In recent years, researchers interested in the origins of anatomically modern Homo sapiens have focused their attention on Omo-Kibish in southern Ethiopia. In 1967, a team led by Richard Leakey recovered three fossil hominin specimens from this region. The age and affinities of these specimens were a matter of much debate for nearly 40 years. In 1999, field expeditions to Omo-Kibish were renewed with the goal of shedding some light on these issues, as well as recovering new fossil specimens and archeological materials.

The geological outcrops at Omo-Kibish, which stretch back approximately 200,000 years, are grouped into what is known as the Kibish Formation. The most important of these outcrops lie west of the Omo River. The sediments in this formation are unique in that they lay flat and have been relatively undisturbed by tectonic activity. Alternating layers of sandstone, siltstone, claystone, mudstone and tuff, which were laid down predominately in deltaic environments, comprise the sequence. The Kibish Formation is divided into four members, numbered I-IV from the base upwards. Each is separated from the next by an erosional break created by the Omo River. Following rapid sediment accumulation, short periods of time allowed the Omo to erode the sediments as much as 25 m before the deposition of the next member. The sequence is not even throughout due to these periods of incision. The entire formation is approximately 105 m thick, with the thickest part of the sequence around 40 m. Each of these members was deposited when Lake Turkana was between 40 and 85 m higher than it is today and extended northward by an additional 100 km. The increased volume of the lake was a direct result of increased rainfall in the Ethiopian Highlands.

Very little fauna was recovered during the original excavations in the 1960s. However, recent fieldwork at Omo-Kibish has produced a rich and diverse collection of mammals, birds and fish. The most common vertebrate fossils found are fish. The large majority of this collection consists of the Nile perch, Lates niloticus, followed by catfish, such as Synodontis. In addition, characiforms, cypriniforms, mormyriforms and tetraodontoforms are also present. The mammalian fossil assemblage consists of extant species found in the region today. Surprisingly, there have been no specimens recovered which can be unequivocally assigned to an extinct taxon. The majority of this assemblage is represented by bovids (>60%), followed by equids and suids. Primate and carnivore fossils are relatively rare. With the exception of guineafowl, the entire fossil bird collection at Omo-Kibish is made up of waterfowl. These include pelicans, darters and herons.

Omo-Kibish is perhaps best-known as the site which has yielded the oldest anatomically modern Homo sapiens to date. The fossils Omo I and Omo II, were first discovered in 1967. Omo I consists of a fragmentary calvaria (i.e., skullcap), a partial mandible, dentition and several elements of the postcranial skeleton. Omo II consists of a nearly complete neurocranium. The fossils were originally reported to have been recovered from comparable stratigraphic levels dated to around 130,000 years BP. However, a considerable amount of controversy surrounded these conclusions. In addition, many researchers claimed that Omo II possesses a number of primitive features not shared with Omo I. This led to the conclusion that Omo II represents an archaic form, possibly having closer affinities with Homo erectus or Homo heidelbergensis, and that Omo I represented an anatomically modern human. These traits include a long and low cranial vault, sagittal keeling (i.e., a thickened ridge of bone along the midline of the top of the skull), a large occipital torus (i.e., a bony projection at the back of the skull) and flat parietal bones. Omo I possesses a suite of modern characters, such as a well-developed chin, a rounded occipital bone and the absence of a sagittal keel. However, primitive features, such as a prominent supraorbital torus (i.e., brow ridge), can be found in this specimen as well. Despite these apparent primitive features, there is an overwhelming consensus that both specimens represent fully modern humans. A third specimen, Omo III, consists of a few cranial and facial fragments with very few diagnostic features. This specimen has not been confidently attributed to any species and the exact location from which it was discovered is unknown.

