Africa is the origin.

"This inference comes from the observation that one of the two primary branches leads exclusively to African mtDNAs while the second primary branch also leads to African mtDNA."

per Cann, Stoneking and Wilson

Moreover, the Africans display the most diverse DNA types - again, the longer time a lineage exists the more mutations it collects and thus the more diversity within such a population. This is a fairly compelling indication that Africans are the originals or oldest with a divergence of 0.47 percent. Asia is second with 0.35, followed by Australian aborigines, New Guineans and finally Caucasians whose divergence is only 0.23 percent. The aborigines of Australia appear to be genetically closer to Europeans than Africans.

"If there was hybridization between the resident archaic forms in Asia and anatomically modern forms emerging from Africa, we should expect to find extremely divergent types of mtDNA in present day Asians, more divergent than any mtDNA found in Africa. There is no evidence for these types of mtDNA among the Asians studied. Africa is a likely source of the human mitochondrial gene pool."

per Cann, Stoneking and Wilson