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2004 Field Season: Day 29

July 21, 2004

I've mentioned several times that much of our work here is to discover what we can about the behavior of early humans. I described a few days ago that the hominins made stone tools largely out of very local rocks. About 98% of the Acheulean tools are from rocks found within 5 kilometers of the sites. You may recall that, when we put all the clues together, we realized that the hominins were probably not trading for other, rarer stone materials. Rather, the groups moved occasionally from one area to another, and brought small bits of distant rocks into the Olorgesailie area.

Another thing we've noticed is that the toolmakers preferred some types of stone material over others when making their tools. We've identified 14 different types of volcanic material that the toolmakers used. Yet the early humans didn't use all the rock types equally. In the Upper Member 1 archeological sites, for instance, the most common raw material was a certain type of basalt, called Ol Keju Nyiro basalt (it's labeled Kb on the image). At most of these sites, the second most common raw material was Mount Olorgesailie phonolite (labeled Oph). You can see in the see in the second image what these raw materials look like. What exactly could explain the preference the toolmakers had for these two materials?

Map of raw materials at Mt. Olorgesailie.

We originally had two ideas. One is that the hominins were choosing the raw materials that made better tools because these were easier to shape or held a sharper edge. The other idea is that the hominins were choosing the raw materials easiest to get - that is, from the closest available sources. To decide between these two explanations, we needed to know several things, including how good the stones were for making tools and the distance between the stone sources and the sites where the tools were discovered. Mike Noll, who worked with us for several years at the Smithsonian and Olorgesailie, performed tests on the raw materials, and Danielle Royer, a graduate student at Stony Brook University, worked with our data on the distances between the sites and the sources.

What they found was interesting. Ol Keju Nyiro basalt was very available, easy to get and close to the sites, but Mt. Olorgesailie phonolite was further away from the sites, and therefore harder to get. Both the basalt and phonolite were the best materials for making stone tools. Mike's engineering tests, conducted when he was a graduate student at the University of Illinois at Urbana-Champaign, showed that, of all the 14 types of volcanic rock, those two materials are the most brittle (good for flaking) and the toughest (giving durable edges). In fact, the ratio of high quality materials to low quality materials doesn't change much at all among the sites spread across the region, no matter if one is closer to a source than another. So this suggests that it wasn't closeness that attracted the hominins, but rather the quality of the raw materials. The toolmakers invested more effort in, and used more of, the high quality raw materials they preferred even if it meant walking and lugging the rocks further.

Some raw materials (Ol Keju Nyiru Basalt and Mt. Olorgesailie Phonolite)

On a camp-life note, Alison Brooks arrived today, and she'll be joining John and Amanda in Locality G, before starting her own excavations over in Locality B. The camp table is starting to feel a little more full now, and in a few days we'll add another table to accommodate a visit by the head of scientific research of the Smithsonian Institution, the Director of the Smithsonian's National Museum of Natural History (where I work), and their families.