The Bouri Formation is a sequence of sedimentary deposits that is the source of
Afar Depression that has provided rich human fossil sites such as Gona and Hadar
.
The Bouri Formation stretches down much of the length and breadth of the Bouri "peninsula", which projects across the dry bed of the Afar Depression. The Formation is sufficiently eroded to expose three geological members or layers: the Hatayae, the Dakanihylo, and the Herto. Human remains with signs of having been prepared for burial have been found in the Upper Herto layers.
hominins across the most recent few million years known informally as the Plio-Pleistocene. These habitats, laid down in sedimentary rocks, have since been uplifted, which allowed their erosion over time and their accessibility today to paleoanthropologists.[2] Volcanic eruptions have left volcanic tuff layers that enable accurate dating of the sedimentary deposits via Argon–argon dating
.
Hatayae
The Hatayae layer is 40 m thick at its base and is composed of variegated
gastropod shells, and mudstone. It was deposited in a floodplain alongside river delta channels and a shallow fluctuating lake dated to around 2.5 mya.[2]
In the Hatayae member have been found the remains of Australopithecus garhi, the most complete specimen of which is BOU-VP-12/130. According to Asfaw and White, et al. (1999), that species is "..descended from Australopithecus afarensis and is a candidate ancestor for early Homo."[3]
Excavations in general have failed to disclose large numbers of stone tools. The likely explanation is the lack of raw materials on lake margins, which, in turn, is likely due to the lack of streams strong enough to carry hand-sized pebbles and the absence of nearby outcrops of basalt.[2]
In spite of rarity, some isolated and widely scattered cores and flakes of Mode I technology have been found. The excavators (de Heinzelin, et al., 2002) report: "..our surveys and excavations have demonstrated that early hominids [hominins] were actively using stone tools on the Pliocene-Hata landscape"; and: "It is not currently possible to positively identify the creators of the earliest stone tools here or at Gona, even though A. garhi is currently the only recognized hominid [hominin] taxon recovered from Hata sediments."[2]
Existence of stone tools is also evidenced by bones of large mammals—such as
bovids) and Hipparion, an extinct genus of three-toed horse—that show butchery cut marks by hominins including those made in removing an animal's tongue. "These are the earliest documented percussion marks made by hominids [hominins], who were presumably processing these bones for contained fatty marrow"; and, "These are the earliest documented cut marks made by hominids [hominins]."[2] As concluded by the excavators, the evidence from the site shows "..a major function of the earliest known tools was meat and marrow processing of large carcasses."[2]
This pattern of butchery processing of large prey was maintained by hominins well into the Pliocene.
Dakanihylo
The Dakanihylo (Daka) layer is 22 to 45 m thick and composed of
bovids (including three new species and two new genera); and water-margin habitats—species of Kobus antelope and abundant Hippopotamus.[4]
endocranial capacity of 995 cm3.[4] These fossils (of H. erectus) are important to the current debate as to whether Asian and African H. erectus were actually different human species. Asfaw, et al. (2002) reports: 1) that these fossils do not support "..the hypothesis of a deep cladogenesis between African and Asian H. erectus ..."; and 2) "..that geographic subdivision of early H. erectus into separate species lineages is biologically misleading, artificially inflating early Pleistocene species diversity."[4] Further, they suggest that "..by 1 Myr the taxon had colonized much of the Old World without speciating. A finding of considerable biogeographic and behavioural significance".[4]
Herto
The Herto layers consist of a 15–20 m thick main sequence found in the southwestern part of the Bouri peninsula. The division between the Lower and Upper Herto layers is characterized by an erosion surface filled with rounded pebbles.
The Upper Herto member changes to yellow sandstone (from the
volcanic sandstone and gravel deposits of variable thicknesses. It shows cross-bedded sedimentation containing pumice rocks and has produced all the human fossils and tools found in the Upper layer. The Upper layer is topped by a volcanic tuff.[1] Two volcanic layers of very fine ash occur, one just below the hominin fossils and one just above; this important feature allows accurate argon–argon dating of adjacent sedimentary layers and their fossils, as reported above. This is valuable "..because the accurate dating of faunas and artefacts of many sites of this general antiquity in Pleistocene Africa has proved notoriously difficult."[1]
In this layer have been found early
Homo sapiens idaltu. Most of the tools are scrapers, cleavers, and various lithic cores; but hand axes, picks and blades are rare. Most stone tools are made of fine-grained basalt except for points and blades made from obsidian. Many are made with the Levallois technique; these are comparable to those found in the Garba III layer at Melka Kunture.
As at Herto, Garba III includes terminal Acheulean hand axes, typical Levalloisian method, and many retouched tools on flakes (side-scrapers and end-scrapers, backed knives, burins, unifacial and bifacial points). The Garba III assemblage has been considered transitional between the Acheulean and the MSA.[1]
In this layer are found a large number of Hippopotamus bones: "One occurrence shows abundant remains of several hippo calves, mostly newborn to a few weeks old, scattered together with butchered adults".[1]
Mortuary practices
Of 15 of the 24 recovered fragments of humans in the Upper Herto layer have cut marks due to soft tissue removal. Clark, et al., (2003) reports that "The latter pattern of bone surface modification is almost never present in hominid [hominin] or nonhuman faunal remains processed for consumption, and is therefore unlikely to represent evidence of utilitarian or economic behaviour." On one skull, "..this defleshing manipulation must have occurred after removal of the mandible. The intentional and deliberate removal of soft tissues such as basicranial vessels, nerves and muscles is therefore indicated. The specimen lacks the entire occipital region surrounding the foramen magnum, and the edges of this broken region are smooth and polished, as are the specimen’s unweathered parietal surfaces."[1]
Ethnographic study upon modern cultures suggests that such post-mortem manipulation could be due to "..curation of human remains as part of mortuary practices".[1]
