2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 7
Presentation Time: 8:00 AM-12:00 PM


ADELSBERGER, Katherine A., Department of Geology, Beloit College, 700 College St, Beloit, WI 53511, WIRTH, Karl R., Geology Department, Macalester College, 1600 Grand Ave, St. Paul, MN 55105 and MABULLA, Audax Z.P., Archeology Unit, University of Dar es Salaam, PO Box 35050, Dar es Salaam, Tanzania, adelsber@stu.beloit.edu

Much information used to reconstruct early human behavior has been inferred from the analysis stone tools. The objective of this study is to determine whether the raw materials used by "archaic" Homo sapiens (200 Ka) at the Laetoli Archaeological Site (LAS), Tanzania were derived locally, or whether they were transported from distant sources. More than 200 Middle Stone Age stone tools (hammerstones, cores, and flakes) were collected by surface survey from six LAS localities. These stone tools erode from the upper Ngaloba Beds, which have also yielded a cranium of an archaic Homo sapiens (LH18) dated to about 200 Ka. Most stone tools were manufactured from basalt (>60%), and these are compared with 60 samples of nearby exposures of basaltic lava (Ogol Lava Beds; 2.41 +/- 0.12 Ma). On the basis of texture, mineralogy, and major and trace-element compositions (see Wirth and Adelsberger, this volume), we conclude that at least two non-Ogol sources were used in the manufacture of the Laetoli basaltic tools.

The basaltic stone tools are aphanitic and non-vesicular whereas samples of Ogol Lavas contain abundant phenocrysts of olivine and clinopyroxene and are commonly vesicular. Furthermore, the tools and Ogol lavas have distinct major ( e.g., SiO2, Na2O, Mg#) and trace (e.g., La/Nb, Nb/Y) element compositions. These data indicate that the basalts utilized for toolmaking were derived from non-Ogol sources. Possible nearby sources included Lemagrut, Sadiman, and Oldeani volcanoes which are 20-30 km distant from Laetoli suggesting that "archaic" H. sapiens at Laetoli had a wide geographic range. Alternatively, basalt from non-local sources may have been locally available to early humans as transported fragments in streams. Although the source of the basalt used in the manufacture of stone tools at the LAS was not be determined in this study, it is clear that the widely-available and local Ogol basalts were not preferred by archaic Homo sapiens. Recent studies have shown that tools produced from vesicular and porphyritic basalt produce a poorer cutting edge than do tools produced from fine-grained rocks and apparently the archaic Homo sapiens of the Laetoli region comprehended these attributes as early as 200,000 years ago.