All About Eve: Biologists Offer a Variation on the Theory of Evolution
About 200,000 years ago there lived one woman who was a maternal ancestor of every human being living today, a team of biologists has concluded after analyzing special genes in the cells of people from all the world’s major racial and ethnic groups.
The scientists have taken to calling the woman Eve because she is thought to be the one maternal ancestor common to all the family trees of every member of the human race.
The name may be misleading, however, because she is not the sole maternal ancestor. The claim is not that Eve was the only woman having children 200,000 years ago. She had many contemporaries who were each among the ancestors of many living today.
The claim is simpler: If each person could trace a family tree far enough back, everyone’s ancestors doubling in number with each generation into the past, the tree would reach a time in which there were several thousand ancestors sharing one generation. Among each person’s many ancestors living 200,000 years ago, according to the claim, the same woman would appear on all the charts.
Controversy Likely
The claim is likely to be controversial. But the scientists behind it, from UC Berkeley, have considerable stature in the study of evolutionary relationships as they can be interpreted from genetic studies.
Their report, in the prestigious British journal Nature, is accompanied by an independent commentary that takes the claim seriously and calls it “the strongest molecular evidence so far in favor of the African population being ancestral (to all living humans).”
The claim does not contradict the general understanding of human evolution. It accepts the fossil evidence that the human lineage diverged from that of the apes a few million years ago into a species called Australopithecus, which evolved into Homo habilis, followed by Homo erectus and, about 400,000 years ago, by early forms of Homo sapiens.
The claim also generally agrees with the view widely shared among anthropologists that anatomically modern forms of Homo sapiens, the species to which all living people belong, arose more than 100,000 years ago and probably in Africa.
One Population, One Place
The findings suggest that while there were primitive forms of Homo sapiens living throughout Africa, Europe and Asia, fully modern humans arose in only one small population in one place and their descendants eventually spread throughout the Old World, replacing the earlier population.
The genetic evidence cited by the Berkeley group also implies that all of today’s racial differences evolved after descendants of Eve had grown quite numerous and migrated out of Africa into Eurasia. The differences arose after various populations of the descendants had become geographically separated and could no longer interbreed to any significant degree with other populations.
The claim is being advanced by Allan C. Wilson, Mark Stoneking and Rebecca L. Cann.
Wilson is a pioneer in the use of genetic differences among living organisms to study evolutionary relationships. He was a developer of the “molecular clock” method of determining when various related species diverged in the course of evolution.
Like the new claim, the molecular clock depends on counting the number of mutational differences between comparable genes in different species. For example, a human gene for a protein such as hemoglobin differs from a chimpanzee gene for hemoglobin by a certain number of mutations.
Such differences could begin to accumulate only after the ancestral lineage common to both had split. Assuming that the mutations, being random, arise at a relatively steady rate, one can estimate that it takes a specific amount of time to produce a specific amount of genetic difference.
Lineages Diverge
In this example, Wilson and another colleague, Vincent Sarich, have long argued, the most recent common ancestor of humans and apes lived 4 million to 6 million years ago, at which point the ape and human lineages diverged.
In making the interpretation about Eve, Wilson and his colleagues have examined a special set of genes possessed by all human cells but not carried in the nucleus, where the vast majority of every cell’s genes reside. The special genes are carried in structures within cells, called mitochondria.
Mitochondria function as a cell’s powerhouse, converting the chemical energy from food into a form that the cell can use. They are the only structures in cells that carry their own genes and which reproduce in the cell by splitting exactly as if they were bacteria inhabiting the cell. (Many biologists believe mitochondria originated as bacteria that became permanent symbiotic partners of cells.)
At conception, when a sperm, which lacks mitochondria, fertilizes an egg, which has many thousands, the resulting embryo inherits only its mother’s mitochondrial genes. At each subsequent cell division, the new cells acquire their mitochondria simply by apportioning the mother’s mitochondria between them.
“It’s this special kind of inheritance--we all get our mitochondria only from our mothers--that makes it possible to come to this kind of a conclusion,” Stoneking said. “And, if you believe in evolution, it’s not remarkable to say that we are all descended from a common ancestor. What we’ve done is find a way to estimate when the most recent common maternal ancestor lived.”
Most Probable Number
The method involves comparing mitochondrial genes from a wide variety of humans to count the differences. From other evidence, Wilson, Stoneking and Cann have concluded that in every million years there is about a 2% to 4% change in the mitochondrial genes.
Working backward from the amount of difference among all peoples today as sampled in 147 people from all over the world, the group concluded that it would have taken 140,000 to 290,000 years for today’s amount of difference to accumulate. The most probable number in the range is about 200,000 years.
The conclusion that Eve lived in Africa emerged from comparing the amount of difference between every possible pairing of individuals among the 147 tested. The researchers then drew a “family tree” linking all the individuals according to how closely their genes were related.
The diagram turned out to have two major branches, one including only certain Africans and the other including other Africans and all other groups.
The common ancestor of the two branches, the scientists concluded, must have been African.
The researchers emphasize that their claim covers only the ancestry of mitochondrial genes. It is virtually certain that the nuclear genes, the ones that control most of human heredity, were contributed by many mothers and fathers.
Their claim, however, implies that the mitochondrial genes of other mothers died out long ago. This could happen, for example, if other mothers contemporaneous with Eve produced only sons, whose sperm have no mitochondrial genes, or if their daughters had no children.
The findings suggest that about 200,000 years ago, the founding population of modern humans was quite small but highly successful in an evolutionary sense.
