This case was developed for use in a general biology course where evolution is taught. Instructors of courses in anthropology and paleontology might also find it appropriate. We have used it in a freshman evolutionary biology course as the last case in the term after studying the general principles of evolution, genetics, and biodiversity.

Case Objectives

To examine:

• the general principles and causes of extinction;
• possible causes for the extinction of mammoths, especially the "overkill" hypothesis and the climate change hypothesis;
• possible causes for the extinction of the Neanderthals, especially the replacement hypothesis and the interbreeding hypothesis;
• the recent phylogeny of humans;
• how to distinguish between speculation and evidence;
• how to marshal evidence in support of an argument; and
• the relative merit of fossil vs. DNA evidence.

BLOCKS OF ANALYSIS

The theme of this case is extinction. Within that overall theme, the major thrusts of this case fall along two lines: what happened to the mammoths, and what happened to the Neanderthals?

Mammoths. When we taught this case study in the summer of 2000, the focus was on the problem of what happened to the Neanderthals. Mammoths were a side issue, although in another setting they could be the dominant focus.

The woolly mammoth, like the Neanderthal, is one of the quintessential symbols of the Ice Age. Tall and shaggy with a hump back and tusks that twist in an upward spiral, it ranged over northern parts of Eurasia and North America. It was hunted for food and clothing by early humans, who also used its bones and tusks as building materials.

The case opens with the actual recent discovery of a woolly mammoth carcass frozen in the Siberian tundra. The dramatic recovery in the fall of 1999 of the Jarkov mammoth (named for the nomadic reindeer herder who originally found it) was the subject of numerous news stories and articles as well as a two-hour Discovery Channel television special. Although not the first discovery of its kind, the Jarkov mammoth was believed to be intact and its internal organs preserved.

Mammoths figure in this case in a number of ways. As a food source for early humans in Eurasia, scientists have speculated that their availability--and eventual disappearance--may have played a role in the extinction of the Neanderthal as well as the evolution and development of modern man. In addition, it has been hypothesized that mammoths along with other large Ice Age mammals became extinct because they were over-hunted by humans (the "overkill" hypothesis), although other scientists argue that climate shifts that occurred during the Late Pleistocene as the Ice Age was coming to a close led to the mammoth's extinction.

Whatever the cause of their demise, mammoths and Neanderthals are linked in the minds of many experts, if for no other reason than that they lived at the same place and time and both became extinct. The dramatic discovery and recovery of the Jarkov mammoth, then, provides a hook into the case at the same time it helps to set the stage for exploring the causes, questions, and controversies concerning the extinction of species.

Neanderthals. The central focus when we taught the case was "What happened to the Neanderthals?" Originally cast as a stooped, shuffling, ape-like brute, we now know that the large-brained Neanderthals successfully occupied Europe and western Asia for over 200,000 years, surviving the bitter cold of the Ice Age and the daily vicissitudes of prehistoric life. Once thought too stupid and too slow to do more than scavenge for food, recent evidence from Croatia indicates that they were skilled hunters able to kill large animals, including rhinos. Analysis of animal and human remains as well as stone and bone tools found in a cave in Croatia inhabited by Neanderthals suggests that they hunted medium to large mammals (Richards, et al., PNAS, June 20, 2000).

There are two general hypotheses put forward on the extinction of the Neanderthals:

1. They were eliminated by either indirect competition or "war" with the more advanced Homo sapiens. The argument runs this way: Back in Africa at the time of Homo erectus, some populations migrated into Europe and Asia. There they developed into specialized regional populations. One line of Homo erectus, primarily in Europe, became specialized for surviving the cold and this group became the Neanderthals. They survived for hundreds of thousands of years, developing stone tools and a culture we can only guess at based upon scanty evidence, such as the presence of fossilized pollen at sites where their remains have been found, suggesting to some that they buried their dead with flowers.

Meanwhile, back in Africa about 200,000 years ago, Homo erectus gave rise to Homo sapiens. One or more populations of Homo sapiens migrated from Africa into Europe and Asia (and eventually beyond). They reached the areas inhabited by the Neanderthals. In some regions it appears that they displaced Neanderthals; in others, such as in Spain, they apparently lived side by side for at least 10,000 years before the Neanderthals disappeared.

Several theories have been put forward (again with scanty evidence to support them) to explain how Homo sapiens may have caused the Neanderthals' demise: Homo sapiens may have out-competed Neanderthals with their more sophisticated technology and advanced hunting skills or via faster breeding and greater longevity; diseases carried by Homo sapiens may have eliminated the Neanderthals; or Homo sapiens may have waged "war" against the Neanderthals, annihilating them.

