Bouri Formation

The Bouri "peninsula" is a geological fault-raised horst that diverts the Awash River, forming a partial dam and creating Lake Yardi; and it contains the Bouri Formation. The peninsula is about 4 km wide by 10 km long, and lies in a NNW-SSE direction in the Quaternary-period rift zone in the southern part of the Afar Depression (Afar Triangle).

The Bouri Formation is a sediment area some 80 m thick that stretches down much of the length and breath of the peninsula. The formation consists of three geological units, called members, from which fossils and artifacts of different periods of human evolution have been excavated. They are: the lowest member, named Hatayae, or the Hata, dated to 2.5 million years ago (mya)—and in which fossils of Australopithecus garhi were found; the Dakanihylo, or Daka, member, one (1) mya—fossils of Homo erectus were found; and the Herto member, Lower layer at 260 kya, and Upper layers at 160 to 154 kya—fossils of Homo sapiens idaltu were found.[1][2][3][4]

The area is important because active tectonics in the Afar Depression created varying habitats for early 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.

The Hatayae layer is 40 m thick at its base and is composed 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 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 alcelaphinae (Wildebeest related 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.

The Dakanihylo (Daka) layer is 22 to 45 m thick and composed of pumice sandstone that is cross-bedded; it is situated in the southern half of the Bouri horst and dated to 1 mya. Fossils found in the layer suggest open grassland—some 377 species of bovids (including three new species and two new genera); and water-margin habitats—species of Kobus antelope and abundant Hippopotamus.[4] Early Acheulean stone tools such as hand axes and cleavers have also been found in the Daka member, as well as evidence of butchery processing on equid, bovid and hippo bones.[4]

Homo erectus fossils include the specimen BOU-VP-2/66, or the Daka skull, an incomplete skull that had an 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]

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.

• Omo remains
• List of fossil sites
• List of fossiliferous stratigraphic units in Ethiopia