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In order to understand our universal traits, and how we came to be who we are, and to understand our place in this world and how we came to be here at all, it is fundamental to glean a more complete understanding of our origins. This search for our origins, to determine who the first hominin was, when they lived, and whether they contributed any genetic material to modern humans is a primary objective of paleoanthropology. It requires reconstructing the human lineage back to its roots, beyond the point where hominins and apes diverged from their common ancestor.
Molecular testing indicates this divergence occurred between five and eight million years ago (Lewin & Foley, 2004). More precise timing, the location, and the mechanisms of this split remain debated. The questions remain unresolved partly because the fossil record is inordinately sparse. The visual representation of human evolution remains an unruly tree with many gaping holes. Fossil discoveries in the past fifteen years have produced more questions than they have answered.
The task is rendered yet more difficult by paleoanthropologists inability to agree on a coherent lineage; indeed some of their debate seems more like bickering than scholarly discourse. When Don Johanson and his team discovered Lucy, in 1974, they pushed the human lineage back a million years (Johanson & Edey, 1981). Australopithecus afarensis lived roughly between 3 and 4 million years ago, a million years before its possible successor, A. africanus. In 1992 Ardipithecus ramidus, was discovered in Ethiopia and replaced Australopithecus afarensis as the oldest known hominin (White et. l. , 1994). This discovery ushered in a new focus for many paleoanthropologiss; to discover the earliest hominin, that first bipedal ape that is ultimately an ancestor to modern humans. In the subsequent decade, five potential hominin species, each one older and more controversial, were discovered. Currently there are four contenders for the start of the human race: Ardipithecus ramidus, Ardipithecus kadabba, Orrorin tugenensis, and Sahelanthropus tchadensis (Gibbons, 2006).
None of these can yet definitively be called a hominin, all four species remain embroiled in controversy, paleoanthropologists cannot agree if any of them contributed genetic material to modern humans, or, if they are even hominins. Ardipithecus ramidus was discovered in 1992 by a team led by Tim White, Gen Suwa and Berhane Asfaw. The fossils were found in the Awash River region within the Afar depression at Aramis, Ethiopia (White et. al. , 1994). Associated fossil remains, including wood, seeds, and vertebrate specimens, were all found within a single interval overlying the basal Gaala Tuff complex, and beneath the Damm Autu Basaltic Tuff.
These volcanic strata have produced dates of 4. 389 and 4. 388 million years, respectively (Renne et. al. , 1999). This location definitively places all Ardipithecus specimens just shy of 4. 4 million years old. An interesting discovery coincided with these early ages; the associated floral and fauna were typically found in a heavily forested, flood plain environment. This suggests that early human evolution may have occurred in a woodland environment, rather than in the open savannah, as previously assumed. Hominins likely did not occupy savannah sites until the advent of the australopithecine genera (White et. al. , 1994).
Based on the initial seventeen fossils, which included part of a child’s mandible, some isolated teeth, a fragment of a basicranium, and four arm bones (three which constitute one arm of an individual), the research team considered their find part of the australopithecine genera, and named it Australopithecus ramidus. They distinguished it from the later A. afarensis, and other hominid species, by the following characteristics: a small canine-incisor to postcanine dental ratio is absent, that is both the upper and lower canines are larger relative to postcanine teeth. The canines are low, blunt and less projecting than in all known apes.
The enamel on the canines and molars is thin, this is a distinctly ape trait; all other hominins have thick dental enamel. However A. ramidus can be differentiated from extant and extinct apes by its less projecting crowns, smaller upper central incisors relative to postcanine teeth, and most importantly the lack of a functional honing facet on the third premolar, which means the canines do not rub against the premolar during chewing. This is also a distinct ape feature. It does retain strong crown asymmetries, in particular enlarged buccal cusps on the upper and lower premolars.
The molar shape is similar to those of apes whereas the canines resemble an incisor, and are not sharply pointed like apes. The size of the canines is intermediate between living apes and australopithecines. Its dentition is more primitive than later australopithecines. This mix of primate and derived features suggests that A. ramidus is a transitory hominin that had retained some ape-like characteristics but was evolving more hominin like features (White et. al. , 1994). Another feature that defines A. ramidus from apes is the position of the foramen magnum, which is placed anteriorly relative to the carotid foramen.
