Located west of the Awash River and spanning an area of more than 5,000 km, the Middle Awash area of Ethiopia is one of the most important regions in East African Paleoanthropology. Beginning with the pioneering work of geologists in the 1960s, and continuing through the present day work conducted by the Middle Awash Research Project, this area has been at the forefront of research into human evolution. Within the Middle Awash are numerous fossiliferous localities that span the late Miocene through the Middle Pleistocene (i.e. from about ~6 million years ago to ~200 thousand years ago)1, providing a wealth of insight into hominin evolution that includes some of the earliest hominin species (e.g., Ardipithecus kadabba and Ardipithecus ramidus), several species of australopithecine, Homo erectus, and even our own species Homo sapiens.
Lead by French geologist Maurice Taieb, the International Afar Research Expedition team first explored the Middle Awash region in 1971 and 1972. Investigations in the mid 1970’s were conducted by the Rift Valley Research Mission in Ethiopia (RVRME), created and led by geologist Jon Kalb. A partial hominin cranium was discovered at the Bodo locality during the RVRME’s extensive explorations of the region. Beginning in 1981, work under The Middle Awash Project, led by Tim White, resulted in over 10,000 vertebrate fossils, important artifacts, as well as a suite stratigraphic and paleoenvironmental information1.
Notable sites such as Asa Koma on the western margin of the Middle Awash study area have yielded a small, but highly important sample of hominin fossils. In 1997, a team lead by Yohannes Haile-Selassie and Giday WoldeGabriel discovered the fossilized remains of a very early hominin dating to nearly 5.8 million years ago (Ma), Ar. Kadabba2. Other sites have also yielded many important paleontological discoveries. In 1992, the 4.4 Ma remains of an early fossil hominin were discovered by the Middle Awash Project at Aarmis, and assigned to the species Ar. ramidus3. White’s team is also credited with the discovery of Australopithecus anamensis, an early member of the genus Australopitethecus whose remains were first recovered at Asa Issie, approximately 10 km west of the Aramis locality.
One of the younger sites in the Middle Awash includes Bouri, which is located in the southern part of the region and has been a subject of paleoanthopological research since the 1990s. From 1996-1998, a team led by White recovered the remains of Australopithecus garhi, a previously unknown hominin that emerged at a crucial time between the disappearance of Australopithecus afarensis, and the advent of Homo.
While the paleontological data have been rich, the geology of the region has long been problematic and early attempts to determine an accurate stratographic sequence were hampered by confounding issues like faulting and uplift. In addition, there is very little similarity between same age deposits exposed only short distances apart1. Breakthroughs in radiometric dating, however, coupled with tephrastratigraphic correlation with other East African basins more reliably dated (e.g. Hadar) have helped to resolve many of these issues1.
In addition to sampling a long sequence of hominin evolution, the Middle Awash preserves evidence of paleoenvironmental change over the course of more than 5 million years. These include the shift from largely lacustrine deposits older than 3.9 Ma to fluvial deposits younger than 3.9 Ma1. Additionally, the geological, paleontological, and geochemical data show evidence of a shift from wetter, wooded habitats associated with the earliest hominins, such as Ardipitehcus and A. anamensis, to drier and more open habitats beginning with the emergence of A. afarensis. These major changes in paleoenvironments can provide insight into conditions affected hominin evolution.
Cultural and technological innovation is also documented in the area, including evidence of early stone tool use at the Bouri locality that demonstrates hominins were proficient at processing animal remains for food as early as 2.5 Ma. Later sites, such as Daka-Bouri and Bodo, preserve more advanced archaeological material that documents the transition between early and late stage Acheulean technology, which might be associated with evolutionary shifts from Homo erectus to Homo heidelbergensis.
In total, the hominin material from the Middle Awash currently represents some of the earliest evidence in the fossil record for hominin bipedality and tool. Combined the fossil material with paleoecological and geological information, the Midde Awash provides one of the most complete datasets for inferring about the anatomical adaptations, behaviors, and environmental conditions effecting early hominin evolution.
The hominin record in the Middle Awash constitutes some of the oldest evidence for bipedal hominin evolution. At least 3 species spanning almost across five million years have been identified at different Middle Awash localities.
