Methods, marks, and models for inferring hominid and carnivore behavior

1. What they did

This is a short reply in the “News and Views” section of the Journal of Human Evolution. Capaldo answers points raised by M. M. Selvaggio about his earlier study of FLK 22, a site at Olduvai Gorge in Tanzania where early hominids and carnivores both left marks on animal bones. Archaeologists use these marks (cut marks from tools, tooth marks from carnivores, and percussion marks from hammerstones breaking bones for marrow) to work out who reached a carcass first, what each got to eat, and how much they competed.

Capaldo responds to three of Selvaggio’s points:

• That scraping meat off bones in his experiments may have damaged or reduced the cut marks.
• That her way of counting cut-marked bones is better than his.
• That only a three-stage experiment can be used to infer a three-stage history at FLK 22.

He supports his case with a table comparing the bone mark results of his “whole-bone to carnivore” experiments (people removed the meat, then spotted hyenas broke the bones) with Selvaggio’s “carnivore to hominid” experiments (carnivores ate first, then people removed the scraps with stone flakes and broke the bones with hammerstones).

2. What they found

On scraping

• In his experiments, meat and skin were cut from long bones and only the thin membrane covering the bone (the periosteum) was scraped off, with a metal knife. The scraps of meat left behind were tiny, and the hyenas did not seem to pay them any special attention.
• He did not record scraping damage to cut marks, but does not recall any cut mark being made unrecognisable. Scrape marks covered only part of the bone surfaces.
• Because he counted bone pieces with at least one cut mark, not the number of marks, he argues scraping could not have changed his cut mark percentages.

On counting

• Selvaggio counts only pieces that have cut marks and no other marks. Capaldo argues this removes cut-marked pieces because they also carry tooth or percussion marks, making cut mark results depend on marks made by other actors for other reasons.
• When he followed her counting method, his cut mark means fell within one standard deviation of hers, so her results were replicated. Tooth mark results also matched. For all long bone pieces together, cut marks were 24.8% in his sample and 34.1% in hers; tooth marks were 63.7% and 65.0%.
• His experiments used no percussion, so percussion marks were absent; these are the only data in the table that reliably separate the two experiments.
• He sees this as falsifying the idea that cut mark percentages reflect how much meat was on the bones, and as showing that his use of a metal knife had no measurable effect on cut marks.

On three-stage models

• His two separate two-stage experiments were meant to control for variation in how both hominids and carnivores process carcasses, and to test two-stage models proposed by Bunn and by Binford. FLK 22 did not match either, which agreed with earlier work by Selvaggio and by Blumenschine.
• Like them, he inferred a three-stage history for FLK 22: cats taking the meat first, hominids then breaking bones for marrow, and hyenas finally removing grease.

3. What they concluded

• It is not a contradiction to infer a three-stage history from two-stage experiments. All these models are hypotheses that need more testing, and models can only be separated when they are compared using the same reporting methods.
• Matching results between different experiments do not make those results invalid. In many other cases tooth mark and cut mark percentages differ enough to infer behaviour.
• Cut mark percentages are currently the most ambiguous evidence. New experiments are needed that compare metal and stone tools, and that take account of the butcher’s experience; Capaldo says he is doing these.
• Work on how tooth marks differ between carnivore families should continue. All three-stage models for FLK 22 assume cats were active first and hyenas last, and better knowledge of tooth marks might even show which cat (for example lion or leopard) made them.

6. Other technologies, skills or innovations

• Experimental carcass processing: people defleshing, skinning and scraping long bones with a metal knife, followed by spotted hyenas breaking and feeding on the bones, compared with experiments using stone flakes and hammerstones.
• Recording bone surface marks by bone portion: bone ends, pieces near the ends, and midshaft pieces.