Qnas with Enquye Negash, Zeresenay Alemseged, and Jonathan Wynn QNAS Tinsley H
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QNAS QnAs with Enquye Negash, Zeresenay Alemseged, and Jonathan Wynn QNAS Tinsley H. Davis, Science Writer Fossilized teeth can tell a story of the diets of long- early hominin species in southwestern Ethiopia shifted gone animals and in turn shed light on the environ- toward grasses and sedges between 2 and 3 million ments in which the animals lived. In a pair of recent years ago (1, 2). The University of Chicago paleoan- articles, Enquye Negash, Zeresenay Alemseged, thropologist Alemseged, who has studied the evolu- Jonathan Wynn, and colleagues report that carbon tion of hominins in southwestern Ethiopia, returned to isotope data reveal that the diets of herbivores and studying fossilized teeth with new analytical methods. Enquye Negash. Image credit: Alem Abreha (photographer). Published under the PNAS license. First published November 16, 2020. www.pnas.org/cgi/doi/10.1073/pnas.2021561117 PNAS | November 24, 2020 | vol. 117 | no. 47 | 29253–29256 Downloaded at UNIVERSITY OF CHICAGO-SCIENCE LIBRARY on November 25, 2020 by grassland with time, which is very different from what we observe on the landscape today. Alemseged: If you were to go today to the site where our fossil samples come from, you would see a mostly barren, hot, and dry landscape with acacia trees sparsely distributed and a gallery forest along a major river. If, instead, you invented a time machine and traveled back, say to 2 million to 4 million years ago, the conditions would be much more lush than what you see today. You would have a major river, which we call the proto Omo, surrounded by gallery forests and woodland and also a major lake. At some distance from the river, you would have more grassland envi- ronments within which you would see hundreds of wild animals roaming the landscape. Within this mosaic environment were the hominins: that is, our ancestors, Zeresenay Alemseged. Image credit: Dikka Research Project. who were the minorities sharing the landscape. PNAS: Both studies relied on fossilized teeth collected Negash, a postdoctoral researcher at George Wash- previously in the Lower Omo Valley and housed at the ’ ington University, led the study that focused on the National Museum of Ethiopia. However, you weren t diet of nine different herbivore families, revealing that looking at morphology or wear patterns, so why teeth? herbivores show two distinct shifts to increased grass Wynn: The ancient river system that’s coming down consumption. Wynn, a geologist and program director from the Ethiopian Highlands feeds into Lake Turkana, at the National Science Foundation who led the hom- bringing down lots of sediments and preserving all of inin work, found only one such shift in hominins, these fossils without the sort of gaps in the sedimen- around 2.37 million years ago. tary record that we have further south in the lake. The disadvantage is [that] this river system tends to break PNAS: What can an animal’s diet reveal about its up a lot of the fossils before they get preserved. Most environment and evolution? of the bones get broken up, but the teeth are much ’ Negash: Whether it is feeding on grass, fruits, or more dense, and they just don t break down when they’re being transported by a river system. So, you’ve leaves, herbivores feed on plants that are available on got tons and tons of fossil teeth, and they’re abraded the landscape they live in. Studying their diet gives us but they are preserved very well. Because they’re often information about their ecology and environment. The somewhat fractured, the museum is more likely to give same principle applies to reconstructing the environ- permission to sample. We spent a lot of time combing ment and understanding the evolution of herbivores through the database and combing through the collec- in the fossil record. When studying a fossil herbivore tions to pick out the specimens that we wanted to sam- assemblage, a grazer-dominated assemblage indi- ple. That was actually a fair amount of the work: Just cates an open landscape dominated by grasslands, identifying what would make the most systematic and while one dominated by browsers implies a more representative sample of this very large dataset. wooded landscape. Determining their diet and how it changed through time allows us to better under- Alemseged: When I did my PhD two decades ago, I stand how their environment changed. did not have access to fossil dental enamel powder. Our current isotopic work is exciting because it allows PNAS: The fossils of the herbivores and hominins you me to evaluate my earlier work (3) based on new ev- studied are from the Shungura and Usno Formations idence. What this research also reveals is that in order in southwestern Ethiopia. How is the landscape dif- to make important