Ardipithecus kadabba
Some of the oldest hominin material in the Middle Awash derives from Asa Koma and its associated localities (Saitune Dora, Alayla, and Digiba Dora). The western margin of the Middle Awash has been the subject of study since 1992, but hominin fossils were not found in the area until 1997 when a very early hominin known as Ardipithecus kadabba was discovered by a team led by Yohannes Haile-Selassie and Giday WoldeGabriel. These hominin fossils have been dated by 40Ar/39Ar radiometric dating to ~5.8-5.2 Ma, and represent the earliest member of the Ardipithecus genus.
The Asa Koma area fossil material represents at least 7 individuals and consists primarily of dental and gnathic (i.e. mandibular and maxillary) specimens, including a partial mandible with potentially 5 associated teeth and 8 additional isolated teeth. In addition, postcranial remains have been recovered from Asa Koma, including fragments of the humerus (ASK-VP-3/78) and ulna (ALA-VP-2/101), the clavicle (STD-VP-2/893), and a hand phalanx (ALA-VP-2/11).
The Ar. kadabba fossils were initially attributed to a subspecies of Ardipithecus ramidus, but differences in the dentition eventually warranted a separation at the species level. The hominin status of Ar. kadabba is based on the upper canines, which in both overall shape and cross-sectional geometry appear to be trending in the direction of Ar. ramidus as opposed to chimpanzees. However, there are some apelike attributes to the teeth in that the upper canine sharpened itself, or was “honed”, against the lower canine (C1) and the third premolar (P3), a condition found in living apes but not in later hominins. In fact, it was the presence of this “honing complex” that prompted the specific level designation for Ar. kadabba.
Postcranial remains attributed to Ar. kadabba are less conclusive with regard to its hominin status. The sole Ar. kadabba specimen relating to locomotor behavior is a complete pedal phalanx (AME-VP-1/71) found at Amba East, a locality in the Central Awash Complex. This phalanx is one of the youngest A. kaddaba fossils (~5.2 Ma) and shares traits with both living apes and Australopithecus afarensis. Specifically, the strong apelike curvature seen in the shaft is combined with a dorsally-oriented proximal articular surface, which is similar to the condition found in later hominins and could provide some evidence for bipedal locomotion. However, this dorsal canting of the proximal joint surface is also found in the Miocene hominoid Sivapithecus, which did not engage in bipedalism.
Ardipithecus ramidus
The phalanx specimen of Ardipithecus kadabba is only one of the notable hominin specimens to come out of the Central Awash Complex. In 1992, a team of researchers led by Tim White found the first hominin specimens at Aarmis, arguably one of the most important Middle Awash sites. Subsequent investigations at Aramis have produced over 100 hominin specimens, representing at least 36 individuals. When first reported in 1994, the hominin assemblage consisted of 13 dental fragments, assorted cranial fragments, and postrcranial elements that included an associated lower arm (left radius and ulna)1.
The small sample size and incomplete nature of the material originally led researchers to tentatively attribute the specimens to a new species of Australopithecus, namely Australopithecus ramidus. Just a year later, the Aramis material was reassigned to the new genus Ardipithecus, based on observed distinct differences between Ar. ramidus and other australopiths. For example, the Aramis post canine dentition was much smaller than, and fell outside the range of Australopithecus. This species reassignment to Ar. ramidus also suggested Ar. ramidus as the sister taxon to all other hominins.
It wasn’t until 14 years later, in 2009, that a full description of the hominin material was published in a special issue of the journal Science. This publication described a partial skeleton identified as ARA-VP-6/500, dubbed “Ardi”. The ~4.4 Ma skeleton contains elements comprising the skull and face, arms and hands, legs and feet, as well as the spine and pelvis.
The skeleton’s constituent material was poorly fossilized, highly fragmentary (possibly due to trampling) and required intensive preparation and reconstruction, both physical and virtual (e.g., using 3D scanning and reconstruction methods). These analyses produced a wealth of information about Ardi, as well as the species Ar. ramidus, including suggestions as to their phylogentic position in the hominin lineage.
The digital reconstructions revealed that Ardi stood around 120 cm tall and weighed ~50 kg. The relatively reduced canines suggest that she is female, though her postcrania (e.g. humerus) indicate that she is the 2nd or 3rd largest of the 8 Ar. ramidus individuals for whom measurements could be made, including some males. Both canine size and body size measurements have led researchers to conclude that Ar. ramidus did not display significant body size dimorphism - perhaps similar to what is seen in chimpanzees.
The craniodental anatomy of the Ar. ramidus specimens may be close to that of the last common ancestor hominins shared with the African apes. However, some scientists have argued that extant apes represents the more derived condition. If this view is correct, it would mean that using modern African apes as proxies for the hominin ancestral condition is misguided.
