Case Teaching Notes for
“A Tale of Three Lice: A Case Study on Phylogeny, Speciation, and Hominin Evolution”

Introduction / Background

This case study is a “clicker case.” Clicker cases combine two teaching techniques, case studies and clickers, to offer an instructional strategy that allows active learning in large science classrooms. Case studies put subject matter into context, engaging students with interesting and relevant real life examples, fostering small or large group discussion, and providing genuine problems to solve. Because the case method relies upon discussion, this practice has been primarily used in small classes, tutorials, or lab sessions. With the addition of electronic personal response systems or clickers, faculty can use case study teaching in large classrooms to foster small group discussion and gain immediate feedback from their students.

The case is presented in class using a series of PowerPoint slides punctuated by questions (called “clicker questions”) that students respond to before moving on to the next slide. In this way, students work through (and understand) the material presented in the case. Specifically designed for use in large introductory science classes, clicker cases integrate lecture material, case storylines, student discussion, (clicker) questions, the clarification of answers to those questions, more lecture, and data.

This case poses the questions: When did hominins lose their body hair, and when did they begin wearing clothing? We take a novel approach to answering these questions by examining the surprising phylogeny of human head, body, and pubic lice. Students are led through the scientific process as they are asked to think about hypotheses, predictions, results, and conclusions. The case illustrates another important aspect of the scientific process: sometimes studies provide relevant data that don’t provide definitive answers. Recent phylogenetic research on ape lice has provided some interesting data, but the questions remain unanswered at present. The case integrates topics including phylogeny, speciation, and hominin evolution.

Although developed for a general biology class, this clicker case is also suitable for use in a non-majors introductory biology course. The story may be useful in an upper division evolutionary biology course, although the case is presented at too basic a level for such a course. As written, the case is best taught at the end of a general biology course focusing on evolution and ecology. Prior to teaching this case, students should be able to read a simple phylogenetic tree. Students will also benefit from some background knowledge of evolution by natural selection, speciation, and hominin evolution.

Objectives

After completing this case, students should be able to:

• Read phylogenetic trees and use inferences from these trees to support or reject hypotheses.
• Explain how divergence of ape taxa (including humans) drove speciation in their louse parasites.
• Explain why a host is the major component of the environment of their obligate parasites and describe how primate lice are adapted to their hosts.

Misconceptions

• Students often view scientific studies as “good” or “bad,” and assume that a “good” study will provide a clear answer to the question being asked. In reality, even well designed studies may not provide a definitive answer to a question.
• Humans did not evolve from chimpanzees. Humans and chimps share a recent common ancestor, but that ancestor was neither chimp nor human.

Classroom Management / Blocks of Analysis

This case was designed and tested in a general biology classroom of 300 students, each required to participate with a student response system or clicker. The clicker case was taught in a 50-minute class period in a large, auditorium-style lecture hall. Students were asked to discuss the questions in small groups created informally within the large classroom.

Important note: When an earlier version of this case was tested, students appeared to be simultaneously uncomfortable and yet intrigued. Instructors can shift the balance from discomfort to engagement by giving the students a gentle warning about the taboo nature of the case, and taking advantage of some of the humorous aspects of the case.

Teaching the Case

Slide 1 is the title slide.

Slide 2 is a phylogenetic reconstruction of extant ape taxa, including humans. This tree is supported by comparative anatomy (i.e., morphological similarities), the fossil record, and DNA. If your students are not familiar with cladograms, explain that (1) each line represents a taxon, (2) the root represents the common ancestor of all taxa in the tree, and (3) each branching point represents the divergence of two taxa.

This is a good time to point out that humans are not descended from modern chimpanzees, but share a common ancestor with chimpanzees. It is also useful to point out that this tree includes extant ape taxa, but that many other taxa once existed and are now extinct.

Slide 3 defines the term hominin. Hominins include all species that are part of the human lineage following the divergence from ancestral chimpanzees.¹

Slide 4 introduces the central questions of this case: When did hominins lose their body hair? When did they begin to wear clothing? The fossil record does not directly answer these questions. Slide 5 asks how these questions can be addressed. Hominin distribution may offer some clues. Perhaps hominins lost their body hair when they moved from forest habitat to open savanna. While this is a reasonable hypothesis, it is not yet testable due to insufficient data. The oldest needles found are about 40,000 years old and provide a minimum age for the development of clothing (Kittler et al. 2003). However, the oldest needles are likely an underestimate of the age of clothing. Earlier humans may have worn scraped hides, though scraping tools might have served multiple purposes and can’t be linked definitively with clothing.

Slide 6 proposes a novel approach to addressing these questions about human hair and clothing.

Slide 7 introduces the three types of human lice: the head louse, Pediculus humanus capitus, the body louse, Pediculus humanus corporis, and the pubic louse, Phthirus pubis. The fact that humans have not one but three types of lice is one reason that humans are distinctive within the ape clade. Don’t discuss the taxonomic status of these three types (i.e., whether the head and body louse are one or two species) just yet, as this topic will be dealt with later in the case.

