At the time of his death, Allan Wilson's hypothesis that all living humans had a common maternal ancestor who lived in Africa as recently as 200 thousand years ago was gaining wide acceptance. This appeared to be a second major scientific victory for him. In 1967, he and Vincent Sarich (6) shook the human family tree when they claimed, based on immunological comparisons of serum albumins, that humans, chimpanzees, and gorillas had a common ancestor five million years ago. At the time paleoanthropologists, who had settled on a common ancestor about fifteen million years ago or more, resented and challenged the invasion of their domain by outsiders. Nevertheless, most anthropologists now accept times of five to eight million years ago. Similarly, Wilson's "Eve Hypothesis" (20, 24) was challenged by some paleoanthropologists who would have accepted a common human ancestor closer to one million years ago (28, 29). Other paleoanthropologists agreed with Wilson's conclusions (30).

During the past few decades, powerful computers have been developed, and computer programs for constructing trees based on homologous DNA and protein sequences have become rather sophisticated (31-33). As a consequence most people who use them do not fully appreciate their limitations. Some researchers (34) thought that Wilson's group (20) had over-interpreted their data. Although Wilson's next major paper in the area (24) was published posthumously, reverence for him did not inhibit a full-scale challenge of his methodology (35-40) that elicited a partial retraction by a coauthor (41). While the original data are good, his reanalysis showed that the "Out of Africa" model was somewhat weakened, but not rejected, and the "dating of Eve" not strongly challenged (42, 43).

Assignment:

There are a number of issues that emerge from Wilson's work and the challenges to it. These issues are often difficult to understand. In your group, discuss all of the topics listed below. Select one topic that interests you research it and attempt to explain it to someone not in this class. Having identified the areas of difficulty, write a thoughtful, clear and concise, at least 3-page explanation or discussion with references and illustrations as appropriate to hand in Wednesday, March 20. Please note that in the decade since this case study was last revised, researchers have sequenced the entire genomes of many humans, a Neandertal (52), and a Denisovan (53).

a. Wilson's conclusions relate only to the most recent matrilineal ancestor of all humans. There were many other women living at that time who are also our ancestors. Explain. Is it possible that the most recent patrilineal ancestor lived more recently (44)? What is known about the evolution of the Y chromosome that is inherited paternally (27, 47)?

b. There are a variety of ways to construct phylogenetic trees from sequence data. Because there must be a single phylogeny, why do the various methods sometimes give different answers, none of which may be the true phylogeny? Would the phylogeny be affected if nuclear DNA sequences were used (45)?

c. The estimation of the time when the most recent maternal ancestor lived is based on the molecular clock hypothesis. Describe this concept. What assumptions are made when it is used? How was it calibrated for mitochondrial DNA in Wilson's work?

d. What can the fossil record tell us about our most recent common ancestor? Is it possible for both the paleoanthropologists and the molecular evolutionists to be correct?

e. To many people, the results of Wilson's work have troublesome interpretations. How is it possible that our ancestors swept across the world displacing other human-like relatives, such as the Neandertals, without leaving any survivors or any evidence of interbreeding? Are we descended from an especially blood-thirsty tribe that murdered and plundered their way to world dominance? In 1997, a group of scientists reported the nucleotide sequence of a region of mitochondrial DNA extracted from a 30,000+ year-old Neandertal bone from the type specimen (46). DNA from another Neanderthal was analyzed (51). How would you interpret this sequence in the context of the above speculations? Likewise how would you interpret this in light of the fact that the genetic diversity among chimpanzees is almost four times greater than in humans (48)?

f. Mitochondrial DNA provides insights into human migrations such as the peopling of the Pacific Islands and the New World (49). How do these studies compare to the archaeological and linguistic analyses of human migration? Would nuclear DNA be a better choice for studying migrations (50)?

References cont.

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