The position of the foramen magnum results from piecing together various skull fragments. The cranial fragments represent adult temporal and occipital regions. Few post-cranial remains have been found, and none that can indicate bipedalism. These fossils evince a strikingly chimpanzee-like morphology, which suggests A. ramidus maybe have existed shortly after the ape-hominin split. The post-cranial remains, which consist of a right humerus, and a left humerus, ulna, and radius, display a mosaic of primitive and derived characteristics, which suggests the animal still spent time in trees (White et. l. , 1994).
In a rare move, seven months after the initial publication, Tim White and his team decided to change the genus name of their discovery to Ardipithecus. ‘Ardi’ means ‘root’ in the Afar language, and ‘pithecus’ means ape. The reason for this genus change is likely due to the several chimp-like morphologies that were noted during subsequent examinations (Gibbons, 2006). They still consider Ardipithecus to be a hominin that is distantly related to modern humans. They believe it is ancestral to Australopithecus anamensis, a hominin found by Meave Leakey and her team (Leakey et. l. , 1998), shortly after Ardipithecus was announced. A. anamensis is represented by 21 fossils, most of which are teeth, a humerus, and part of a tibia, which provides the oldest known evidence for hominin bipedalism.
A. anamensis dates from about 3. 9 to 4. 2 million years ago and is considered to be the ancestor of A. afarensis, which is believed to be distantly ancestral to modern humans. If White et. al. are correct in their phylogeny, then Ar. ramidus would be the oldest known ancestor of modern humans. However, other researchers consider Ar. amidus a sister taxon to australopithecines, that, is the two species diverged from a common ancestor. They believe that Ardipithecus is a dead genus that did not contribute any genetic material to modern H. sapiens (Pickford & Senut, 200). Although the phylogeny and evolutionary location of Ar. ramidus remains debated, very few question its hominin status, even though the limited post-cranial remains cannot definitively prove a bipedal mode of locomotion (Gibbons, 2006; see Pickford & Senut, 2001 for countering argument).
Because it was found in East Africa and does not push the boundaries of the split, Ar. ramidus can probably be placed in the human family tree, even if it is an evolutionary dead-end. However, the discovery of additional fossil remains could just as easily remove it. Not far from the location of the original Ar. ramidus discoveries, in the western margin of the Middle Awash of Ethiopia, a similar but unique jawbone was unearthed in 1997 by two Ethiopian scholars, Yohannes Haile-Selassie and Giday Wolde-Gabriel. They initially considered it a subspecies of Ar. amidus, but wanted to find additional specimens to avoid naming a new hominin on the basis of its dentition.
Four years of searching yielded 11 more fossils representing five individuals and enabled the team to classify their find as a new species: Ardipithecus kadabba (Haile-Selassie, et. al. , 2001). The majority of the fossils are teeth, but they are very distinct from ape dentition. All great apes, both extinct and extant, have large, shearing canine teeth that project tusk-like and can be used as weapons. All hominins lack this adaptation, and even the early Ar. adabba has considerably smaller canines that are not at all chimpanzee-like. Hominins also lack the functional honing facet that apes have, which occurs when the upper canine continually rubs against the lower third premolar during chewing to keep the teeth sharp.
However, a single specimen of Ar. kadabba has wear facets on the premolar, which distinguishes it from Ar. ramidus, and also makes it the most primitive canine ever found among hominins (Gibbons, 2006). The persistence of primitive dental and postcranial characteristics in these new fossils indicates that Ar. adabba was phylogenetically close to the common ancestor of chimpanzees and humans (Haile-Selassie, et. al. , 2001). The dental enamel of Ar. kadabba is thicker than that of Ar. ramidus, but it is thinner than in all other known hominins. This raises questions about the phylogenetic position of Ar. ramidus, as the general trend of human evolution is towards an increase in enamel thickness. The presence of four distinct cusps and the absence of distal tapering on the third molar are both primitive features found in most Miocene hominoids.
However, the lower canines show the development of the distal tubercle and the mesial marginal ridge, which can be interpreted as representing the earliest manifestations of the evolution of an incisiform canine, which is a definitive feature of later hominins. A few post-cranial fossils have also been uncovered, including some pedal remains which are good indicators for mode of locomotion. Foot fossils were found in later excavations and extend the dates of Ar. kadabba to as early as 5. 2 million years. A complete left fourth proximal foot phalanx with similar curvature to A. farensis shows a mosaic of features shared with both apes and early Australopithecus (Haile-Selassie, et. al. , 2001).
