ASU student develops solution to common archaeological problem
Bone fragments collected from Kobeh Cave in Iran, a Middle Paleolithic site. Photo courtesy Patrick Fahey
Patrick Fahey is very good at identifying ancient animal remains.
He has analyzed and identified tens of thousands of fossils from archaeological sites like Pinnacle Point, a designated World Heritage Site located on the southern coast of South Africa. The archaeological site consists of a series of caves and rock shelters where modern humans lived 50,000 to 160,000 years ago.
As an anthropology PhD candidate, Fahey uses information from these bone fragments to help reconstruct human ecology from 100,000 years ago at Pinnacle Point 5-6, a rock shelter at the site.
However, he — like other zooarchaeologists and paleontologists — has faced a frustrating problem: Most animal bones found at ancient sites are badly broken into small fragments. Teeth and skulls, useful for identifying species, are rare. What survives most often are fragments from the dense middle sections of long bones, known as shafts.
“If the bone is complete, it’s pretty easy to get information from. You simply look at the bone and you compare it to modern skeletons,” said Fahey, an affiliated student at the Institute of Human Origins and PhD candidate at the School of Human Evolution and Social Change. “But that is hardly ever the situation. In reality, a single bone may break into dozens or even hundreds of tiny pieces, leaving researchers to work only with those small bits that have survived for millennia. This fragmentation, and the loss of many parts of the original bone, makes direct comparison with modern skeletons difficult or impossible.”
Though the fragmentation makes it difficult to identify the species the bone fragments came from or to even estimate the animal’s size, even small fragments can contain important information about the past — and ignoring them can lead to biased conclusions.
So, Fahey found a solution.
He developed a mathematical formula that helps scientists worldwide do their jobs more accurately. The formula uses a measurable feature of the bones — the thickness of the walls that form the middle shaft of long bones referred to as cortical thickness — to estimate an animal’s body size.
The new method is based on a straightforward idea: Bigger animals need stronger long bones to support their weight. That strength comes, in part, from thicker bone walls. The precise relationship between cortical thickness and body mass, defined mathematically, makes it possible to predict the animal’s size from fossil remains.
A simple measurement with big potential
To develop and test the new method, Fahey measured cortical thickness on more than 400 modern bones from 122 species of land mammals, ranging from tiny shrews to African elephants. There was a remarkably consistent relationship between wall thickness and body mass, regardless of the animal’s species or which limb bone was measured.
Two mathematical models were created, one estimates an animal’s body weight and the other places the animal into a general size category. Tests show that these models remain accurate even when bones are highly fragmented. Fahey also created a web application for the formula where scientists can upload their data and receive results.
“It’s a pretty simple way to estimate animal size from fossils, and one that has been applied by other scientists using different parts of the skeleton, but not before using cortical thickness,” said Fahey. “I think it was generally assumed that the relationship wouldn’t be very strong, given the variability between species and between bones. But I found there was less variability in cortical thickness than expected, and the approach worked surprisingly well. Sometimes super useful scientific discoveries come from simply testing our assumptions.”
Overall, this new approach improves accuracy, reduces subjectivity and makes it easier to compare findings across different archaeological sites.
“Thirty-five years ago, my students and I showed that these small shaft fragments are critical to our analysis of ancient human behavior, but we lacked formal methods for assigning them to body size, which is critical,” said ASU paleoanthropologist Curtis Marean, Fahey’s faculty advisor and co-author of the study. “So I helped lead Patrick to the important question, and he sealed the deal with great work.”
Marean is a research scientist at the Institute of Human Origins, and Foundation Professor and Virginia M. Ullman Professor of Natural History and the Environment at the School of Human Evolution and Social Change.
“Prior to this formal model, we used the ‘eyeball’ method, which is what we call more formally ‘expert knowledge,'” Marean said. “The problem is that expert knowledge varies by expert and is difficult to test for accuracy and precision. Patrick’s paper shows that the experts varied a lot in their abilities, and his model takes out the guesswork and gives everyone a tool they now can use to get it right.”
Knowing the size of animals humans interacted with in the past is crucial, Fahey explained. It helps scientists understand what early humans ate, how they hunted and how they responded to changes in climate and environment.
At an evolutionary level, humans are unusual predators: Most predators avoid prey much larger than themselves, but humans regularly hunt the largest animals on the landscape. Understanding how this behavior evolved may hold important clues to our ecological success as a species.
“I’m really excited because this formula is going to simplify the process of identifying animal remains so much,” Fahey said. “It is scalable to massive assemblages, it's super cheap and you don’t need any equipment besides calipers. I hope it becomes widespread. We made it usable by anybody across the world, it doesn’t matter about the geographic location or the time period of the specimens.”
The article, “Diaphysis cortical thickness as a predictor of body-size in terrestrial mammals and its application to zooarchaeology,” was recently published in the Journal of Archaeological Science.
Along with this recent work, Fahey was also a co-author on a new publication in Science Advances and produced some of the study’s informational graphics. The article, “Earliest evidence for intentional cremation of human remains in Africa,” sheds new light on the mortuary practices of a 9,500-year-old hunter-gatherer community in central Africa.
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