Lice are highly host specific, and have evolved to resist the immune system of their host and to obtain all their nutrition from the host’s blood. All human lice are sucking lice (as opposed to chewing lice) and are obligate ectoparasites, with mouthparts specialized for feeding on the blood of their hosts. The distribution of parasites on hosts can result from several processes: (1) co-speciation of parasites with the speciation of their hosts, (2) host switching, or (3) duplication and speciation on a single host (Reed et al. 2007). In this case, we seek to understand how and why it is that humans are host to three types of lice.

Slide 8 introduces the relationship between human head lice and chimp lice. Encourage your students to think of a hypothesis, and to suggest a means of testing their hypothesis. You may generate discussion by taking up their suggestions.

Slide 9 presents the first hypothesis: human head lice and chimp lice speciated at the same time that hominins and ancestral chimps separated from their common ancestor. This divergence occurred about 6 million years ago. The tree should help students to visualize the relationships among humans and the other apes. This tree is similar to the earlier tree, but has a different orientation, and a time scale. The time scale might not be intuitive to students; point out that smaller numbers represent more recent events. You might need to remind your students that modern humans are currently the tip of the human lineage, but that they were preceded by other hominins (e.g., Australopithecines, Homo erectus). Generate discussion by asking the class how they would test the co-speciation hypothesis.

Slide 10: To test the hypothesis, researchers compared two genes (the mitochondrial cytochrome c oxidase subunit I [Cox1] gene and nuclear gene elongation factor 1 alpha [EF-1α] gene) from human head lice and chimp lice, to see when they last shared a common ancestor (Reed et al. 2007).

Slide 11 is the first clicker question (CQ#1). Note that students are not asked to identify the factually accurate piece of information, but rather the predicted result for the co-speciation hypothesis. Clicker questions #1–5 demonstrate that a multiple-choice format can be used to ask predictive and synthesis questions, not just to test factual recall.

Slide 12 presents the results. Human head lice and chimpanzee lice both belong to the genus Pediculus. Phylogenetic analyses using DNA indicate that these two species diverged approximately 6 million years ago. This provides strong evidence of co-speciation of the lice with their hosts (Reed et al. 2004; Reed et al. 2007; Wade 2007).

At this point, you might want to discuss the concept of co-speciation, which is speciation of a parasite lineage at the same time as (and as a result of) speciation of the host lineage. The conditions for speciation are met with the separation of host lineages, with the accompanying erection of barriers to gene flow and changing environments for the parasites with evolutionary changes in the hosts.

Slide 13 presents a second louse: the human pubic louse. Encourage your students to think of a hypothesis to explain the origin of pubic lice, and to suggest a means of testing their hypothesis. You may generate discussion by taking up their suggestions.

Slide 14 presents the new niche hypothesis: human head lice and human pubic lice speciated because hominins lost their body hair. The loss of body hair, and the retention of head and pubic hair, would have created two, isolated hair “niches” with different environments for lice.

Slide 15: There are many approaches to testing this hypothesis. You might point out to the students that it is difficult to test the cause of historical events. The first test we suggest is to compare human head and body lice to see when they last shared a common ancestor.

Slide 16 is CQ#2. Note that students are not asked to identify the factually accurate piece of information, but rather the predicted result for the co-speciation hypothesis.

Slide 17: Human head lice and pubic lice belong to different genera. Phylogenetic analyses using DNA indicate that these two species diverged approximately 12 million years ago ((Reed et al. 2004; Reed et al. 2007). Divergence of these two lice lineages was not driven by loss of body hair; the new niche hypothesis is rejected. Be careful not to provide too much information here as the next question (CQ#3; Slide 18) probes student understanding further.

Slide 18 is CQ#3 and tests students understanding of the conclusion that human head and pubic lice did not diverge on a hominin.

Slide 19 introduces CQ#4. Students will be shown a phylogeny on the next slide (Slide 20), then asked the same question again on Slide 21.

Slide 20 compares the (surprising) phylogeny of selected apes and their lice. You might ask how many students recognize the human. Of course this is not to imply anything unfavorable about Darwin’s personal hygiene!

Slide 21 is CQ#4. You might wish to discuss the phylogeny in Slide 20 after viewing, with your class, the student responses to CQ#4. If the majority of your students answer this question correctly, a brief review of the phylogeny will suffice. If the majority of students answer it incorrectly, it may be appropriate to take them through the phylogeny carefully and perhaps try CQ#4 a second time before advancing to Slide 22.

Slide 22: Human and gorilla Pthirus lice last shared a common ancestor 3–4 million years ago, long after divergence between gorilla and hominin lineages (Reed et al. 2007). This suggests speciation caused by a host switch from the gorilla lineage to hominins.

