Over 200,000 years ago, a woman gave birth to the ancestors of every member of the human race... Or did she? In 1987 the proposed discovery of our most recent common ancestor jolted the world of evolutionary biology. The debate continues...

Mapping the Issue: Applying a teaching and learning framework to promote reasoned student inquiry

Issues such as Eve provide an opportunity to help secondary science students understand how science works in a meaningful context. Eve is socio-scientific. At her core lies new science—not yet completely accepted—dealing with heredity and biological evolution and blending imperceptibly with social perspectives such as population growth. She demonstrates the impact of technology on our search for solutions, but more so the nature of science, as she begs solutions that require application of scientific criteria to critically assess claims, make decisions, and construct solid lines of argumentation related to the issue. The MAP Teaching/Learning Framework (Figure 1) provides a structure for this type of exploration in accordance with National Science Education Standards (NRC, 1996), and Eve serves to demonstrate how the framework, a sequential approach consisting of three phases—motivate, assess, and propose—can apply to that issue and many others like it.

The Motivate Phase: Why should students know about Eve?

The motivate phase of the MAP Framework establishes the relevance of the issue, instills in students a "need-to-know," and creates a reasonable structure to guide their upcoming investigations. With any socio-scientific issue, we (and students) engage and seek to learn when presented with compelling need.

In 1987 Allan Wilson, Rebecca Cann, and Mark Stoneking changed our views on human origins and the evolution of populations. Sans fossils, they analyzed loops of genetic code called mitochondria (mtDNA) from 147 people on various continents, and traced the evolution of the human species back 200,000 years to what they surmised was the most recent common matrilineal (as mtDNA is acquired maternally) ancestor, an African woman—Mitochondrial Eve. The find seemed to remove a stumbling block in our attempts to make sense of our ancestral lineage in the larger scheme of human evolution.

For evolutionary biologists, there is clear enough need to explore the value and accuracy of the mitochondrial "clock" for tracking lineage through millennia and charting early migrations and population phenomena. With our students, however, we must be more explicit. For instance, few students (or adults) realize that mtDNA studies are now helping us to examine evolutionary histories of disease-carrying parasites, effects of population sizes on susceptibility and resistance, how to coordinate conservation efforts, and even factors that may have contributed to visual distinctions among races. Still fewer realize that decisions we make in these undertakings are based on assumptions that rely on the accuracy of theories—in this case the single-point origin or out-of-Africa theory. When the theory is inaccurate our assumptions are wrong and solutions based on the assumptions falter. To help students acquire contextual background and see a need to learn about the underlying science, provide an overview and one or more prompts that elicit discussion. Modeling the theory (Figure 2) and diagramming the out-of-Africa migration routes with divergence times and locations works well to launch students' investigation of Eve. It is especially effective when capped by discussion of distinctions between it and the multi-regional origin hypothesis—the notion that our species slowly developed in many regions and that present population differences resulted from breeding and inheritance from hominid species occupying those regions.

As students develop a need-to-know that sustains their drive to investigate, they also generate questions that serve to structure their investigations. The process requires care for follow-up investigations to be successful, as students must construct the questions, and they must be designed so that they establish reasonable goals for investigations in the next phase. We can facilitate that by helping students to structure the wording of their questions. The samples in Figure 3 are effective examples.

The Assess Phase: Of Motherhood, Mitochondria, and Migration

In the assess phase of the MAP Framework students explore claims and counterclaims that inform their questions while teachers guide their effort to determine the adherence of claims to accepted scientific norms such as precision, consistency, and openness—constitutive values in science.

Eve's scenario involves opposing claims that students can address with as much structure as teachers wish. A good strategy is to provide a starting point with original findings and a small set of claims and counters that highlight uncertainties. Note that socio-scientific issues are complex, and Eve and her relationship to human migration and diversity is no exception. While there is inherent complexity in discerning levels of scientific support for claims and how variables balance when proposing lines of argumentation, there is not a need to fully assess every nuance. Overwhelming your students will only compromise the true goal.

The Search for Eve
Everyone has inherited mtDNA, but code from a single person mutates over generations. These mutations occur in different locations so that eventually, by looking at differences in mtDNA, scientists can reconstruct the points at which ancestral lines branched. Enter dating. If we know how much a person's mtDNA has changed from that of an ancestor, and if we assume mutations occurred at a steady rate (as did Wilson, Cann, and Stoneking), then we only need to know the rate to determine when the ancestor lived. For example, if three mutations separate a person's mtDNA from an ancestor, and mutations occurred every 1000 years, we can infer that the ancestor lived 3000 years ago. That inference requires known quantities, so Wilson and company used Australian aborigines whose island arrival was calculated at 30,000 years and whose divergence they could measure in order to obtain a reliable rate. Given a rate, they determined that Eve lived around 200,000 years ago, and was part of a small population that would eventually cover every region in the world, replacing the hominid species inhabiting these regions. No consorting with the natives. Therein lies the rub.