This fossil is consistent with an early form of terrestrial bipediality, but additional pedal remains are needed to either confirm or deny this assumption. The distal half of an intermediate hand phalanx was found. The fingers have minimal curvature, and are larger but morphologically similar to most A. afarensis hand phalanges. Two humerus fragments and an ulna were also uncovered. The left distal humerus fragment is smaller than Ar. amidus remains, but larger than A. afarensis. The ulna, from a different individual, is the size of the smallest A. afarensis fossil. These bones also show more curvature than is typical of many later hominins. The lateral half of a left clavicle that is more robust than a chimpanzee was also located.
The area in which Ar. kadabba was found has a long sedimentary history, spanning from about five to eleven million years ago, indicating that these fossils are at least this old. More careful dating placed them between 5. 54 and 5. 7 million years old, which pushed hominid occupation back another million years, right into the time of the proposed ape-human split. And like Ar. ramidus, this location has presented geological and paleobiological data that demonstrates this earliest hominin derives from a relatively wet and wooded environment that was modulated by tectonic, volcanic and litoral processes. The accompanying fossil faunal remains support this (Wolde-Gabriel et. al. , 2001). Ar. kadabba’s combination of derived and primitive traits has led some to contest its hominin status and claim that it is an ancestor of modern chimpanzees.
Early hominins can be expected to show a mosaic of ape- and human-like characters. Ar. kadabba’s classification is based on its dental fossils. The severe lack of postcranial remains and the transitional teeth makes it impossible to prove that this new discovery is a hominin. Two paleoanthropologists who dispute Ar. kadabba’s status as a hominin are Martin Pickford and Brigitte Senut, who discovered even older fossils in the Lukeino formation at the Tugen Hills of Kenya. The volcanic tuff layer these fossils were found in dates to between 5. 8 and 6. 1 million years ago.
They designated a new genus for their potential species, naming it Orrorin tugenensis, which translated to ‘Original Man from Tugen’ (Pickford & Senut, 2001). So far 20 fossils have been found, most from the expeditions in 2000 and 2001, but one from 1974 whose phylogenetic status has been contentious since its discovery (Gibbons, 2006). The bones come from at least 5 individuals and consist of several limb bone fragments, including a femur and humerus, several mandibular fragments, a single manual phalanx, and some assorted, isolated teeth. As with Ar. kadabba dental remains of O. ugenensis are the most plentiful fossils. The upper incisor is described as robust, and is smaller in size than Australopithecus and similar in size to Ardipithecus. Like all other known hominins (except Ardipithecus) O. tugenensis has thick enamel. It is about the same thickness as modern H. sapiens. Its small molars are also comparable in size and shape to modern humans (Pickford & Senut, 2001).
A humerus is the only representative of upper limb bones and shows a straight lateral crest which the m. brachioradialis muscle inserts into. This feature is present in modern chimpanzees as well as in A. farensis and is linked to climbing adaptations. Also like A. afarensis, the manual phalanx is curved which further supports the idea that O. tugenensis had climbing adaptations. Pickford and Senut (2001) have concluded that their new hominin was bipedal and although it retained arboreal adaptations it could not brachiate. The three femora are the best preserved specimens; one preserves the femoral head and two-thirds of the shaft, but both are missing the greater trochanter. The femoral head is rotated slightly anteriorly, and both it and the shaft are flattened anterior-posteriorly.
An intertrochantric groove is present, and runs from the moderately deep trochanteric fossa to just above the lesser trochanter. This furrow is found in modern humans and occurs when the thick tendons of the leg put pressure on the bone when the hip is fully extended. Pickford and Senut (2001) claim this grove proves a bipedal mode of locomotion and possibly obligate bipedalism. Additional leg bones or pelvic remains need to be discovered before this hypothesis can be proven.
Many researchers (eg: Haile-Selaisse et. al, 2001) have difficulty accepting that Orrorin tugenensis may have been bipedal, particularly due to the treatment and interpretation of the femora since their discovery. The femur that retains its head was found in three pieces with unclean breaks, and was glued together before internal structural measurements could be taken. Although CT scans were done to reconstruct the bone without surface deformities and cracks, the resolution was too poor to be certain of the pattern of bone thickness.