Though found in numerous scattered pieces, a replica of Ardi’s nearly complete cranium was produced using microCT scanning. Based on these digital reconstructions, the Ar. ramidus specimen’s cranial capacity sits between 300 and 350 cc. This cranial capacity is similar to female chimpanzees and both sexes of bonobo, but smaller than all later hominins (e.g., australopiths are typically between 400-550 cc). The basicranium of Ar. ramidus is similar to australopiths in that it exhibits a short cranial base and a downturned posterior aspect of the skull. Both features are thought to be primitive hominin traits, and are also shared with the older (~6-7 Ma) purported hominin Sahelanthropus tchadensis from Chad.
The skull lacks other australopithecine traits such as large cheekbones indicative of strong chewing musculature. Ar. ramidus exhibits significant prognathism in the face, though the lower face projects comparatively less than chimpanzees. The upper canines and incisors are also reduced when compared to chimpanzees. The dentition differences in Ar. ramidus may indicate less ripe-fruit frugivory specializations (as in chimpanzees) and a trend toward a more omnivorous diet. Further, Ar. ramidus molars have a more generalized molar occlusal morphology than modern apes. This is similar to the morphology seen in australopiths, though Ardipithecus species lack the postcanine megadonty seen in later hominins. The thickness of the enamel in Ar. ramidus molars is also relatively less than the Australopithecus condition. Both the increased molar size and enamel thickness suggests a shift to a diet consisting of more abrasive foods.
Stable isotope analysis has also shed light on the feeding behavior of Ar. ramidus. Carbon isotope analysis has shown that the percentage of C4 plants exploited by Ar. ramidus is significantly lower than that found in Australopithecus species, indicating feeding in a more closed environment (i.e., woodland). However, the proportion of C4 plants Ar. ramidus exploited is not as low as that found in chimpanzees, indicating Ar. ramidus’ diet was less restrictive.
Aspects of the canine/third premolar complex show further distinction between Ardiptiehcus and extant African apes. Unlike modern chimpanzees, Ar ramidus exhibits little to no canine sexual dimorphism, and lacks a honing complex between the upper canines and lower third molar. Ar. ramidus also exhibit an australopith-like canine eruption schedule (as seen on a Ar. ramidus juvenile male specimen), which is delayed when compared to chimpanzees. These features could provide evidence that enlarged canines were not important sociobiological aspects of adult group dynamics. The fact that the upper canine crown projects less than the lower canine crown provides further evidence that the canines were not as important for fighting or display purposes.
Ardi’s postcrania is considerably more primitive than later hominins, with a few derived anatomical aspects. Ar. ramidus retains a fully abductable big toe (primitive), yet also exhibit a relatively inflexible midfoot and other anatomical structures indicative of bipedalism (i.e. adaptations for propulsive movement such as effective heel strike and toe off).
A mosaic of primitive and derived features are also present in Ardi’s pelvis and femur. For example, the relatively primitive pelvis of Ar. ramidus has comparatively mediolaterally broad ilia, a derived trait that results in the formation of the anterior inferior iliac spine as a separate growth site. The ischium, however, is largely the same as the African apes.
The Ardipithecus femur shares traits with later hominins and earlier Miocene apes to the exclusion of the extant African apes. These include retention of homologs of the hypotrochanteric fossa, a third trochanter, and the lack of a lateral spinal piliaster.
The Ardi skeleton exhibits limb length proportions closest to the values found in above branch quadrupeds (i.e. the hindlimbs and forelimbs of Ardipithecus were roughly the same length). This is in contrast to later hominins which have longer legs relative to arms, and the reverse of extant African apes which have longer arms relatively to their legs.
Meanwhile, the hands of Ar. ramidus exhibit flexible wrists, palms and fingers, which may be interpreted as primitive retentions from an arboreal quadrupedal common ancestor shared with the extant African apes. The flexibility of the wrist, compared to the less flexible African apes, would have allowed Ardipithecus to support its body weight in a palmigrade posture. This would indicate that the orthograde positional behavior of later hominins (related to bipedality) and the living apes (related to suspensory locomotion and vertical climbing) is a case of homoplasy.
The proportions, along with other aspects of the postcrania, suggest that Ardipithecus engaged in terrestrial bipedality and above branch arboreal palmigrade locomotion.
