Athena Review Vol. 5, no. 1
Records of Life: Fossils as Original Sources
42. Early Hominins: Sahelanthropus to Australopithecines
Central Africa
The discovery of the earliest known hominin fossils in the Sahel desert of Chad from the Late Miocene period has opened up the possibilities of further discoveries in other regions of Africa, wherever Miocene and Pliocene strata are accessible.
Sahelanthropus
Sahelanthropus tchadensis ("man from Sahel in Chad") is represented by six specimens from the western Djurab Desert in northern Chad, including a nearly complete cranium (fig.1a,b) and fragmentary lower jaws. Discovery of these fossil remains in central Africa, 2,500 km from the East African Rift Valley, suggest that the earliest members of the hominid clade were more widely distributed than previously thought (Brunet et al. 2002).
Fig.1a,b: Skull of Sahelanthropus tchadensis (after Brunet et al. 2002).
Associated fauna suggest the fossils are between 6 and 7 million years old, indicating that the divergence between the human and chimpanzee lineages was earlier than indicated by most molecular studies.
The Sahelanthropus fossils display a mixture of primitive and derived characters. Compared to chimpanzees, it had a shorter face, a longer braincase, and thicker molar enamel. The molar enamel is also possibly thicker than in Ardipithecus ramidus, but thinner than in Australopithecus. Compared to gorillas, it had a smaller body size, and a narrower and less prognathic lower face. Compared with all great apes, it had smaller canines with apical wear, lacking evidence of the honing complex. It had a relativly small brain compared with both modern apes and other early hominins, with an endocranial capacity estimated at 320–380 cm3 (Brunet et al. 2002).
Information is still insufficient to know whether Sahelanthropus was a habitual biped, a key attribute of early hominins. But given the skull’s other basicranial and facial similarities to later fossil hominids that were clearly bipedal, Brunet at al. (2002) consider bipedalism likely.
East Africa: the Rift Valley
The
great majority of early hominin fossils from Africa have been found in
Late Miocene through Pliocene strata exposed in the East African Rift
Valley (fig.2). These geological contexts have formed the basis of fossil
discovery and interpretation, and the Rift Valley exposures show continuities in both fossil and tool forms from north to south.In the southern portion of the Rift Valley, the most productive fossil-finding areas range from Olduvai to the Lake Turkana region. Fossils include both hominins and related fauna, providing clues on the Late Miocene and Pliocene environments.
To the north, the exposures in Ethiopia along the Awash River and the Hadar desert region have yielded numerous early hominin fossils of Australolpithecus and Ardipithecus. The sites of Australopithecus garhi, dating from about 2.5 mya at the end of the Pliocene, have producd the oldest known stone tools. Lithic artifacts found at Gona and other Ethiopian sites represent the Oldowan flake tool industry, defined in the 1950s by Louis and Mary Leakey at Olduvai Gorge in Tanzania.
Fig.2 Map of Rift Valley region in East Africa, showing sites of major Late Miocene and Pliocene hominins.
Ardipithecus
Ardipithecus ("basal ape") lived during the Late Miocene and Early Pliocene periods in the Afar Depression of Ethiopia. Two fossil species include A. ramidus, which lived about 4.4 mya during the early Pliocene, and A. kadabba, dated to approximately 5.6 mya in the late Miocene.
The first A. ramidus fossils were found in 1992–1994 by White and his colleagues (1994) in the Afar Depression in the Middle Awash river valley of Ethiopia, including the skull, mandible, teeth, and up to 45% of the skeleton. This was originally described as a species of Australopithecus, but later renamed as the separate genus Ardipithecus. This fossil was dated to 4.4 mya on the basis of its stratigraphic position between two volcanic layers. Thereafter, in 1999-2003, bones and teeth of nine A. ramidus individuals were discovered in the Gona area of Ethiopia's Afar Region, dated to between 4.35 and 4.45 mya.
