Q&A: Dating fossils from hotbed period of early human evolution

UNIVERSITY PARK, Pa. — Erin DiMaggio, associate research professor of geosciences at Penn State, was part of the international team of that recently discovered fossils belonging to our genus Homo and a new, yet identified, species of Australopithecus from a site in Ethiopia. The fossils, dated to between 2.6 and 2.8 million years ago, indicate that Australopithecus and the oldest specimens of Homo co-existed in the region. The team, led by researchers at Arizona State University, published their findings in Nature. The work was chronicled in several outlets, including Scientific American, Science, Smithsonian Magazine, New York Post, Reuters and The Washington Post.

The study points to growing evidence that the evolutionary path to our own existence isn’t linear, DiMaggio said. Australopithecus, a genus of early hominins, was perhaps more diverse than previously thought in the evolutionary path to humans, before dying off.

DiMaggio, a co-author of the paper, has worked at the site of the Ledi-Geraru Research Project since 2006. She explained the role geoscientists play in working with paleontologists to chronicle evidence of our early human ancestors.

Q: What’s your role in the research?

DiMaggio: I work with our geology team to piece together the timing and geologic history of the region to learn about when and how the landscape changed through time as hominins were running around and interacting with their landscape. This requires a deep understanding of the rock layers. We measure and describe sequences of rocks, document faults and sample ancient volcanic ash layers that can be dated. These methods were all used to learn about the geologic context and age of the hominin teeth in this paper.

Q: How do geoscientists work with paleontologists to discover such important fossils? How do you find the needle in the haystack?

DiMaggio: Well, it is like finding a needle in a haystack! It begins with paleontologists surveying a potentially interesting area for fossils and geoscientists studying the rock layers and looking for volcanic ash layers. Our paleontology and geology teams work together to figure out what’s next; our work informs and guides the other team. Once an important locality is identified we plan out what data needs to be collected. For us, the geologists, that might be high resolution geologic mapping of a site, collecting samples to refine the age of the rock layers or detailed stratigraphy. Our geology and paleontology teams are constantly talking and that improves our science.

Q: How do geoscientists date fossils that are discovered?

DiMaggio: Good question! Since we can’t date the fossils directly, they are too old, we look for clues in the rocks. We are lucky that the Afar depression is a very geologically active place and has been for a long time. As a result, the rock layers that we study tend to have a lot of ash layers preserved from ancient volcanic eruptions. We look for those layers because they often contain crystals called feldspars that can be radiometrically dated to determine a numerical or "absolute" age for the layer. We bracket the age of the fossils using those ash layers. 

Q: How confident are you about the age of the new Australopithecus and Homo fossils?

DiMaggio: Very. The fossil teeth presented in this paper were all discovered within about 10 meters of dated volcanic ash layers, meaning there wasn't a lot of time, geologically, between the date of the eruption and the time the hominin died and was preserved on the landscape. Furthermore, the fossils were discovered in well-mapped regions of the field area where the faults, stratigraphy, as well as volcanic ash, have been studied extensively.

Q: What are some challenges that you face working in this area?

DiMaggio: Well, there are geological challenges as well as physical challenges. As I mentioned earlier, since the Afar depression, where our field site is located, is an active area it requires extensive geological work. We have been working at this site for 20 years now and are still finding new faults, new volcanic deposits, there is always something more to see! On the physical side of things, we are living and working in 90 to 110 degrees Fahrenheit heat with little shade and in extremely dry and dusty conditions. Staying healthy, hydrated and not getting sunburn is important.

Q: This team has been working at this site for two decades with a series of great discoveries to show for it. What’s that been like?

DiMaggio: Like I hit the science jackpot! Not to sound too sappy but our team is really a family — a family of outstanding, dedicated and kind-hearted people. I get to work with the Top Guns of paleontology, archaeology and geology, with the best-of-the-best support crew and local colleagues and friends who make the project possible each year. It is very rewarding to see the hard work of so many people pay off with the publication of this important paper.

Q: How have Penn State students contributed to your work?

DiMaggio: I have had excellent undergraduate students who have contributed to various aspects of our work in Ethiopia. Although students did not directly work on this paper, students in my lab prepared volcanic ash samples and interpreted correlative deposits, selected feldspar grains for radiometric dating and analyzed the sedimentological characteristics of rocks to learn about past environments. The contributions by Penn State undergraduate students have been crucial to improving the resolution of the geologic story at Ledi-Geraru; more publications to come! Their work underscores the importance of undergraduate involvement in research.