Australopithecus anamensis
Australopithecus anamensis remains have been found at 2 Middle Awash localities, Asa Issie and Aramis 14. Asa Issie is located approximately 10 km west of the Aramis localities where Ardipithecus ramidus was discovered. Asa Issie first came to prominence in 2006 with the publication of an article by Tim White and colleagues that announced 30 newly found Au. anamensis fossils at the site. The 4.12 Ma fossils extended the geographic range of Au. anamensis from its previously known localities in Kenya into Northeastern Ethiopia.
The Asa Issie fossils are comprised of 2 partial maxillary dentitions, a partial metatarsal shaft, a foot phalanx, a hand phalanx, 4 vertebral fragments, and a proximal femur. At least 2 individuals are represented in the dentition material recovered from Asa Issie. Compared to Ar. ramidus, the cheek teeth of Au. anamensis appear substantially larger, but similar in size to their Au. anamensis and Australopithecus afarensis counterparts. Relatively large canines are present for both individual, and the absolute crown dimensions for these teeth are larger than the ranges of variation seen in Au. anamensis and Au. afarensis. The low crowned molars are also different from Au. afarensis, in that they exhibit flaring buccal and lingual crown faces. Au. anamensis also exhibits relatively thick molar enamel when compared to Ar. ramidus.
Few Au. anamensis postcranial elements were discovered among the Asa Issie fossils. The single intermediate hand phalanx found appears morphologically similar to Australopithecus fossils from Hadar, including slight dorsal curvature of the shaft and heavy attachment sites on the proximal palmar surface for the m. flexor digitorum superficialis. Four vertebral elements include an atlas larger than the Hadar Au. anamensis specimen and a distinctly larger thoracic arch when compared to any of the vertebrae in the Hadar A.L. 288-1 specimen (i.e. the 3.2 Ma specimen of Au. afarensis nicknamed Lucy).
Only the proximal portion of the Asa Issie femur specimen is present, which preserves the lesser trochanter and a portion of the neck-shaft junction. The anteroposterior curvature of the femoral shaft is artificially emphasized as a result of taphonimic damage. The cortical bone on the shaft is extremely thick when viewed in cross-section. The heavily roughened superioinferiorly directioned posterior attachment for the m. gluteus maximus differs from the rest of the relatively smooth shaft; lacks a linea aspera but exhibits blunt outlines of the adductor attachments medially and laterally. The Asa Issie femur is comparable to the AL 288-1 femur in the aspect of the “minimal linea aspera” morphology. Based on these traits, the Asa Issie specimen is located on the more primitive end of the range of variation seen in Au. afarensis.
Other Au. anamensis specimens outside the Asa Issie locality have been discovered and investigated. Postcrania (including elements from the hindlimb and the forelimb) exhibit morphology indicative of bipedalism and powerful arms for tree climbing.
Australopithecus garhi:
The Bouri peninsula is located in the southern part of the Middle Awash and has been a subject of paleoanthopological research since the 1990s. From 1996-1998, a team led by Tim White recovered hominin remains that helped fill a crucial temporal gap in the fossil record. Prior to the discovery of the Bouri hominins, the record for East African hominins between 2-3 Ma was relatively scarce. These new fossils, dubbed Australopithecus garhi by its discoverers are dated to ~2.5 Ma and appear most similar to Australopithecus afarensis, though there are some important differences, especially in the dentition. The characters traits of Au. garhi, and subsequently the discovery of a new Australopithecus species, was so unexpected that the discovers used the Afar word for “surprise” as the new species name. Au. garhi is represented by a partial cranium derived from the Hata Member of the Bouri Formation. Postcranial elements suspected as Au. grahi, including a partial skeleton, have also been recovered at Bouri.
The holotype of Au. garhi is a partial skull (ARA-VP-12/130), which preserves portions of the frontal, parietals, and maxilla including the complete dentition with the exception of the left second and third molars. It exhibits an intriguing mix of features that share similarities with both gracile and robust australopithecines as well as those that presage morphological characteristics seen in Homo. Much of the cranial morphology recalls that seen in Au. afarensis, including the a significant degree of subnasal prognathism, the shape of the braincase, and a relatively small cranial capacity of 450 cc.