In 2009 was announced the relatively complete A. ramidus fossil skeleton of a 50-kilogram (110 lb) female, nicknamed "Ardi", discovered in 1994 at Aramis in the Afar desert (White et al. 2009). Layers of volcanic ash encasing the deposits have been radiometrically dated to 4.3-4.5 million years ago. This date, however, has been questioned by ot
hers who propose Ardi should be dated
at 3.9 mya, much closer to the dates of Australopithecus afarensis (Kappelman and Fleagle 1995).The relation of Ardipithecus ramidus to human ancestors is a matter of debate (Stanford 2012). Behavioral analysis showed that Ardipithecus could be very similar to chimpanzees. Like most hominids, but unlike all previously recognized hominins, it had a grasping big toe (hallix) adapted for locomotion in the trees. It is not confirmed how much other features of its skeleton reflect adaptation to bipedalism on the ground as well.
Fig.3 Skull of Ardipithecus ramidus.
A. ramidus had a relatively small cranial capacity of 300 -350 cm3, slightly smaller than a modern bonobo or female common chimpanzee brain. It is smaller still than the brain of Australopithecus afarensis (~400 to 550 cm3), dating from about 3.3 mya; and only 20% the size of the modern Homo sapiens brain.
As in common chimpanzees, the face of A. ramidus was much more prognathic than that of modern humans. The teeth of A. ramidus lacked the specialization of other apes, indicating it was a generalized omnivore and fruit eater with a diet that did not depend heavily on foliage, fibrous plant material, or other abrasive food. Like later hominins, meanwhile, Ardipithecus had reduced canine teeth. The size of the upper canine tooth in A. ramidus males was not distinctly different from that of females, which contrasts with the sexual dimorphism observed in common chimpanzees. Since A. ramidus is in some ways unlike chimpanzees, the common ancestor of humans and chimps may have differed significantly from the modern chimpanzee.
The second species of Ardipithecus, A. kadabba, is defined from teeth and fragments of skeletal bones, and is dated at 5.6 mya. In 2004 it was proposed as a separate species, based on tooth evidence showing primitive morphology and wear pattern distinct from those of A. ramidus (Haile-Selassie et al. 2004).
Australopithecus anamensis
Australopithecus anamensis, known from fossils recovered in 1995-7 from Kanapoi and Allia Bay in the Turkana Basin of northern Kenya (fig.2), dated 4.2 -3.8 mya, may represent the earliest upright-walking human ancestor, The species is named for the Turkana word anam (“lake”), applying to the ancient Lake Lonyunum in the time of A. anamensis. Its remains from Lake Turkana come from two occupation levels bracketed by volcanic strata above and below, dated by the argon-argon (40Ar-39Ar) method from 4.07±0.03 to 4.17±0.02 myr. The species is proposed by its discoverer, Maeve Leakey (1999), to be an intermediate form between Ardipithecus ramidus and Australopithecus afarensis. The latter species may have emerged from A. anamensis, and the two species lived concurrently. This has been borne out by fossils recovered in 2016 from Ethiopia, described below (Haile-Selassie et al. 2019) .
Based on limited postcranial evidence, A. anamensis may have been habitually bipedal, although it retained some primitive features of its upper limbs, as in the case of A, afarensis. Australopithecus anamensis was first classified in 1995 from 21 bone fragments including skull pieces, a mandible, and teeth. The first mandible came from a female with all teeth fortuitously present. These, particularly the canines, are larger than teeth of the later A. afarensis found at both Hadar, Ethiopia (including the partial female skeleton of “Lucy”), and at Laetoli near Olduvai in Tanzania, where footprints preserved in volcanic ash helped confirm the upright posture of these early hominids. One complete lower jaw bone closely resembles that of a common chimpanzee (Pan troglodytes), with canines larger than those of A. afarensis, but thicker molar enamel than chimps. A. anamensis molar fossils show a pattern of long striations similar to the microwear on the molars of gorillas, suggesting a similar diet. A. anamensis had a smaller cranial capacity (estimated 365-370 c.c.) than A. afarensis.
A. anamensis shows considerable sexual dimorphism, based on comparison of a female mandible and canine teeth found in 1995 compared with those of a male mandible found in 1997. Tooth rows in the A. anamensis mandible are more closely parallel than those of A. afarensis. The more primitive parallel jaw form preserved in gorillas and chimpanzees is contrastive to the rounded dental arcade of humans.