Senut, who reconstructed the bone, has been accused of ‘creating’ the current position of the bone to demonstrate its bipedal aspects (Gibbons, 2006). Pickford and Senut remain adamant that the CT scans prove the bipediality of O. tugenensis, and refuse to allow further examination of their discoveries. This ensures many questions about this femur will remain unresolved; this is doubly unfortunate since O. tugenensis is the only early hominin specimen with significant postcranial remains. Pickford and Senut have also been criticized regarding their placement of O. tugenensis in human phylogeny (Aiello & Collard, 2001).
They claim that because of dental and postcranial morphological similarities with later Homo, and size and shape differences with australopithecines, O. tugenensis is a direct ancestor of modern humans, and that the Australopithecus genera was a dead end. They also believe that the Ardipithecus taxon is an ancestor of modern chimpanzees. They have proposed a simple two-branch evolutionary tree, with one branch leading to Australopithecus and extinction, and the other leading from Orrorin to the novel, intermediary genus Praeanthropus and then to Homo (Pickford & Senut, 2001).
If Orrorin is indeed a hominin, its many similar traits may indicate that it did lead, possibly directly, to Homo. Another possibility is the traits found in Orrorin were lost in the Australopithecine genus then evolved again several million years later. Although the same characteristic evolving twice is not unheard of (e. g. wings in birds and insects), Pickford and Senut seem resolved in rendering all other hominin genera obsolete. Many other paleoanthropologists believe that Orrorin should be classified as Ar. adabba as the two species are of similar ages, were discovered in close proximity, and have a few similar dental features. However, bipediality cannot yet be confirmed in O. tugenensis, and the similar dentition to modern humans may be a result of a similar, omnivorous diet (Aiello & Collard, 2001). It appears that this early hominin shared a similar wetland/forested environment with Ardipithecus, which has provided further support for the hypothesis that the first hominins did not evolve in a plains/savannah environment (Pickford & Senut, 2001).
Despite the similarities to modern humans and the proposed phylogeny, O. tugenensis is not yet definitively proven to be a hominin. Like both species of Ardipithecus, additional fossil remains must first be uncovered. While paleoanthropologists were bickering over the phylogeny of Ardipithecus and Orrorin, Michel Brunet, a French archaeologist working in the Djurab Desert of Chad, uncovered the nearly complete cranium of what may be the oldest possible hominin (Brunet et. al. , 2002). The cranium was found in several hundred fragments but when pieced together it is 95% complete, except it was missing the lower jaw.
The cranium was found in the Toros-Menalla layer and associated floral and faunal remains of extinct species have placed the date of this potential hominin at six to seven million years ago. Shortly after the initial find a lower jaw was discovered in two pieces not far from the remainder of the skull. Brunet is arguing hominin status based on facial features alone as no postcranial remains have been discovered (Brunet et. al. , 2005).
Like those before him claiming they had found the earliest human ancestor, Brunet was the focus of intense criticism, primarily suggesting that his reconstruction and interpretation of the skull was biased (Wolpoff et. al. , 2002). In attempts to counter these accusations Brunet subjected the cranium to intense CT scanning which reconstructed it at hundreds of different angles and showed what it would look like without cracks and distortions (Zollikofer et. al. , 2005).
Brunet and his colleague Pilbeam maintain that S. tchadensis walked upright and is the earliest hominin, but Wolpoff and the discoverers of O. ugenensis believe it is an early form of gorilla or possibly the ancestor of an extinct ape and have suggested renaming it Sahelpithecus tchadensis (Wolpoff et. al. , 2002). Although the CT images are available to everyone, only Brunet and his team have actually examined the skull. The cranium needs to be measured and interpreted by other individuals to remove biasing. A problem with the S. tchadensis skull, the O. tugenensis femur, and many other early remains is that paleoanthropologists are all looking at the same fossil and seeing different things because they are clouded by their preconceived judgments.