With regard to the dentition, while the canines and incisors are well within the range for Au. afarensis, the postcanine denition (premolars and molars) are larger than almost any other known Australopithecus or Paranthropus specimen. The only hominin with larger premolars and molars is Paranthropus boisei. Unlike Paranthropus, however, Au. garhi exhibits a much thinner palate, relatively non-flaring zygoma (i.e., cheek bones), and the incisors and canines are more similar to the gracile condition as seen in Au. afarensis. Futhermore, though the size of Au. garhi’s premolars and molars are in the paranthropine range, Au. grahi’s wear patterns are quite different. Paranthropine teeth tend to wear flat and obliterate the morphology of the tooth cusps, while the cusp morphology of cheek teeth in Au. garhi remains intact. In fact, the wear pattern of the cheek teeth in Au. garhi has similarities with that found in Homo.
The partial skeleton located 278 meters (m) from the holotype skull, includes fragments of numerous limb elements, such as the left femur, right humerus, radius, and ulna, as well as parts of the fibula and foot. While the femur was found in situ, the other elements were collected on the surface only 2 m from each other, prompting some researchers to argue that the elements came from the same individual. Found alongside this postcranial material was a partial mandible that lacked teeth or any diagnostic characters. Another 9 km away, at a locality designated Esa Dibo, White's team found more hominin material including a nearly complete mandible with preserved dentition. Additional humeral fragments were found ~1 km north of the mandible.
The postcranial material, which can't be absolutely attributed to Au. garhi, also shares similarities with both australopiths and Homo. The single pedal phalanx is indistinguishable from Au. afarensis, both in size and curvature. The arm and leg material are a mosaic of human and australopith proportions. The femur is long relative to the upper arm bone (humerus), which is similar to humans, but the lower arm (radius and ulna) is relatively longer than the humerus. Longer lower arm relative to the upper arm is similar to the australopith and extant ape condition. This mix of features may indicate a combination of arboreal and terrestrial (bipedal) locomotor strategies. Other tantalizing clues about group composition and social structure come from size differences in the fragmented fossils recovered. For example, 2 recovered mandibles are much too small to fit with the ARA-VP-12/130 maxilla. In addition, some of the recovered arm bones (e.g. a proximal ulna and humeral shaft) are significantly larger than corresponding elements preserved in the partial skeleton found at Bouri. Both of these examples could be evidence of sexual dimorphism in Au. garhi.
No archaeological material has been recovered at the Asa Koma, Aramis, and Asa Issie sites. The archaeological record within the Middle Awash complex begins ~2.5 Ma at the Bouri locality, which provides important zooarchaeological evidence of stone tool use and early hominin foraging behavior, though no actual stone tools have been recovered at the site. Along with lithics found ~90 km away at Gona, the zooarchaeological material at Bouri represents some of the earliest evidence for stone tool use by hominins. The relationship between the Australopithecus garhi fossils and the tool assemblage is inconclusive; the modified animal material, however, is reliably dated to the same 2.5 Ma age as the hominin remains.
Remains of 1equuid and 2 bovid elements were recovered at Bouri, all of which preserve evidence of modification via stone tool. First recovered was a partial mandible of a medium sized antelope-like animal (i.e. alcelaphine bovid). Striae were clearly visible near muscle attachment sites, suggesting the marks were made when the tongue was removed from the animal. The tibia of a larger bovid covered with cut marks, chop marks, and impact scars was found ~197 m from the mandible. The marks may result from the use of a hammerstone, took often utilized to break open the tibial shaft and gain access to the nutritions marrow inside. A femur from an early horse species, Hipparion, was found near the tibia, and also exhibited cut marks interpreted as evidence for butchery.
While the Bouri zooarchaeological discoveries are unambiguous evidence for hominin tool use, it remains the case that the actual tools used to make the cut marks are absent. One argument asserts that Bouri lacks the raw materials needed to manufacture stone tools, so that the hominins produced the lithics elsewhere and transported the finished tools in and out of the site as needed. Either way, the Bouri assemblage offers the supporting evidence that hominins used stone tools for processing animal carcasses almost from their earliest appearance in the archaeological record.
Asa Koma and the western margin of the Middle Awash represent one of the most diverse late Miocene faunal assemblages in East Africa. Sixty four mammalian species representing 23 families across 8 orders have been identified from the region. In terms of some of the primary mammalian groups, cercopithecid primates are rare in the late Miocene of sub-Saharan Africa, and the Asa Koma area is no exception, though fragmentary remains of a minimum of 3 (and possibly 4) species have been recovered. The carnivore fauna on the other hand is quite diverse, with species representing 18 genera having been recovered. These genera span 6 family groups including large cats, dogs, bears, and weasels (i.e. felids, canids, ursids and mustelids respectively). Some genera, including the mustelid Plesiogulo and the viverrid Helogale (i.e., a type of mongoose) represent the earliest known occurrences of those taxa. Bovids are also well represented with 8 tribes comprising 13 genera found at Asa Koma. The most abundant of the bovid tribes are the tragelaphines (similar to antelopes) and reduncines (reedbucks and waterbucks). These 2 groups are suggestive of woodland and grassland environments. The fauna at Asa Koma are also important for biostratigraphic dating, with mammals such as the fossil pig, Nyanzachoerus syrticus, and the primitive elephant Primelephas corroborating the radiometric dating of the volcanic tuffs.