2. The second major hypothesis is that Neanderthals and Homo sapiens interbred and became modern humans. (Before we had such clear evidence of the two groups living together, an earlier version of this hypothesis was that Neanderthals were a direct ancestor of Homo sapiens.) Two lines of evidence are used to bolster this "fusion" argument. First, that the two coexisted and thus may have traded with one another, perhaps even lived together, in which case why wouldn't they have interbred as well? Second, the recent discovery of the fossilized skeleton of a 24,500-year-old child in Portugal with apparently a mixture of Neanderthal and H. sapiens traits suggests to some that they were interbreeding.

General Concepts and Principles of Extinction. Before engaging the case, students should have a basic understanding of the principles and causes of extinction. Important points to cover, either in lecture or supplemental reading, include the distinction between taxonomic extinction, in which one species evolves into another, (e.g., Homo erectus isn't around any more because they became us, Homo sapiens) and "true" extinction, in which an entire species (e.g., T. rex), genus, family, order, class, or phylum is wiped out.

Underlying principles of extinction should be covered so that students realize that extinction is a natural, inevitable, and ongoing process; that it can be localized (e.g., in prehistoric times horses were found throughout Europe, Asia, North America, and South America, but 10,000 years ago they became extinct on our continent and were only later reintroduced in modern times by the Spaniards) or global (like the worldwide extinction of the dinosaurs at the end of the Cretaceous); that extinction can occur at any taxomonic level, with it more commonly occurring at the population level and more rarely at the phylum level (thousands of vertebrate species have become extinct but of the eight vertebrate classes only one--placoderms--has gone extinct); and that the extinction of one group opens the way for other groups (when the dinosaurs became extinct, mammals, which hitherto had been small rodent-like creatures, expanded and took over their niches).

Causes of extinction include environmental changes (it gets hotter or colder or lighter or darker, or oxygen levels increase or decrease); the rise of a new predator (like Anomalocaris or like us); or a new disease appears on the scene and wipes out species with no resistance. When such changes occur, there are three possible responses: you can migrate (if the change is localized and not global); evolve (if the change is slow enough, giving you time to change, too, and stay one step ahead of the game, e.g., by developing body armor); or become extinct (which happens when the change is very fast--although, if there is enough genetic variation in a population, some individuals might make it).

You might also want to set the stage for this case study by touching on mass extinctions. There have been about six to a dozen of these. Although they get a lot of attention, only about four percent of all extinctions can be accounted for by these mass extinctions. Here are the ones that get the most press:

• The first major extinction probably occurred with the oxygen revolution, when blue-green algae began producing oxygen as a waste product of photosynthesis, wiping out the anaerobic organisms. The cause for this mass extinction, we might posit, was O₂ pollution.

• Another mass extinction occurred when the stromatolite populations of blue-green alagae living in mounds were wiped out. This coincided with the appearance of the first animals, so this one may have been due to predation.

• During the Cambrian Period, 590 to 505 million years ago, there was a dramatic increase in the types of organisms. Most of the animal phyla suddenly appear in the fossil record, including trilobites. There were several mass extinctions of trilobites during this time, one in which they and 80 percent of all genera on the planet became extinct in the span of five million years. There was a lot going on during this time: sea levels were falling, currents in the oceans were changing, continents were shifting--and so maybe we can chalk this one up to plate tectonics.

• The Late Permian mass extinction was the most dramatic. Two hundred and fifty million years ago, the continents were all jammed together in one land mass called Pangea. When Pangea formed, sea levels dropped, killing off a lot of organisms along the shore. Organic matter increased -- there was more death, more rotting, more oxygen used, which produced anoxia. So, this one could have been due to reduced oxygen levels.

• Sixty-five million years ago, at the end of the Cretaceous, dinosaurs became extinct, as did 50 percent of all plankton and 10 percent of the freshwater genera. Most scientists now agree that the cause was an asteroid or comet that collided with our planet, causing a massive, apocalyptic change in the earth's climate.

• Then, 10,000 years ago, during the Late Pleistocene, about 70 species of large land mammals disappeared, including giant sloths, mammoths, mastodons, woolly rhinos, Irish elk, and the giant moa. Looks like humans did it, although there are other theories.

CLASSROOM MANAGEMENT

This case was handled in one 80-minute class period. The class was small, consisting of 20 students who were divided into four groups of five students each. The groups were permanent throughout the term and were well acquainted with group work.

This course is the first semester of a general biology sequence. In the summer it is taught using team learning, a technique devised by Larry Michaelsen of the University of Oklahoma (see Michaelson, L.K., 1992, "Team learning: A comprehensive approach for harnessing the power of small groups in higher education," To Improve the Academy, 11:107-122). No lectures are given. Instead, students have reading assignments and take both individual and group quizzes over the reading. They then work on case studies that pertain to the reading.

