When population geneticist Rebecca L. Cann and her colleagues postulated the existence of an African woman who was the source of all the mitochondrial DNA present in the modern-day human population, their assertions raised controversy and stimulated further study of humankind’s origins.
Prior to 1987, molecular biologists had used new technology involving mitochondrial deoxyribonucleic acid (DNA) to devise a molecular clock that allowed them to calculate when apes and humans separated from a common ancestor. The DNA found in the mitochondria of cells is inherited only through the mother, whereas the DNA found in the nucleus of cells is inherited from both parents. DNA from both parents involves recombination and reveals slower mutation rates than its mitochondrial counterpart, making it less suitable for the establishment of a molecular clock.
Molecular biologist Allan C. Wilson devised a primate clock that showed that the date for ape-human divergence was only five to seven million years ago, as opposed to the twenty-five million years paleontologists had estimated, thus demonstrating the utility of the new mitochondrial DNA (mtDNA) technology. When population geneticists Rebecca L. Cann and Mark Stoneking, along with Wilson, presented the results of their research findings using mitochondrial DNA in 1987, they effectively challenged prevailing ideas about the evolution of modern humans (Homo sapiens sapiens)—how long that evolution took and where it first occurred. Not only did they posit the existence of a single woman (Mitochondrial Eve) from whom the mtDNA of all current human beings had come, but they also located her and the origin of modern humans in Africa. This gave new life to competing theories about the origins of modern humans.
In an article titled “Mitochondrial DNA and Human Evolution,” which appeared in the January 1, 1987, issue of the journal Nature, Cann, Stoneking, and Wilson presented the results of an analysis of the mtDNA of 147 individuals representing five different populations. Based on their analysis, the researchers offered answers to some basic questions about the source of the human gene pool.
Cann and her colleagues studied DNA taken from placentas and two cell lines. The sample included African Americans and sub-Saharan Africans; Asians from six different populations; people from Europe, North Africa, and the Middle East; and aborigines from Australia and New Guinea. The researchers mapped these mtDNA samples and developed evolutionary family trees. Because each of these genetic family trees showed a branch with only Africans and another with all other populations plus Africans, Cann and her colleagues concluded that Africa was the source of the mtDNA genes. They also found slightly greater variability in mtDNA among the African population, which implied that the African populations had been around longer, making Africa the likely origin of modern humans.
The researchers were able to establish a time line that demonstrated that the first group of modern humans may have left Africa between 90,000 and 180,000 years ago, although they considered these findings tentative. As a result of their analysis, they estimated that all of the mtDNA in their study could be traced back to a single African woman who lived approximately 200,000 years ago. She was dubbed African Eve, or Mitochondrial Eve.
Although Cann and her colleagues were tentative in relating their findings to the fossil record, their views did tend to accord with the paleoanthropological approach based on fossil evidence that had placed the oldest anatomically modern humans in Africa—the “out of Africa” or single-origin hypothesis. This contrasted with the alternative view that modern humans had evolved from an earlier human ancestor (Homo erectus)
After their article was published, Cann, Stoneking, and Wilson faced a barrage of criticism for their claims. Some complained that the number of individuals involved in their study was too small and that the number of Africans was too small. Others questioned the methodology involved in creating the family trees and the researchers’ statistical analysis, arguing that the conclusion about the African origin was not definitively proven by their approach. Still others questioned the use of the particular part of the mitochondrial DNA they had chosen to use for their study. In an effort to test the data provided by Cann and her colleagues, other researchers did additional experiments with mtDNA. New studies based on nuclear DNA and DNA from the Y chromosome were also conducted. Over a period of years, however, additional studies tended to uphold the more recent time frame for the origin of modern humans, as well as the likelihood of their African origin.
In an article titled “Mitochondrial Genome Variation and the Origins of Modern Humans,” which appeared in the December 7, 2000, issue of Nature, Max Ingman,
Ingman and his colleagues devised an improved “clock” that substantiated the original time frame proposed by Cann, Stoneking, and Wilson for the emergence of modern humans, coming up with an estimate of 175,000 years ago (plus or minus 50,000 years). By comparing mtDNA with DNA from the X chromosome, they were able to verify in two different genetic sequences both the time of emergence of modern humans and the genetic variability among Africans.
Through their research, Cann, Stoneking, and Wilson reaffirmed the usefulness of mitochondrial DNA as a new and more accurate DNA technology based on discoveries and developments in molecular biology. Applied to the field of human evolution, it solidified the importance of the relatively new field of genetic anthropology. The findings of Cann and her colleagues and subsequent studies forced paleontologists to reconcile the fossil evidence with the powerful new genetic evidence by recalibrating the time line of human evolution based on the more recent dates for the origin of modern humans that the mtDNA research indicated. In addition, this research led to even more certain proof that modern humans originated in Africa and immigrated to other parts of the world more recently than had been suspected, ultimately reinforcing the knowledge that Africa is the birthplace of all humanity. It also showed that outward differences among humans are superficial, given the short period of time in which they developed.
The reaction to Cann, Stoneking, and Wilson’s study demonstrated how science works: Questions or findings stimulate debate and give rise to alternative explanations, and scientists test the possible explanations, finally reaching resolution. By postulating the existence of an African Eve, Cann and her colleagues also raised awareness of the role of women in human evolution and captured the imaginations of many people outside the field of anthropology, encouraging them to investigate and to take more interest in science.
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Barinaga, Marcia. “’African Eve’ Backers Beat a Retreat.” Science 255 (February 7, 1992): 686-687. Presents a summary of three studies criticizing Cann, Stoneking, and Wilson’s methodology in establishing family trees based on their mitochondrial DNA sample. Makes the problem easy for nonscientists to understand. -
Cann, Rebecca L., Mark Stoneking, and Allan C. Wilson. “Mitochondrial DNA and Human Evolution.” Nature 325 (January 1, 1987): 31-36. The article that first presented the idea of an African Eve. -
Ingman, Max, Henrik Kaessmann, Svante Paabo, and Ulf Gyllensten. “Mitochondrial Genome Variation and the Origins of Modern Humans.” Nature 408 (December 7, 2000): 708-713. Reports on the authors’ substantiation of Cann, Stoneking, and Wilson’s findings through research that used different and comparative mtDNA methodology. Outlines the problems with previous studies, discusses the power of mtDNA methodology, and clearly explains how the authors designed their study to overcome some of the previous shortcomings. -
Lewin, Roger. “Africa: Cradle of Modern Humans.” Science 237 (September 11, 1987): 1292-1295. Explains why the contribution of molecular biology to the problem of the origins of modern humans is important. Gives background of the work done in the study of hominid origins using mitochondrial DNA. Also explains and illustrates the meaning of the fossil evidence in relation to the molecular biological approach. -
_______. “The Unmasking of Mitochondrial Eve.” Science 238 (October 2, 1987): 24-26. Reviews the criticisms of Cann, Stoneking, and Wilson’s study, especially the misunderstandings of the meanings of the Mitochondrial Eve postulated by Cann and her colleagues. -
Stringer, Christopher, and Robin McKie. African Exodus: The Origins of Modern Humanity. New York: Henry Holt, 1996. Provides a comprehensive look at the entire issue of the origins of modern humans. Chapter 5 presents a detailed discussion of the controversy caused by the publication of Cann, Stoneking, and Wilson’s article in 1987. Written for the nonscientist.
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