Variability in the Middle Stone Age of Eastern Africa

In 1967 the Kibish Formation in southern Ethiopia yielded hominid cranial remains identified as early anatomically modern humans, assigned to Homo sapiens. However, the provenance and age of the fossils have been much debated. Here we confirm that the Omo I and Omo II hominid fossils are from similar stratigraphic levels in Member I of the Kibish Formation, despite the view that Omo I is more modern in appearance than Omo II. 40Ar/39Ar ages on feldspar crystals from pumice clasts within a tuff in Member I below the hominid levels place an older limit of 198 +/- 14 kyr (weighted mean age 196 +/- 2 kyr) on the hominids. A younger age limit of 104 +/- 7 kyr is provided by feldspars from pumice clasts in a Member III tuff. Geological evidence indicates rapid deposition of each member of the Kibish Formation. Isotopic ages on the Kibish Formation correspond to ages of Mediterranean sapropels, which reflect increased flow of the Nile River, and necessarily increased flow of the Omo River. Thus the 40Ar/39Ar age measurements, together with the sapropel correlations, indicate that the hominid fossils have an age close to the older limit. Our preferred estimate of the age of the Kibish hominids is 195 +/- 5 kyr, making them the earliest well-dated anatomically modern humans yet described.

Archaeology's main contribution to the debate over the origins of modern humans has been investigating where and when modern human behavior is first recognized in the archaeological record. Most of this debate has been over the empirical record for the appearance and distribution of a set of traits that have come to be accepted as indicators of behavioral modernity. This debate has resulted in a series of competing models that we explicate here, and the traits are typically used as the test implications for these models. However, adequate tests of hypotheses and models rest on robust test implications, and we argue here that the current set of test implications suffers from three main problems: (1) Many are empirically derived from and context-specific to the richer European record, rendering them problematic for use in the primarily tropical and subtropical African continent. (2) They are ambiguous because other processes can be invoked, often with greater parsimony, to explain their character. (3) Many lack theoretical justification. In addition, there are severe taphonomic problems in the application of these test implications across differing spans of time. To provide adequate tests of these models, archaeologists must first subject these test implications to rigorous discussion, which is initiated here.