In 1999, a team of researchers returned to Omo-Kibish with the hope of resolving some of the controversy surrounding the dates and locations of the hominin fossils found there. The first step was relocating the site using old photographs and crude maps from the original 1967 expedition. Researchers were then able to confirm that both Omo I and Omo II did indeed come from comparable stratigraphic levels. Both were recovered from Member I of the Kibish Formation. Dating on feldspar crystals from pumice clasts in Member I provided an older limit of around 196,000 years BP. Unfortunately, there were no dateable materials from Member II so researchers were forced to use those from Member III. This provided a younger limit of approximately 104,000 years BP. Direct isotopic dating techniques were unable to narrow this age bracket, but researchers noticed that the dates from various stratigraphic levels in the Kibish Formation were remarkably similar to the dates recorded from Mediterranean sapropels. Sapropels are organically rich dark-colored muds at the bottom of the ocean which result from anoxic conditions caused by increased inflows of water from the Nile River system. These increases were themselves a direct result of higher levels of rainfall in the Ethiopian Higlands. Interestingly, these increased periods of rainfall are the same which caused fluctuations of the depth of Lake Turkana and the subsequent deposition of each member of the Kibish Formation. With this discovery, researchers were able to reliably correlate dates from the Kibish Formation with those from the Mediterranean sapropels. They concluded that the age of Omo I and Omo II was 196,000 +/- 2 years. This new date makes the Omo-Kibish hominins the oldest dated anatomically modern Homo sapiens yet discovered. This title was previously help by the human fossils from Herto, dated to around 154,000 years BP.

In addition to obtaining new dates for Omo I and II, recent field expeditions have recovered additional fossils and archeological materials. New postcranial elements associated with the Omo I skeleton include a virtually complete left clavicle and several bones of the hand and foot. These new finds provide additional evidence for the specimen’s status as an anatomically modern human. For example, a comparative study of the Omo I clavicle concluded that the degree of curvature fits comfortably within the modern human range of variation. Recent, more detailed, comparative studies of the original material also identify modern features previously unrecognized in the Omo I postcranial skeleton. These include a large projection of the olecranon process of the right ulna. These studies also suggest that the morphology of the acetabulum (i.e., the concave articular surface where the head of the femur meets the pelvis to form the hip joint) indicates the individual may have been female. They have also identified several peculiar features shared with Neandertals and modern humans from Eurasia which once were interpreted as evidence for gene flow between these two groups. The presence of these features in the much older Omo I specimen indicate that they may actually have an earlier African origin. In addition to the new Omo I fossils, a partial left tibia and a fibula contemporaneous with the Omo I and Omo II fossils, as well parietal and occipital fragments from Member III or IV, have been recovered.

The region today is characterized by an arid to semiarid climate with high temperatures and low precipitation. Bordering the Omo River are patches of closed to open woodland which open laterally into vast floodplains. These floodplains are much drier and consist of mainly grassland, scrubland and bushland. Bovids are the most common mammalian taxa at Omo-Kibish. In addition, hippo, several species of suid, giraffe, zebra, black rhino, guerza monkey, blue monkey, De Brazza’s monkey, vervet monkey, anubis baboon, galago, elephant, wild dog, fox, jackal, cheetah, leopard, lion, and hyena can be found here. Paleoenvironmental reconstructions of Omo-Kibish suggest habitats very similar to present day conditions. Fossil taxa, such as the giant forest hog, indicate closed wet habitats with extensive canopy cover. Fossil fish from Member I suggest shallow swampy water, whereas those from Member IV suggest well-oxygenated open water. The large number of waterfowl is also indicative of bodies of water with an abundant source of fish. The presence of grazing bovids clearly suggests that the floodplains of the Omo River supported vast edaphic grasslands.

The 1967 excavations at Omo-Kibish originally recovered a relatively small number of stone tool artifacts. Recent excavations at three sites in the Kibish Formation have yielded a more extensive collection. The majority of stone artifacts are represented by debris. However, flakes, cores, choppers, scrapers, blades, points, awls and handaxes have also been found. Retouched tools are relatively rare. The most commonly used raw materials at these sites were chert, jasper and chalcedony.

References

Assefa Z, Yirga S, Reed KE. 2008. The large-mammal fauna from the Kibish Formation. Journal of Human Evolution 55: 501-512.