Like Asa Koma, the associated vertebrate faunal assemblage at Aramis is also extensive, numbering more than 6000 catalogued fossils at or below the family level. In terms of the large mammals recovered, these fossils represent nearly 40 species and 34 genera. Interestingly, almost one third of the macrovertebrate fauna is made up of 2 species of cercopithecoid monkeys (primarily colobines), which supports the more closed habitat designation for Aramis. Tragelaphus, the spiral horned antelope or kudu, is the most common large mammal in the assemblage and the dentition of the fossil specimens is congruent with modern dietary preferences for this group of browsers, preferring woody habitats over open ones. Open habitat associated antelopes (grazers) are not common in the Aramis assemblage.
The Aramis micromammal and avian assemblages tell a similar story to the macrovertebrates. Most of the microvertebrate remains are believed to be the result of owl pellets based largely on the assemblage composition and patterns of observed damage. The small mammal fossils, representing 32 genera, are primarily indicative of a woodland environment and include the porcupine Atherurus and the murid Oenomys (i.e. the rusty-nosed rat). Other mammals corroborate this view, including woodland bats, shrews and mice. The avian fossils at Aramis represent 16 families, and support inferences that Aramis was a woodland during the Pliocene. Open country birds and waterfowl are both rare, while woodland associated fowl (guineafowl and peafowl), lovebirds, and parrots are more common, as are raptors that hunt in emergent forests.
Unlike the Aramis 14, another Middle Awash Australopithecus anamnesis locality, Asa Issie is notable in that hominin fossils were found in close association with a sizeable fauna assemblage (>500 vertebrate fossils). Since many of the associated taxa are fairly well-constrained temporally, the assemblage is important for biostratgraphic dating of Asa Issie relative to the faunal composition of the Aramis localities. These chronologically sensitive animals include bovid, suid and primate species that are more derived than those found in the Gaala Tuff (4.4 Ma) of the Sagnatole Formation (at Aramis) yet older than the overlying Moiti Tuff (3.9 Ma). The Asa Issie fauna is dominated by tragelaphine bovids (27% of the total fauna) and colobine monkeys (24% of the total fauna), including a new species of colobine monkey, Kuseacolobus hafu. A notable characteristic of the Asa Issie assemblage is that aquatic fauna are rare and the assemblage is dominated by terrestrial animals.
While the dating for the Bouri locality is based primarily on 40Ar/39Ar and paleomagnetic dating, some of the recovered faunal remains help to corroborate the absolute dating results, including the bovids Tragelaphus pricei, Beatragus whitea, and Damaliscus ademassui. Two biochronologically sensitive species of extinct pigs, Metridiochoerus andrewsi and Kolpochoerus limnetes, have also been found at Bouri. Aside from the previously mentioned fauna, alcelaphine bovids are the most abundant. The faunal assemblage also includes the early three-toed horse, Hipparion, 3 genera of carnivore, 2 genera of non-human primate (Therapithecus and Papio), as well as birds, reptiles (e.g., Crocodilia), and 2 genera of the elephant-like Proboscidian.
Asa Koma:
The localities where Ardipithecus kadabba have been found span a period of more than 600,000 years (Ka) and occur in two different geologic formations. The formation of the region is due to a series of volcanic, sedimentary, and tectonic processes. The majority of the localities are found in the Adu-Asa Formation, and more specifically from the Asa Koma Member of the Adu-Asa Formation. The younger localities, such the Asa Koma site from which a complete pedal phalanx was derived, are found in the Kusaralee Member of the Sagantole Formation. The fossiliferous sediments of both members are comprised of lacustrine and fluvial units, indicating that lakes and rivers, respectively, were in existence when these members were deposited. Hominin fossils in the Asa Koma Member were found in silty clay sediments bounded by basaltic tephra and diatomite. The Asa Koma Member is dated securely to a maximum of 5.77 ± 0.08 Ma based on the Ladina Basaltic Tuff that underlies the member and a minumum of 5.54 ± 0.17 Ma based on basalts at the locality of Saitune Dora. Dates for the Alayla and Digba Dora localities fall within the range of these two dates.