This is a major weakness of many paleoanthropologists; researchers tend to not see what is there, or not there, but only what they want to see. It is doubtful than any of these issues will be resolved without the aid of many additional postcranial fossil remains. The cranium, which Brunet assumes is from a male, has an orthognathic face with weak subnasal prognathism, a small ape-size braincase (from 320 to 380 cc), a massive brow ridge (comparable to a male gorilla), and a long and narrow basicranium.
Sahelanthropus tchadensis is marked by other features: the upper part of the face is wider than the lower; a large canine fossa is present; and the specimen has a small and narrow U-shaped dental arch; orbits are crowned with a thick, large, and continuous supraorbital torus; marked postorbital constriction; small posteriorly located sagittal crest and a large nuchal crest; large mastoid process; relatively small incisors; upper canines longer mesiodistally than buccolingualy; no diastema; and the enamel thickness of the cheek teeth intermediate between Pan and Australopithecus (Brunet et. al. , 2002).
Brunet distinguishes it from gorillas by its smaller size, narrower and less prognathic lower face, and smaller canines and cheek teeth, and from chimpanzees by the narrow basicranium with a flat nuchal crest, the anteroposteriorly shorter face, and the thick enameled cheek teeth. S. tchadensis is distinguished from Miocene apes by its small teeth with thinner cheek-teeth enamel and thicker molar enamel and by the dimensions of the face. It is clearly distinct from the Australopithecus, Kenyanthropus, Paranthropus and Homo genera, primarily because of the date of the fossils and the minimal cranial capacity.
It can be distinguished from Ardipithecus by its less incisiform upper canines that are not diamond-shaped and have a mesiodistal long axis, and by its lower canines with a stronger distal tubercle; and from Orrorin by additional dental differences including non-chimp-like upper canines (Brunet et. al. , 2002). Because the face shows a mosaic of derived and primitive features it is difficult to distinguish whether it is an ape or a hominin based on facial characters alone.
According to Brunet the CT scans show that the reconstructed cranium has a forward positioned foramen magnum, located between that of modern day humans and chimpanzees. He believes this means the head was supported on an upright body and thus S. tchadensis was a bipedal hominin (Zollikofer et. al. , 2005). Pickford, Senut, and Wolpoff (Wolpoff et. al. , 2002) have measured this angle differently and see it as much closer to an ape and suggest it may be a female gorilla.
Only the discovery of additional postcranial remains will resolve such controversies. In 1993 Michel Brunet also discovered a new species of australopithecine, A. bahrelghazali, in Central Africa, and expanded the known range of hominins by 2500 km west of the Rift Valley (Brunet et. al. , 1996). This hominin falls within the range of variation of A. afarensis and may eventually prove to be A. afarensis, but regardless Brunet has demonstrated that early hominins occupied a much wider area than previously thought. If S. chadensis is proven to be a hominin, it may indicate that human evolution occurred in central Africa rather than West Africa as previously assumed (Gibbons, 2006).
The fossils from Chad provide further evidence that the earliest humans likely inhabited a wooded, forested environment. The rich fauna from the Toros-Menalla layer includes a significant aquatic component including fish, crocodiles and amphibious mammals, as well as those associated with gallery forest and savannah, such as primates, rodents, equids, bovids, and elephants (Vignaud et. l. , 2002). Brunet assumes that S. tchadensis lived near a lake but not far from a desert; this is the oldest record of desert conditions in the Neogene of northern central Africa. Paleoanthropologists have already begun to reconsider the environment in which the ape-hominin split occurred and they may soon have to reconsider the geographical location as well. New fossil discoveries raise new debates regarding the origins of our species and its evolution. Old assumptions are tossed out, new hypothesis are supported.
None of these four species can yet be definitively labeled a hominin, and the human phylogenetic tree revised and redrawn. The paleoanthropologists must first unearth further postcranial remains to prove conclusively that ‘their’ potential hominins were obligate bipeds, which is the defining characteristic of humankind (Gibbons, 2006). Many fossil finds have undergone periods of intense criticism where notable researchers have denied the remains belonged to a hominin, these arguments were only resolved with the discovery of new fossil evidence.
Re-examination of previous finds can seldom resolve complex issues of fossil morphologies and evolutionary relationships. Until this new evidence is found the paleoanthropological community will debate on. And, while it remains unsure which species may have contributed what traits to human evolution, it seems certain that the more we can learn of the roots of our phylogenetic tree the more we can learn of the roots of what defines us as human beings.