The postcranial skeleton of A. anamensis, however, shows more hominid-like or derived traits. Portions of a 3.5 mya humerus (upper arm bone) found at Kanapoi in 1965 reveal, along with wrist bones discovered in the late 1990s, that the hands and fingers of A. anamensis had a mixture of ape- and hominid-like features. Notably, the relatively straight shaft of the tibia or shinbone (the larger of the two lower leg bones), and the shape of both of its ends, indicate bipedalism, central to the designation of A. anamensis as a hominid. If true, A. anamensis walked upright an estimated half million years before A. afarensis made the celebrated footprints preserved in stone at Laetoli. The tibia ends have also helped to determine the weight of A. anamensis at 47- 55 kg.
A. anamensis may be seen as a transitional hominin linking A. afarensis with the earlier Ardipithecus ramidus. The latte shows tooth enamel thinner than either A. anamensis or A. afarensis. The sample for Ardipithecus ramidus includes seven of eight wrist bones, and most fingers. It is not yet determined whether it walked upright, but the foramen magnum or hole attaching the skull base to the spine is relatively far forward, suggesting bipedalism.
A. anamensis is more similar to the earlier A. afarensis from Laetoli of 3.6-3.4 mya than it is to the younger 3.3-3.0 mya Hadar sample. The Laetoli specimens, known only from crania and mandibles, show very close canine sizes and molar sh
apes to the A. anamensis
teeth. More recently, significant A, anmensis fossils found in Ethiopia, including a nearly complete skull, provide critical evidence that A. anamensis is a transitional fossil between Ardipithecus and A.afarensis (Haile-Selassie et al. 2019). A skull dated at 3.8 mya, named MRD, was found in 2016 at a location in the sandstone near the Woranso-Mille district of Ethiopia, 56km from of the site in the Afar region of Ethiopia where the remains of "Lucy" wrere found in 1974. Yohannes Haile-Selassie (2019) reports the find as ."the most complete cranium of an early human ancestor ever found in the fossil record older than 3 million years."
Fig.5 Skull of Australopithecus anamensis MRD, in profile view (photo: Cleveland Mus.Nat.Hist.; Haile-Selassie et al.2019).
The skull measures about 20cm front to back and 11.5cm wide. The almost complete skull had been split into two pieces, with the upper jawbone was separated from the rest of cranium. Based on the MRD skull, A anamensis was much smaller than modern humans. Apparently an adult male, it had a long, robust face with prognathic jaws, and a well-developed sagittal crest indicating strong jaw muscles. Some primitive features are similar to those in Sahelanthropus, including a long and narrow shape of the braincase and the shape of the location on the back of the skull where the neck muscles attach.
MRD also shows a reduction in canine size from older species, though these teeth are still larger than in A. afarensis, as canine size progressively diminished among later hominins. Some aspects of its face foreshadowed traits in later species such as the structure of the cheek bones, which anchor important chewing muscles.Until now, the earliest Australopithecus anamensis fossils were 3.9 million years old. The MRD fossil's age indicates this species co-existed for approximately 100,000 years with Lucy's species, challenging previous notions that the earlier species had evolved into the later one with no overlap.
Australopithecus anamensis: Scientists reveal the face of our oldest known ancestor
Australopithecus afarensis
Australopithecus afarensis ("southern ape from Afar") was a Pliocene hominin who lived in East Africa ca.
3.9–2.9 mya. The first fossils were discovered in the Afar region
of Ethiopia in the 1930s. A 1972–1977 expedition in Hadar, Ethiopia by
M. Taieb, D. Johanson, and Y. Coppens discovered a well-preserved
skeleton (AL 288-1), called "Lucy", and at site AL 333 the
remains of several individuals, called "the First Family" (Johanson et
al 1978). Meanwhle, starting in 1974, Mary Leakey led an expedition to
Laetoli, Tanzania (just south of Olduvai gorge), and found fossil
tracks attributed to A. afarensis, along with and fossil jaws and teeth (Leakey, M.
et al. 1976). The leg bones of A. afarensis as well as the Laetoli fossil trackways suggest it had bipidal posture, although it was less efficient at walking than humans. The relatively long arm and shoulder bones show similarities to those of orangutans and gorillas. This suggests the retention in A afarensis of primitive traits related to tree climbing inherited from the human–chimp last common ancestor, possibly no longer with adaptive functionality.