Aramis:
The geology surrounding Aramis, which is separated from Asa Issie by a northwest to southeast oriented fault, has been studied since the early 1980s. The exposed Middle Awash sediments span a period from approximately 6 Ma to a little more than 150 Ka and are in excess of 1 km thick. The Aramis member of the Sagantole Formation is located in the Central Awash Complex (CAC) of the Middle Awash. The CAC is a domelike horst near the southwestern rift margin. It consists of 300 m of exposed sediments that date between ~3.9 and ~5.6 Ma. Sediments at the Ardipithecus locality are within this section in silt and clay deposits on a floodplain, which is exposed along a 9 km arc. The hominin-bearing sediments are very tightly constrained in their dating, occurring between two volcanic tuffs which are both dated to ~4.4 Ma.
The lower tuff, which is known as the Gaala Tuff Complex (GATC) and is heavily inundated with pumice and crystals, is dated to 4.419 ± 0.068 Ma. Overlying this tuff is the Aramis Member of the Sagnatole Formation where the hominin fossils are found. These sediments are floodplain deposits containing salmon to red-brown colored silt, clay, and sand. The deposits are between 3 m and 6 m thick, with the thickest portions situated toward the east. The Aramis Member is capped by the Daam Aatu Basaltic Tuff (DABT) that dates to 4.416 ± 0.031 Ma. The Lower Aramis Member has been identified as the GATC and DABT tuffs and all the strata in between. As the GATC and DABT are extremely close in age, it's possible that these sediments and fossils were deposited over as little as 100 years, though it could have taken as long as 10,000 years. In any event, the depositional area represents a relatively short time period and is as securely dated as one can hope for in a paleoanthropological setting.
The massive and bioturbated silty clays that dominate the Aramis Member indicate that these floodplain deposits occurred at some distance from the ancient river. There is very little evidence of river channels (e.g., sandstone lenses) and much of the bioturbation evidence (e.g., dung beetle burrows) points to a predominantly terrestrial environment that received some seasonal moisture from springs. The sediments containing the Ardipithecus fossils are considered drier and more terrestrial than similarly aged localities where other Ardipithecus fossils have been found (i.e., Gona). Evidence of wetter conditions at Aramis appears after the deposition of the poorly consolidated DABT tuff, which is overlain by sediments and faunal deposits that suggest a shift to a wetter environment.
Asa Issie:
The Asa Issie locality is located approximately 10 km from Aramis. The sediments are of similar age with those of the Adgantole Member of the Sagantole Formation, and biostratigraphic dating of key index fossils confirms their contemporaneity. The sediments at Asa Issie are interpreted as floodplain deposits lying distal to the main river channel and are a mix of silts and clays. Paleosol nodules are found interbedded within these sediments. The overall sedimentology along with the faunal makeup is characteristic of an assemblage that was affected taphonomically by carnivores, but was covered quickly and not subjected to fluvial transport.
Bouri:
Bouri is located on a tilted fault block formed by a series of multiple, tightly packed normal faults and associated grabens. It lies perpendicular to the Quaternary Rift Zone and diverts the Awash River while acting as a dam for Yardi Lake. The geology of the area is complex, with the fault systems causing the older Daka Member to appear stratigraphically above the younger deposits where the Bodo cranium was discovered. The Daka Member is also directly atop the Hata Member at Bouri, obscuring this previously unknown Member where the Bouri hominins were found. Though the type locality for the Hata Member is comprised of fluvial and deltaic sediments, the portion of the Hata Memeber where the Bouri hominins were found is comprised mainly of lacustrine and lake margin sediments, and nearly devoid of fluvial deposits.
The primary volcanic unit associated with Bouri is the Maoleem Vitric Tuff (MOVT), which was deposited on shallow lake margin environs. The MOVT is approximately 1 m thick and is exposed for ~10 km. The MOVT was dated using laser-total-fusion 40Ar/39Ar and paleomagnetic dating. Of the 60 MOVT grains that were analyzed radioisotopically, 57 yielded dates that were statistically the same and provided an average date of 2.496 ± 0.008 Ma. In addition, 21 samples were tested using magnetostratographic techniques that showed a reverse polarity signature for the MOVT. Combined with the 40Ar/39Ar dates, this places the MOVT within the Matuyama Chron. Paleomagnetic and radioisotopic data suggests an estimated sediment deposition rates below the MOVT at 10.9 cm/1000 years. Given the similarity between the lithologies above and below the MOVT, using the same rate of deposition is appropriate and indicates that the minimum age for the Bouri hominin is 2.45 Ma.
