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Butchery marks on bones get more accurate test

A new approach accurately measures animal bone cut marks made by prehistoric human butchery.

“In archaeology, butchery marks on animal bones are a key piece of evidence…”

The combinations of 3D imaging, shape analysis, and Bayesian statistics to identify butchery cut marks with an 88 percent success rate in classifying butchery behaviors. The 3D-imaging technology is similar to what engineers use to measure scratches on microchips and surgical blade sharpness.

“This approach represents a major improvement in accuracy when compared to many archaeological methods, and improving this technique will help us get the human evolution story correct,” says research team leader Erik Otárola-Castillo, assistant professor in the anthropology department at Purdue University.

“By strengthening quantitative methods to evaluate archaeological evidence, we will be able to learn more about early humans much more quickly.”

Sheep, deer, and bison

Human-made stone tools leave cut marks on animal bone, such as sheep, deer, or bison, through butchery. These cut marks can vary in size from 1 to 5 centimeters, but the depth of the cuts is often tiny, measuring at approximately 1/15 of a millimeter. Archaeologists often attempt to distinguish between cut marks made by a human’s stone tool, which leaves a “V” shape, and other damage, such as trampling by a hoofed animal, which can leave marks with more of a “U” shape.

“In archaeology, butchery marks on animal bones are a key piece of evidence used to answer questions about food acquisition in prehistoric hunter-and-gatherer populations,” says Otárola-Castillo, who studies hunters-gatherers’ diets to answer questions about human evolution.

Tools, hooves, or crocodile teeth?

Archaeologists have attempted to identify butchering marks since the 1800s. While the methods of analyzing bone marks have improved over the decades, there is still quite a lot of uncertainty and lack of consensus on how to best measure them. Current measurement techniques range from naked eye qualitative assessments to high-powered microscopy, such as scanning electron microscopy or micro-photogrammetry.

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There are cases when marks can be ambiguous. In 2010, a couple of research teams published contradicting findings regarding bone marks from a prehistoric site, Dikika in Ethiopia. Results from their analyses varied so much they disagreed whether the marks resulted from humans’ tools or animals’ hooves, or perhaps scratches from sand particles or other rock edges. Fresh debate now contends that feeding crocodiles could have made some of these marks.

“The tension over this debate has been great, because the source of the cuts matters significantly. If made by stone tools, then this is an example of stone tool-use by some of the earliest human ancestors,” Otárola-Castillo says. “Recently, researchers sent bones with cut marks to various experts for analysis. The goal was to evaluate the convergence of different experts’ assessments of the characteristics of the bone marks using traditional methods. Alarmingly, results showed that the qualitative assessments were inconsistent between all experts. Similar concerns also relate to modern forensics evidence, so our work is a finding that may be of interest to that field as well.”

Otárola-Castillo, Curtis W. Marean from Arizona State University, Jessica C. Thompson from Emory University, and Shannon P. McPherron from the Max Planck Institute for Evolutionary Anthropology began working on methodological improvements for bone mark analysis in 2010. McPherron and Marean were members of one of the teams evaluating bones from the Dikika site in Ethiopia.

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In their new study, Otárola-Castillo and the team used more than 40 cuts and slices made by volunteer butchers on sheep bones using stone tools. A “cut” represents a 90-degree incision angle, and a “slice” represents a 45-degree incision angle. The researchers then measured the cuts with a profilometer, a 3D microscope that measures topography, roughness, and layer thickness in the micro- and nanometer ranges. After measuring them, the researchers conducted a 3D shape and size analysis of the curves and surfaces to compare the cut marks.

“Once the digital data were compared, we used Bayesian statistics, which provide a quantitative measure of scientific believability. For example, given the evidence, the probability of accurately determining the identity of a mark on a bone is 88 percent in this case,” Otárola-Castillo says.

Emma James from The University of Queensland is also a coauthor of the study, which appears in the Journal of Archaeological Science.

Funding came from a variety of sources, including the National Science Foundation, the Margo Katherine Wilke Undergraduate Research Fund at Purdue University, Purdue’s College of Liberal Arts ASPIRE, and a grant from the John Templeton Foundation to the Institute of Human Origins at Arizona State University.

Source: Purdue University

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