Fig.4: Skull of Australopithecus afarensis (Cleveland Museum of Natural History, cast).
Based on its jaws and dentition, A. afarensis was probably an omnivore. The molar crests are similar to those of chimps and female gorillas. Compared to Ardipithecus and Sahelanthropus, the incisors of A. afarensis are reduced in breadth. The canines are reduced in size, and lack the honing mechanism seen in great apes which continually sharpens them. The molars of australopiths are generally large and flat with thick enamel, adapted to crushing hard and brittle foods, in contrast with thin enamel in the molars of chimpanzees and gorillas, who are primarily fruit eaters.
Marked sexual dimorphism is evident in the relatively large sample of A.afarensis fossils. The skull (fig.4).had a relatively high face, brow ridges, and marked prognathism, with the jaw jutting outwards. One of the biggest skulls (AL 444–2) is about the size of a female gorilla skull (Kimbel et al.), and another deep and robust jawbone strongly resembles that of a gorilla. Unlike gorillas, however, the sagittal and nuchal crests which supported the jaw's temporalis muscle do not vary much between sexes.
The brain volume of A. afarensis was 365–526 cc, in the size range of great apes, and its brain was probably structured like those of apes. The precise relation of A. afarensis to the nearly contemporary A. anamensis from Tanzania is much debated, particularly over which could have given rise to the Early Pleistocene hominin Homo habilis. Some researchers believe that H. habilis descended more directly from A. anamensis (Leakey M.G. 2001),.
Kenyanthropus platyops
Kenyanthropus platyops ("flat-faced Kenyan man"), a Pliocene hominin contemporary with Australopithecus aferensis, lived around Lake Turkana, Kenya. In 1999 a relatively complete crania (KNM-WT 40000) was discovered in the Nachukui Formation at Lomekw (fig.2), along with three fragmentary mandibles, two partial maxillae, a temporal bone, and numerous teeth. The skull KNM-WT 40000 (fig.6a,b), dated at 3.5 mya, overlay another specimen, KNM-WT 38341, dated at 3.53 mya (Leakey 2001).
Kenyanthropus had small molars and a relatively flat face, with a steep nasal cavity and moderate mandibular depth, and thick molar enamel, These and other features of its cranial structure bears some similarities to those of Australopithecus afarensis, including brain size, parts of the nasal area, and the suborbital and the temporal regions. The relatively flat face for which the K platyops species is named indicated to Leakey et al. (2001) that hominins were more taxonomically diverse during the middle Pliocene, and that less prognathic jaws evolved earlier that previously thought. White (2003), meanwhile, considered the Kenyanthropus platyops skull a distorted specimen of A. afarensis. This was contested by Spoor et al. (2010) who cited the maxilla of KMN-WT-40000 is distinct from that of all australopithecines.
Fig.6a,b Skull of Kenyanthropus platyops KNM-WT 40000.
Fossilized remains attributed to K. playtops were also found on the south bank of the Turkwel River by Lake Turkana (Ward et al. 1999) . Associated faunal remains appear similar to those from other Pliocene sites in the Turkana basin. The fauna provide evidence of varied habitats dominated by thick bushland, riparian forest, as well as more open country. Permanent water sources were available from the ancient Turkwel and Omo Rivers.
The Turkwel K. platyops specimens included a partial juvenile mandible (KNM-WT 22936) retaining parts of a permanent right canine and a right third premolar. Associated fossil remains (KNM-WT 22944 A-K) consisted of jawbone fragments, a number of teeth, cranial pieces, and
several postcranial elements. Postcranial bones in
sample KNM-WT 22944 G-K were of particular interest, including a
partial left metacarpal (finger bone), three wrist bones (the capitate,
hamate, and lunate) and a section of a pedal phalanx or toe bone. These
bones provided important information on locomotion, forearm
musculature, and more speculatively, on the likelihood of tool-use.The metacarpal and capitate bones indicate that K. platyops were not knuckle-walkers like modern apes. Furthermore, the pedal phalanx indicates that their toes were wider and straighter than those of apes, and therefore structurally adapted for bipedalism. Other characteristics of the wrist bones indicate that the Turkwel individuals had powerful forearms, with a gripping strength comparable to living apes. The capitate head, which makes up almost 50% of the bone is much larger than those of A. afarensis and A. africanus. A deep lateral groove on the capitate produces a constriction, or “waisting,” almost unknown in humans but prominent for chimpanzees.