Sometime between 5.8 and 4.4 Ma, parts of the Middle Awash were covered in forest and/or woodlands with elements of wooded grasslands. In particular, between 5.8 and 5.4 Ma Asa Koma was a moderately wet wooded environment, possibly riparian, with some grassland areas. The overall composition of the large mammal communities found within the Asa Koma faunal assemblage appears quite similar to the extant communities living in woodland environments today. For example, the Bovid present at Asa Koma point toward habitats with dense vegetation, and wet floodplain grasslands and swamps. Bovid taxa typical of drier, more open conditions were relatively rare in the analysis. Meanwhile, rodent fossils such as the root rat (Tachyoryctes) and cane rat (Thryonomys) suggest higher elevation and wetter (e.g., lakes and rivers) surroundings. The scarcity of lagamorphs (e.g. hares and rabbits) reflects the lack of open grasslands.
Other lines of evidence, including the study of non-mammalian fauna, geology and radio metric dating stable isotopes concur with the mammalian assemblage analyses regarding the woodland conditions at Asa Koma. Stable carbon isotopes show a grass biomass proportion of between 20-45%, which is indicative of woodland or grassy woodland environs. Stable oxygen isotopes indicate that the environment was humid or cool at a high altitude.
Like Asa Koma, the Aramis paleoenvironment at 4.4 Ma was also predominately woodlands with areas of grassland and forest. Woodland associated animals such as colobine monkeys, tragelaphine antelope, and species of rodents like porcupines and murids (e.g. rats) all point to a woodland environment. The presence of invertebrates and lower vertebrates were also analyzed. For example, the dominant gastropod at Aramis is a genus of land snail (Maizania) whose modern analogs inhabit lowland forest environments. On the other hand, lower vertebrates rarely appear in the assemblage, and their appearance is hypothesized to be a result of periodic flooding and/or raptor kills.
Among the Aramis paleobotanical data analyzed, researchers found evidence of fossil wood from palm trees, endocarps of hackberry fruits, evidence of fig trees, and phytoliths (i.e., redeposited silica from plants) from woody dicots and palms. These remains are also consistent with a woodland or woody grassland reconstruction.
Chemical analysis corroborates the faunal and floral analyses. Results of carbon isotope data extracted from fossil teeth suggests there was a low percentage of C4 plants in the area, indicating that the animals present at Aramis 4.4 Ma did not exploit open grassland food resources to a large degree. Isotopic studies on paleosol carbonates taken from the depositional layers at fossil localities showed 13C levels consistent with woodland or grassy woodland environments.
The woodland designation for Aramis is not without contention. While the published data indicates that Ardipithecus existed in a predominately woodland environment, other researchers argue that Aramis was more savanna-like, with patches of riparian woodland and forest. The critique claims that the initial results relied too heavily on isotopic data gleaned from paleosol organic matter and carbonate. However, the most recent analysis, while relying on isotopic data, also incorporated other paleoenvironmental indicators including the geology, sedimentology, taphonomy, and paleontology of the area. The results of this analysis are specific to the Ardipithecus habitat at Aramis, and are not a generalization of the Middle Awash.
Another Middle Awash locality, Asa Issie (~4.12 Ma) may represent an intermediate environment between the earlier closed woodland habitats and later periods trending towards a more open, warmer wooded grasslands. Analysis of stable δ18O and δ13C from paleosols derived from the vertebrate-bearing sediments suggest the environment at Asa Issie was more like a humid woodland savannah. Faunal analysis suggests the Asa Issie habitat at 4.12 Ma was similar to the Aramis environment during the time of Ar. ramidus at ~4.4 Ma.
The majority of the faunal and sedimentalogical data suggest that the relatively younger Bouri site (~2.5 Ma) was located on a lake margin with grasslands along the water's edge. The absence of rivers and streams capable of transporting cobbles into the area may account for the lack of raw materials available for local stone tool manufacture. The faunal assemblage at Bouri reflects a lake margin habitat and comprises a mix of grazers and aquatic or water-dependent forms. The sedimentology of the area is lacking in channel sands and riverine invertebrates, which further supports the characterization of the area as being on the margin of a shallow lake.
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