This case was the culmination of a section on human evolution. In preparation for the case, the students had read a chapter detailing hominid and primate phylogeny, with the following question posed: "What happened to the Neanderthals?" In the lab preceding this case, students in their groups examined a large number of primate skulls and were asked to make up a phylogeny based upon their observations (see http://ublib.buffalo.edu/libraries/projects/cases/missing_link_intro.html). At the end of this lab, the mammoth case was handed out to the students with instructions that they were to research individually the various hypotheses that had been suggested to account for the disappearance of the Neanderthals. They were told to list the hypotheses and give evidence for and against each one. Also, they were told to be ready to discuss these at the next class period.

During the next class, after the daily quizzes were given, we asked the students in their groups to compare notes and discuss the relative merits of the different hypotheses about the Neanderthals. After about 15 minutes, we then had student scribes from each team write down all possible hypotheses on the board. They listed about eight. Among these were: "Homo sapiens exterminated the Neanderthals," "Neanderthals evolved into Homo sapiens," "climate changes killed off the Neanderthals," "Homo sapiens out-competed the Neanderthals," and "Neanderthals and Homo sapiens interbred,"as well as a few "fringe" theories, such as "Neanderthals were still living." We then asked the groups to choose the hypothesis that most appealed to them (each group had to choose a different hypothesis) and gave them about five minutes to marshal evidence in support of their chosen hypothesis.

Next, we ran a discussion, listing on the board for each of the four hypotheses the group's assertions and their evidence for these, allowing other groups to challenge the points made. This lasted about 30 minutes, although more time should have been allotted to fully develop these ideas.

At this point we asked the students to imagine that an isolated group of Neanderthals had been found. We asked them: "What do you think we should do with them? Put them in zoos? Use them in experiments? Enroll them in our schools? (After all, they appear to be a different species.) Study them? (Suppose they didn't want to be studied?)."

After posing these questions, we gave the groups five minutes to think of a series of questions they would like to know the answers to before making a decision about the fate of the surviving Neanderthals. We then let each group ask us, the experts, their questions. The questions they asked had to do with culture, diet, cognitive abilities, language, and whether or not they could mate with us and produce viable offspring. After a couple of question-and-answer rounds, we asked the class to vote on the choices.

Extra Credit Assisgnment. Students can earn up to 25 points of extra credit if they write a dialogue paper on the disappearance of the Neanderthals. Writing a good dialogue requires students to have a firm grasp of the facts and simultaneously understand several sides of a sensitive and controversial issue. We find that this kind of an assignment is a good way of stimulating critical thinking.

The paper must follow a particular format: it must be at least five type-written, double-spaced pages and in the form of a dialogue between two people arguing on opposite sides of the question. In addition, it must be written in the form of a script or play. For example:

Students are instructed that there should be at least 20 exchanges between the people speaking, i.e., as shown in teh example above, Sam should speak at least 20 times and Shirley should speak at least 20 times. The exchanges must be serious, substantive, and courteous comments made between two intelligent people grappling with an important issue. A short introductory paragraph is required indicating where the dialogue is occurring and what the circumstances surrounding the discussion are. In addition, the paper must end with a paragraph in which the student gives his or her personal opinion and the reasons behind it. The paper is graded primarily on the basis of the number of issues explored and the depth of the coverage.

REFERENCES

When we taught this case we didn't give the students references but told them to search the library and the Internet for information. Below are citations to a number of articles you might want to give your students to get them started researching these topics. The books listed after the articles are both informative and highly accessible, particularly the books by Ward and Shreeve, which make for fascinating reading.

References for Students

Articles
• Bower, B. "Salvaged DNA adds to Neandertals' mystique." Science News 157(14):213 (April 1, 2000).
• Bower, B. "Neandertals' diet put meat in their bones." Science News 157(25):389 (June 17, 2000).
• Norris, S. "Family secrets." New Scientist 162(2191): 42-46 (June 19, 1999).
• Stone, R. 1995. "Siberian mammoth find raises hopes, questions." Science 286(5441):876-877 (October 29, 1999).
• Tattersall, I. "Once we were not alone." Scientific American 282(1):56-62 (January 2000).
• Wong, K. "Who were the Neanderthals?" Scientific American 282(4):99-107 (April 2000).
• Zilhao, J. "Fate of the Neandertals." Archaeology 53(4):24-31 (July-August 2000).