One missing human-like trait is a third metacarpal styloid on the capitate bone (KNM-WT 22944). This feature, present in both modern Homo sapiens and more recent hominid ancestors, is associated by some with tool use. Absence of the styloid suggests the Turkwel K. platyops were not regular tool users or makers.
Australopithecus garhi
Australopithecus garhi from Ethiopia, dating from about 2.5 mya (Asfaw et al 1999), is named garhi after the Afar word for "surprise." A. garhi has anatomical traits suggesting an ancestry somewhere between A. afarensis and A. africanus, and transitional features anticipating the first examples of Homo habilis, H. rudolfensis, and H. ergaster/H. erectus. Notably, the A. garhi fossils are linked with the earliest stone tools yet discovered at hominid sites, dating to the Pliocene/Pleistocene transition.
The first A. garhi remains were found in 1990 in the Hata Member of the Bouri geological formation, part of the rift valley located just west of the modern Awash River (fig.2). Fossils include a left parietal skull fragment, a partial mandible, and portions of a left humerus or upper arm bone, with associated volcanic tuff dated by Argon isotope decay methods to 2.496 mya. The humerus and parietal bones resembed those of the smaller, gracile A. africanus or A. afarensis. Additional 1996 finds from the Bouri deposits included scattered remains of arm, leg, foot and mandible bones, probably representing a single individual. These were associated with catfish (Siluriform) remains and bovi
d (wild cattle) fossils with distinct cutmarks, the
earliest evidence yet known for stone tool use. In 1997, two partial A.
garhi skulls were recovered nearby. About 10 km north at Esa Dibo, a
mandible and humeral fragments were also found.Classification of the Bouri Hata hominid by Asfaw et al. (1999) as the new species A. garhi is based solely on the cranial and dental remains. The teeth of A. garhi have remarkably mixed traits. While its molars and pre-molars are very large, exceeding even those of robust australopithecines in size, their morphology seems closer to those of the genus Homo. The A. garhi face otherwise appears primitive in shape, prognathic, with a protruding jaw like that of A. afarensis or A. africanus (fig.7). The cranial capacity, meanwhile is about 450 cm3, at the lower end of Australopithecine ranges.
Fig.7: Skull of Australopithecus garhi (Ethiopian Nat.Mus., cast).
The postcranial Bouri remains are only the third Plio-Pleistocene hominid specimen allowing the accurate determination of limb proportions. The femur (upper leg bone) shows the earliest known appearance of femoral elongation, becoming longer relative to the upper arm bone or humerus, a trait developed in modern humans. The brachial (arm length) index, however, is still apelike.
Concerning evidence for tool use, de Heinzelin et al. have identified more than 400 vertebrate fossil specimens from the Hata locale where the A. garhi remains were found. Bone modifications in two excavations show large mammals were defleshed and disarticulated, with long bones broken for marrow extraction in the vicinity of the Bouri skeleton. Several bovine and Hipparion (three-toed horse) bones had prominent and unmistakable cut marks. A large bovid’s tibia (lower leg bone) showed percussion marks by hammerstones. A bovid mandible, meanwhile, had cut marks on its medial surface, evidence that the tongue was removed using sharp stone tools.
While it is still unknown who the first toolmakers were at Hata, they may be related to the nearby Gona site 96 km to the north (fig.2), where a large number of Oldowan tools were found in nearly contemporary 2.6 myr levels. At Hata, the few simple cores and flakes were not excavated in situ, but eroded out of the ground. Unlike Gona, raw materials were lacking here, and there is no prominent tool assemblage.
A. garhi appears to be younger than, and derived, from A. afarensis. Based on the crania it may not be so closely related to the nearly contemporary A. africanus. Also problematic is its relationship to early forms of Homo, further complicated by lack of knowledge of whether there is a single, sexually dimorphic H. habilis or two separate species (H. habilis and H. rudolfensis).
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Glossary