Slide 23: This figure (Reed et al. 2007) is optional and requires a good grasp of phylogeny and speciation. The phylogeny indicates a duplication approximately 13–12 million years ago (estimates vary) that lead to the divergence of Pthirus and Pediculus. This occurred before the divergence of gorillas, chimps, and hominins. At approximately 7 million years ago, ancestral gorillas and the shared ancestor of chimps and humans diverged. Following this, each lineage lost one louse species such that the gorilla lineage retained only Pthirus, while the chimp/human lineage retained only Pediculus. Approximately 3–4 million years ago, Pthirus diverged into two species when a population switched from ancestral gorillas to hominins. Note that the tree shows only extant louse and ape lineages. The real tree would likely have been more complicated with many more ape and louse species and speciation events.

Slide 24 explains how hominins likely picked up pubic lice from ancestral gorillas: not the way you and your students might have imagined! This is a good opportunity to discuss with your students the challenges of studying the past, and the fact that many interesting questions have not yet been answered.

Slide 25 suggests that hominins may have picked up gorilla lice because they had lost their body hair and now had an additional “hair niche” for a louse capable of hanging on to coarse pubic hair. This is similar to, but different from, the new niche hypothesis that was considered earlier. In this case, the new niche made speciation possible, but the mechanism was a host switch rather than ecological speciation in two different habitats. This is a good time to point out to your students the difference between possible explanations and conclusive evidence. Note that students are expected to incorporate the first central conclusion of this case: hominins already had one type of lice from chimps, so only one niche was available to the gorilla louse.

Slide 26 introduces the human body louse as a different morphotype belonging to the same species as the head louse. The two morphotypes have a number of biological and ecological differences (Light et al. 2008), but are impossible to distinguish by photo (Reed 2009, pers. comm.). Head lice require more frequent blood meals. Body lice are attracted to areas with other body lice or body lice feces. Both types of lice tend to return to their preferred habitat if disturbed. They have not been observed to interbreed in nature, but can interbreed in the lab, producing viable and fertile offspring. When reared in the lab under conditions similar to those of body lice, head lice change into the body louse morphotypes within a few generations. This transformation is associated with high initial mortality, and likely results from selection against alleles responsible for the head louse phenotype. Genetically, head and body lice do not form reciprocal monophyletic groups, and are thus not considered separate species.

Slide 27 asks students to think about the relationship between human head and body lice.

Slide 28 presents a third hypothesis: body lice arose when humans started to wear clothing.

Slide 29: It is difficult to test hypotheses about historical causes. However, it would be useful to determine when body lice first arose by estimating a date for their most recent common ancestor.

Slide 30 is CQ#5. Clarify for your students that they should answer the question by thinking about which species was most likely the first to wear clothing: the common ancestor of humans and chimps, a hominin that had lost some or all of its hair, or a modern human.

Slide 31: In a phylogenetic analysis of human head and body lice, all body lice were contained within a single clade with a most recent common ancestor dating to 72,000 +/- 42,000 years ago. This provides strong evidence that body lice arose on modern humans. One interpretation of the data is that they might suggest the origin of clothing around 72,000 years ago (Kittler et al. 2003). However, the data should be interpreted with caution, especially given the large (though typical) error.

The students are then asked whether these data are also consistent with other hypotheses. Spend a few minutes entertaining alternative explanations proposed by your students. For example, your students may suggest that the ancestors of body lice moved from our heads to the bedding materials used by our ancestors, and then moved to clothing later, i.e., 40,000 ya. Ending the case with a request for students to generate novel testable hypotheses encourages them to continue thinking about the problem and moves them away from the notion that every question has one clearly correct answer.

Slide 32 summarizes the data provided by studying the phylogeny of lice. While lice haven’t provided definitive answers to our questions (When did humans lose their hair? When did they start to wear clothes?), they have shown that they can provide relevant information.

Slide 33 leaves the questions unresolved, as they are in the literature. It also provides a great opportunity to discuss the nature of scientific research. Some results were predictable, while others were surprising. The data presented in this case have provided some answers, but have also introduced new questions. In reality, research is often much more complex than asking a question, gathering data, and producing a clear answer to your question.

Answer Key

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References

• Kittler, R., Kaysar, M., Stoneking, M. 2003. Molecular evolution of Pediculus humanus and the origin of clothing. Current Biology 13: 1414–1417.
• Light, J.E., Toups, M.A., Reed, D.L. 2008. What’s in a name? The taxonomic status of human head and body lice. Molecular Phylogenetics and Evolution 47: 1203–1216.
• Reed, D.L, Smith, V.S., Hammond, S.L., Rogers, A.R., Clayton, D.H. 2004. Genetic analysis of lice supports direct contact between modern and archaic humans. PLOS Biology 2: 1972–1982. Available at http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020340. Last accessed October 11, 2009.
• Reed, D.L., Light, J.E., Allen, J.M., Kirchman, J.J. 2007. Pair of lice lost or parasites regained: The evolutionary history of anthropoid primate lice. BioMedCentral Biology 5:7. Available at http://www.biomedcentral.com/1741-7007/5/7. Last accessed October 11, 2009.
• Reed, D. 2009. Personal Communication.

Slide Credits

Acknowledgements: This material is based upon work supported by the NSF under Grant No. DUE-0618570. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of NSF.

Date Posted: October 29, 2009.

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