Books
• Lister, A., and P. Bahn. 1994. Mammoths. New York: Macmillan.
• Shreeve, J. 1995. The Neandertal Enigma: Solving the Mystery of Modern Human Origins. New York: Morrow.
• Tattersall, I. 1999. The Last Neanderthals: The Rise, Success, and Mysterious Extinction of Our Closest Human Relatives. Boulder, CO: Westview Press.
• Ward, P.D. 1997. The Call of Distant Mammoths: Why the Ice Age Mammals Disappeared. New York: Copernicus.

Internet Sites
• What Killed the Mega Beasts? (Discovery Channel Online)
  http://dsc.discovery.com/convergence/megabeasts/megabeasts.html

• Humans and Other Catastrophes: The Late Pleistocene Extinctions (American Museum of Natural History)
  http://www.amnh.org/science/biodiversity/extinction

• Woolly Mammoths: Evidence of Catastrophe? (The Talk.Origins Archive)
  http://www.talkorigins.org/faqs/mammoths.html

• Fossil Hominids: The Evidence for Human Evolution (Talk.Origins Archive)
  http://www.talkorigins.org/faqs/homs/

• Homo Sapiens Neandertalensis: Forcing a Radical Reconsideration of Human Origins (Washington State University)
  http://www.wsu.edu:8001/vwsu/gened/learn-modules/top_longfor/timeline/neander/neander-a.html

Additional Reading for Instructors

• Bar-Yosef, O. "Eat what is there: Hunting and gathering in the world of Neanderthals and their neighbours." International Journal of Osteoarchaeology 14(3-4): 333-342 (May-August 2004).

• Duarte, C., et al. "The early Upper Paleolithic human skeleton from the Abrigo do Lagar Velho (Portugal) and modern human emergence in Iberia." Proceedings of the National Academy of Sciences of the United States 96(13): 7604-7610 (June 22, 1999).

• Estevez, J. "Vanishing carnivores: What can the disappearance of large carnivores tell us about the Neanderthal world?" International Journal of Osteoarchaeology 14(3-4): 190-200 (May-August 2004).

• Evans, P.D., et al. "Evidence that the adaptive allele of the brain size gene microcephalin introgressed into Homo sapiens from an archaic Homo lineage." Proceedings of the National Academy of Sciences of the United States (November 7, 2006).

• Krings, M., et al. "DNA sequence of the mitochondrial hypervariable region II from the Neandertal type specimen." Proceedings of the National Academy of Sciences of the United States 96(10):5581-5585 (May 11, 1999).

• Jankovic I., et al. "Vindija cave and the modern human peopling of Europe." [Review article] Collegium Antropologicum 30(3): 457-466 (September 2006).

• Janokvic, I. "Neandertals....150 years later." [Review article] Collegium Antropologicum 28 (Suppl. 2): 379-491 (2004).

• Krings, M., et al. "Neandertal DNA sequences and the origin of modern humans." Cell 90(1): 19-30 (1997).

• Larkin, M. "More to speech than a big hypoglossal canal." The Lancet 353(9153): 649 (February 20, 1999).

• Lieberman, P. "Silver-tongued Neandertals?" Science 283(5399): 175 (January 8, 1999).

• Richards, M.P., et al. "Neanderthal diet at Vindija and Neanderthal predation: The evidence from stable isotopes." Proceedings of the National Academy of Sciences of the United States 97(13)7663 (June 20, 2000).

• Smith, F.H., I. Jankovic, and I. Karavanic. "The assimilation model, modern human origins in Europe, and the extinction of Neandertals." Quarternary International 137: 7-19 (August 2005).

• Stringer, C., and C. Gamble. In Search of the Neanderthals: Solving the Puzzle of Human Origins. London: Thames and Hudson, Ltd. (1993).

• Straus, L.G. "On the demise of the Neandertals." Quarternary International 137: 1-5 (August 2005).

• Tattersall, I., and J.H. Schwartz. "Hominids and hybrids: The place of Neanderthals in human evolution." Proceedings of the National Academy of Sciences of the United States 96(13):7117-7119 (June 22, 1999).

• Trinkaus, E., and P. Shipman. The Neanderthals: Changing the Image of Mankind. New York: Knopf (1993).

• Wolpoff, M.H., et al. "Why not the Neandertals?" [Review article] World Archaeology 36(4): 527-546 (December 2004).

• Wolpoff, M.H. "Multiregional evolution: The fossil alternative to Eden" in P. Mellars and C. Stringer (eds.), The Human Revolution: Behavioral and Biological Perspectives on the Origins of Modern Humans, vol. II, p. 62-108, Edinburgh: Edinburgh University Press (1989).

Acknowledgements: This case study was developed with support from the National Science Foundation and The Pew Charitable Trusts.