Brown FH, Fuller CR. 2008. Stratigraphy and tephra of the Kibish Formation, southwestern Ethiopia. Journal of Human Evolution 55: 366-403.

Butzer KW. 1969. Geological interpretations of two Pleistocene hominid sites in the Lower Omo Basin. Nature 222: 1133-1135.

Butzer KW, Brown FH, Thurber DL. 1969. Horizontal sediments of the lower Omo Valley: the Kibish Formation. Quaternaria 11: 15-29.

Clark JD, Beyene Y, WoldeGabriel G, Hart WK, Renne PR, Gilbert H, Defleur A, Suwa G, Katoh S, Ludwig KR, Boisserie J-R, Asfaw B, White TD. 2003. Stratigraphic, chronological and behavioural contexts of Pleistocene Homo sapiens from Middle Awash, Ethiopia. Nature 423: 747-752.

Day MH. 1969. Omo human skeletal remains. Nature 222: 1135-1138.

Day MH, Stringer CB. 1982. A reconsideration of the Omo-Kibish remains and the erectus-sapiens transition. In Congres International de Paleontologie Humaine, ed. MA de Lumley, pp. 814-846. Louis-Jean Scientific and Literary Publications: Nice.

Day MH, Stringer CB. 1991. Les restes crannies d’Omo-Kibish et leur classification a l’interieur du genre Homo. Anthropologie 95: 573-594.

Day MH, Twist MHC, Ward S. 1991. Les vestiges post-craniens d’Omo (Kibish). Anthropologie 95: 595-610.

Feibel CS. 2008. Microstratigraphy of the Kibish hominin sites KHS and PHS, Lower Omo Valley, Ethiopia. Journal of Human Evolution 55: 404-408.

Fleagle JG, Assefa Z, Brown FH, Shea JJ. 2008. Paleoanthropology of the Kibish Formation, southern Ethiopia: introduction. Journal of Human Evolution 55: 360-365.

Leakey REF. 1969. Early Homo sapiens remains from the Omo River region of south-west Ethiopia. Nature 222: 1132-1133.

Louchart A. 2008. Fossil birds of the Kibish Formation. Journal of Human Evolution 55: 513-520.

McDougall I, Brown FH, Fleagle JG. 2005. Stratigraphic placement and age of modern humans from Kibish, Ethiopia. Nature 433: 733-736.

McDougall I, Brown FH, Fleagle JG. 2008. Sapropels and the age of hominins Omo I and Omo II, Kibish, Ethiopia. Journal of Human Evolution 55: 409-420.

Pearson OM, Fleagle JG, Grine FE, Royer DF. 2008. Further new hominin fossils from the Kibish Formation, southwestern Ethiopia. Journal of Human Evolution 55: 444-447.

Pearson OM, Royer DF, Fleagle JG, Grine FE. 2008. A description of the Omo I postcranial skeleton, including newly discovered fossils. Journal of Human Evolution 55: 444-447.

Rightmire GP. 1984. Homo sapiens in sub-Saharan Africa. In The Origins of Modern Humans: A World Survey of the Fossil Evidence, eds. FH Smith and F Spencer, pp. 295-325. Alan Liss co: New York.

Rightmire GP. 1989. Middle Stone Age humans from eastern and southern Africa. In The Human Revolution, eds. P Mellars and CB Stringer, pp. 109-122. Edinburgh University Press: Edinburgh.

Shea JJ. 2008. The Middle Stone Age archaeology of the Lower Omo Valley Kibish Formation: excavations, lithic assemblages, and inferred patterns of early Homo sapiens behavior. Journal of Human Evolution 55: 448-485.

Trapini J. 2008. Quaternary fossil fish from the Kibish Formation, Omo Valley, Ethiopia. Journal of Human Evolution 55: 521-530.

Voisin J-L. 2008. The Omo I hominin clavicle: archaic or modern? Journal of Human Evolution 55: 438-443.