Case alumnus returns to teach SAGES class on human evolution
Robert Walter determined age of famous “Lucy” and is now among geologists in search of modern human origins
At first glance, a handful of dirt looks insignificant to first- and second-year students in the Case Western Reserve University SAGES course “Geological Evolution of Humans.” But Robert Walter, a visiting SAGES Presidential Fellow and a Case alumnus, tells his class that sediment contains a geological clock on human origins.
A world expert in geochronological techniques (the dating of rocks), Walter has played an integral part in determining the age of early human fossils, including several of the major finds of the past century. It was the most famous of these finds—the partial skeleton of “Lucy,” a human ancestor 3.2 million years old—that originally brought Walter to Case as a graduate student in 1975.
Now a faculty member at Franklin and Marshall College in Lancaster, Pa., Walter returned to Case this year to teach a special course for the university’s new undergraduate initiative, the Seminar Approach to General Education and Scholarship (SAGES). His course, which is similar to ones he has taught for upper-level geology majors and graduate students, examines human evolution in relation to the geological record.
“Sediments, not just fossil bones, answer key questions about the age of hominids and the environments they lived in,” Walter explained, “which enable us to understand the rates and nature of evolutionary change”.
Along with geological and human history, Walter’s seminar explores the history of science: He talked about how the evolution of our thinking about evolution is as fascinating as the physical evolution of the species itself. Class discussions and writing assignments focus on works by scientists and theorists such as Charles Darwin, Charles Lyell, and Donald Grayson. In addition, students will visit the Lucy exhibit at the Cleveland Museum of Natural History—an exhibit about which Walter is uniquely qualified to comment.
Pushing the Science
When Lucy was found in the Hadar region of Ethiopia in 1974, geologists faced a dilemma. Radioactive carbon dating, the standard method at the time, could date remains only as far back as 50,000 years. “Lucy pushed the science,” Walter explained. To determine her age, scientists would need another approach.
Walter, as an undergraduate at Franklin and Marshall in the early 1970s, had studied volcanic rocks under the guidance of Stan Mertzman, who had received his Ph.D. from Case just a few years before. Now, a member of the team that discovered Lucy—James Aronson, a former Case geologist currently at Dartmouth College—recruited Walter to apply potassium-argon dating to the volcanic deposits that had surrounded the remains. By dating the sediment above, around and below the find, Walter and his colleagues could “bracket” Lucy’s age.
As geologists excavated additional fossils—some in valleys and river beds contaminated with volcanic material from different eras and areas—the dating methods had to become ever more precise. Walter, as a post-doctoral fellow at the University of Toronto under Derek York, helped developed new technologies, including laser-fusion, argon-argon dating methods that have since been refined to the point where the age of a volcanic particle as small as a grain of salt can be determined with great precision and accuracy. When the age of each grain in a sample is plotted on a graph, geologists can distinguish the contaminants from the primary volcanic material (often a powdery, mineral-rich ash) and determine the age of a fossil unearthed there.
Out of Africa
After receiving his doctorate from Case in 1981, Walter served as chief geologist at the Institute of Human Origins, founded by Donald Johanson who discovered Lucy while he was a member of the Case anthropology department. For the past decade, Walter has done independent research in a region of Eritrea (once northern Ethiopia), at the northern apex of what geologists call the Afar Triangle. This geologically and tectonically active area was the site for some of the last century’s most important hominid finds.
Over millions of years, Walter explained, volcanic activity in the Triangle caused the breaks and fissures of the Great Rift Valley, which extends from Eritrea to Mozambique. In the process, human evolutionary history was preserved in layers of sediment. Recent hominid discoveries in this anciently populated region date back more than 5 million years, making Lucy look like the baby of the fossil group.
Although Walter continues to work with hominid discoveries, he has also branched off into a new and independent investigation, studying early human migrations from Africa to other parts of the world. In a 1999-2000 field exploration of an area in Eritrea called the Danakil Depression, Walter found stone tools and animal fossils along an ancient, perched shoreline that he describes as “the bathtub ring of the Red Sea.” These artifacts and fossils were deposited 125,000 years ago, during the last interglacial period when sea levels were six meters higher than they are today and when the climate was warmer and wetter.
This discovery, Walter said, provided “the earliest evidence of human occupation of a coastal marine environment.” The geological context of these tools suggest that early humans butchered large land animals and harvested edible shell fish in what amounts to the earliest “surf and turf” feast discovered so far. An adaptation to a seafood-rich diet by 125,000 years ago marks an important, perhaps defining, shift in early human behavior. This site also provides some of the first evidence of where the migration of Homo sapiens out of Africa might have taken place. Such coastal routes, it is now thought, allowed rapid expansion of human populations throughout Eurasia and even Australia between 100,000 and 60,000 years ago.
Once he finishes teaching in SAGES, Walter plans to return to the area and conduct excavations that will yield further information about this ancient beach environment. Meanwhile, he has expanded his study of his early finds to consider their implications for the history of evolution. The connection between his research and the substance of his SAGES seminar couldn’t be stronger.
“Even if the students never go into geology,” Walter said, “I want them to have this fascination about early human origins, so that when they read about a new discovery, they will have a deeper understanding about their past.”
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