No More Mrs. Nice Girl
Was there room for debate? Most certainly, and paleoanthropologists—then and now—let it be known. It is understandable. Imagine having spent your entire career collecting, cataloguing, painstakingly studying and correlating fossil remains to study the worldwide development of hominid species, arriving at estimates of human origins ranging from one to 15 million years—and then along comes Eve! Early on, Milford Wolpoff, an anthropologist at the University of Michigan, even referred to Allan Wilson as "public enemy number one" (Shreeve, 1990). Those are strong words, when the debate is supposedly only about science. Most disheartening has been the lingering claim that these migrating peoples—and therefore Eve by association—were just not pleasant folks. To have replaced indigenous species so rapidly as they spread, they would have to have been violent, perhaps even genocidal. Geneticists dispute the assumption that replacement means that these recent migrants are, as the nickname that has stuck, "Killer Africans."

A Mitochondrial Clock-Stopper?
So Eve's young? Many anthropologists counter that the concept of "natural selection," which would eliminate some mutations while favoring others, stops the mtDNA clock. Disadvantageous mutations are lost due to environmental pressures, reducing the genetic variation we see among humans. Random loss—the cessation of the clock every time a male is born instead of a female—would have the same effect. Therefore, adding two and two, or rather thousands, makes poor Eve much older, as if being genocidal isn't bad enough. Meanwhile, geneticists explain that the mutations used for dating only occur at places "neutral" for selection—none of the sequences where they are found code for proteins and so do not affect how individuals being studied adapt to their environment. But over time, times do vary, almost every time... estimates now vary from narrow bands (120,000 to 150,000 years) to wide (50,000 to 500,000 years). Despite continuing support, the variance in the clock calculations nag the model's proponents, recently with an added spin that threatens validity as well as further impugning the mechanism's accuracy. What if dads counted? What if mtDNA was not only inherited maternally? See Counterclaim Exhibits A and B for two brief classroom-ready claims that students can discuss and follow for further leads.

Expanding the Search
To further assess opposing claims, students need sources and assistance. To meet varying needs, the author prepares sources with varying degrees of structure. These include teacher summaries (e.g., Exhibits A and B above), news reports, full study texts, and lists of sources students can track online or through Interlibrary Loan. Exhibits A and B are high structure, something not feasible except at the early stages. News articles, especially from university archives, are a great next step. For instance, to help students assess comparisons of early hominids and humans from Eve's time, try introducing Krings' (1997) findings of genetic differences between one Neanderthal and humans that "laid to rest" the multi-regional hypothesis, then follow with news briefs (Figure 5) showing that the battle goes on. Though not original, news reports are readable and helpful in deciding which studies warrant a full appraisal. They also provide background on claimants themselves—bias that may or may not be prompting claims (recall Wolpoff), why claimants may seek to only reveal certain data, and so forth.

The Propose Phase: Mapping Arguments and Further Investigation

The propose phase of MAP provides students a chance to construct arguments based on assessment of claims, engage in critical peer review and debate in writing, oral presentation, and role play scenarios (e.g., hearing enactments, town meetings, etc.).

With Eve, students first construct supports for the out-of-Africa or multi-regional hypothesis, or a compromise position. Then in pairs they present their main points to their partner and refine arguments based on feedback. Each student or pair then constructs a line of argumentation in written form. Oral presentations provide a nice culmination. Students' work is assessed for three principal components: clarity of concept understanding, incremental nature of scientific argumentation, and the structure (lexical cohesion) of the written or verbal argument. The last two are highly transferable but difficult to teach, largely because of vagueness. Helping students understand what it looks like when they see it is a major step forward.

When effective, scientists' argumentation carefully progresses from factual data to theoretical claims. Evidence stacks one layer on another, each thought or support tied in language and concept to previous ideas, each increasingly inferential. The terms and phrases that describe concepts are used consistently at all levels, and the intent... is never to mystify, but demystify. Befuddling readers may stymie rebuttal, but will not persuade them of the value of a claim. Students can look for these characteristics in claims and articles they assess in the second phase, and become fully competent in their own abilities to construct solid arguments as they work regularly with new issues in a supportive environment. A few practices that can positively contribute to students' abilities to construct quality scientific lines of argument are outlined in Figure 7. The understandings that result also help to refocus investigations as time permits.

Mapping Student Success

Teaching science with issues, whether Eve or another, confers natural advantages. It's nice to address the inevitable student question "Why do we need to learn this?" before it is asked. Equally effective is the explicit focus on the nature of science and the ease with which an issue weaves scientific and societal components into a meaningful whole. Using a framework such as MAP ensures the advantages are not lost, and the students get the benefit from the lessons.

REFERENCES

Cann, R., Stoneking, M., and Wilson, A. (1987). Mitochondrial DNA and human evolution. Nature, 325, 31-36.

Eyre-Walker, A., Smith, N. H., and Smith, J. M. (1999). How clonal are human mitochondria? Proceedings of the Royal Society of London, Series B, 266, 477-483.

Krings, M., Stone, A., Schmitz, R. W., Krainitzki, H., Stoneking, M., and P‹‹bo, S. (1997). Neandertal DNA sequences and the origin of modern humans. Cell, 90, 19-30.

National Research Council. (1996). National Science Education Standards. Washington, D.C.: National Academy Press.

Schwartz, M., and Vissing, J. (2002). Paternal inheritance of mitochondrial DNA. New England Journal of Medicine, 347, 576-580.

Shreeve, J. (1990). Argument over a woman. Discovery, 11(8), 52-59.

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