The Bouri Formation is an area in the Middle Awash Valley, in Ethiopia that has provided a rich source of Australopithecines and Homo fossils, artifacts and bones of large mammal with cut marks from butchery. It is part of the Afar Depression that has created other rich human fossil sites such as Gona and Hadar.
It consists of three geological units called members in which fossils and artifacts from different periods of human evolution has been excavated. The lowest Hatayae member (2.5 mya) in which Australopithecus garhi fossils have been found, the Dakanihylo member (1 mya) and Homo erectus, and the Herto member lower (260 ka) and upper layers (160 to 154 ka) and Homo sapiens idaltu.
Human remains from the Upper Herto layers have been found with signs of having been changed after death by mortuary practices.
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The Bouri Formation occurs in the Bouri “peninsula”, a geological fault raised horst that diverts the Awash River and forms a partial dam creating Lake Yardi. The peninsula is about 4 km wide and 10 km in length and lies in a NNW-SSE direction in the Quaternary period rift zone of the southern Afar Region.
The Bouri peninsula contains the Bouri formation, a sediment area that stretches down much of its length and breath and is 80 m thick. It is eroded to expose three geological members or layers: the Hatayae (also known as Hata), the Dakanihylo (also known as Daka) and the Herto.[1][2][3][4]
The area is of importance since active tectonics in the southern Afar Depression in the last few million years have created varied types of habitats for early hominids during the Plio-Pleistocene. Then more recently these habitats laid down in sedimentary rocks have been uplifted allowing their erosion and so accessibility to paleoanthropologists.[2] Occasional volcanic eruptions have also left volcanic tuff layers that allow the sedimentary deposits to be accurately argon–argon isotope dated.
The Hatayae layer as its base is 40 m thick made up of variegated silt clay and paleosols, zeolitic and bentonitic tuffs, carbonates that are pedogenic, sandstone with bivalve and gastropod shells, and mudstone. It was deposited in a floodplain along 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. These are most complete for the specimen, BOU-VP-12/130. This species is “descended from Australopithecus afarensis and is a candidate ancestor for early Homo.”[3]
Excavations have in general failed to find large numbers of stone tools. The explanation for this is the lack of raw materials for making them on lake margins. This would have been due to the lack of streams strong enough to carry pebbles, and the absence of nearby basalt outcrops.”[2]
However, in spite of rarity some isolated and widely scattered cores and flakes have been found of Mode I technology. As the excavators note “our surveys and excavations have demonstrated that early hominids were actively using stone tools on the PlioceneHata landscape.”[2] Moreover that “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 taxon recovered from Hata sediments.”[2]
Evidence of the existence of stone tools is also provided by bones of large mammals of such as alcelaphinae (Wildebeest related bovids) and Hipparion (an extinct genus of three-toed horse) by hominids showing butchery cut marks including those used to remove an animal’s tongue.[2] “These are the earliest documented percussion marks made by hominids who were presumably processing these bones for contained fatty marrow. …These are the earliest documented cut marks made by hominids.” [2] As noted by its excavators, the evidence from the site shows that “a major function of the earliest known tools was meat and marrow processing of large carcasses. Finally, they extend this pattern of butchery by hominids well into the Pliocene.”[2]
The Dakanihylo layer is 22 to 45 m thick and made up of pumice sandstone that is cross-bedded. It is dated to 1 mya and found in the southern half of the Bouri horst. Fossils suggest open grassland (377 species of bovids including three new species and two new genera, and water-margin habitats (species of Kobus antelope and abundant Hippopotamus).[4]
In the Dakanihylo Member has been found early Acheulean stone tools such as hand axes and cleavers and evidence of butchery upon equids, bovid and hippo bones.[4]
Homo erectus fossils include specimen BOU-VP-2/66 of an incomplete skull that had an endocranial capacity of 995 cm3.[4] These fossils of H. erectus are important since it has been suggested that Asian and African H, erectus were different human species. But these fossils do not support “the hypothesis of a deep cladogenesis between African and Asian H. erectus” and that “that geographic subdivision of early H. erectus into separate species lineages is biologically misleading, artificially inflating early Pleistocene species diversity.”[4] Moreover, they suggest that the H. erectus taxon had “by 1 Myr the taxon had colonized much of the Old World without speciating. A finding of considerable biogeographic and behavioural significance”.[4]
The Herto layer is named after a local village and consists of a 15–20 m thick layer. It is found in the southwestern part of the Bouri horst and consists of a lower and an upper layer. The division between the Lower and Upper Herto layers is characterized by an erosion surface filled with rounded pebbles.
This consists of lignite, pinkish carbonate layers, and silty clays of predominantly lake origin containing gastropods and bivalves. It is dated to 260 ka. Late Acheulean tools are found together with “hominid remains that are as yet unknown”. Humans in this habitat lived next to a freshwater lake and killed large mammals such as hippopotamids.[1][5]
The Upper Herto Member changes from the fluvial and lake-margin deposits of the lower layer to yellow sandstone and date to between 160 ka and 154 ka BP. Immediately above the erosion surface separating the two layers is volcanic sandstone and gravel deposits that have variable thickness. It is a yellow-brown to grey colour and shows cross-bedded sedimentation containing pumice rocks up to 15 cm in diameter. This layer has produced all the human fossils and tools found in the Upper layer. The Upper layer is topped by a volcanic tuff.[1] An important feature is that two volcanic layers of very fine ash occur one just below the hominid fossils and one just above and this allows an accurate date by argon–argon dating to between 160,000 and 154,000 years ago to be given to adjacent sediment layers and their fossils. This is significant “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 Middle Stone Age tools and the remains of Homo sapiens idaltu. Most of the tools are scrapers, cleavers, and various lithic cores. Hand axes, picks and blades are rare. Most stone tools are made of fine-grained basalt, except for points and blades that were made from obsidian. Many are made with the Levallois technique. They 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]
Of 15 of the 24 recovered fragments of humans in the Upper Herto layer have cut marks due to soft tissue removal. It has been noted that “The latter pattern of bone surface modification is almost never present in hominid 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]