discoveries in human evolution, ferent from that of millions of years ago? you don’t always need more fossils, although field- work remains our primary source of data. By applying Negash: The Shungura Formation is a site that covers novel techniques and approaches on existing fossils— the time span between 3.6 and 1 million years ago. one of which is stable isotope analysis—one can an- ’ During this time, what we re inferring from our study is swer some of the most prominent questions in paleo- that there was a major shift in the vegetation on the anthropology. What did our ancestors eat and how landscape. Beginning around 3.6 to 2.9 million years did they use their landscape? ago, the landscape was dominated by more wooded environments. But from about 2.7 to 2 million years ago, PNAS: A small amount of dental enamel was removed there was an expansion of grasslands. Essentially, the for stable carbon isotope analysis. What were you landscape was becoming more open and dominated looking for? 29254 | www.pnas.org/cgi/doi/10.1073/pnas.2021561117 Davis Downloaded at UNIVERSITY OF CHICAGO-SCIENCE LIBRARY on November 25, 2020 Negash: Dental enamel is composed of an inorganic compound that contains carbon derived from food. For herbivores, the carbon isotopic composition of the plants they eat—whether it is C3 plants, which are trees and shrubs, or C4 plants, which are grasses—is recorded in bones and dental enamel. So when we drill the teeth of fossil herbivores and remove a small amount of powder enamel, we are aiming to study what type of plants they were consuming. That allows us to determine whether they were grazers, browsers, ormixedfeeders,whichfeedonbothC3 and C4 plants. Wynn: We already knew that Paranthropus bosei had an almost entirely C4 diet (4). And the question I think that we tried to answer is, “How did they get to that point with earlier species?” The samples from the Omo were sort of unique in that respect because, in the previous data, we had a lot of samples around 2 million years that showed that C4 diet. And then, we had a lot of samples earlier, around 3 million years, that showed it was a mix of C3 and C4 in pre- vious species. But there were just really sparse sam- ples represented in the dataset between 3 and 2 million years. PNAS: What did the analysis reveal? Were there differences in the isotope analysis between herbivores and hominins? Negash: There have been previous isotopic studies that looked at the diet of a few herbivore species from this site (5–7). Instead of focusing on just a few herbi- vore families, what our study provides is a robust com- parative study of multiple herbivore taxa that tracks Jonathan Wynn. Image credit: Dikka Research Project. how their diet changed with time from about 3.6 to 1 million years ago. Our analysis reveals that at about 2.7 million years there is a change in the proportion of The adaptations, the changes in their tooth shape browsers, mixed feeders, and grazers. Prior to 2.7 mil- and other morphological features, don’t correspond lion years, we had a higher proportion of mixed to this time when they changed their diet. I think feeders, but after 2.7 million years, we see a decrease we can say it’s behavioral, but is it because of some in the proportion of mixed feeders and an increase in sort of underlying change in the availability of trees and grazers. Then, at 2 million years, we see a further de- grass? Or do grasses become so much more common crease in mixed feeders and a higher proportion of or do trees just disappear from the landscape at that grazers (1). The timing in the shift in diet of most time? herbivores is different from what we found for the hominins. Alemseged: The two robust hominin species that we considered in our study may have had adaptations for Wynn: Enquye found changes at different times and repetitive chewing, and this would be consistent with [in] all kinds of different families of mammals, but a grass-based diet. Because grasses are not necessar- the hominins, they all change at 2.37 million years, ily rich in their dietary content, a large amount would no matter what species they belong to (2). Every- have to be consumed in order to get needed calories. body, these Paranthropus with the massive chewing Yet, we see a dietary shift within the same species teeth, changed to a C4 dietatthesametimethat without major morphological change. This may sug- early Homo, our own genus, changed to a C4 diet. gest that changes in environmental conditions were They’re living in the same place at the same time, responsible for the dietary shift across species, and they probably have totally different diets that give additional factors besides diet will have to be invoked them their own ecological niches, but both genera, to explain the morphological diversity observed across both categories of species change right at that time and space.