An earth-shattering discovery in Hadar, Ethiopia rocked the anthropological world when on November 30th in 1974 Donald Johanson and Tom Gray unearthed the nearly complete skeleton of an approximately 3.18 million-year-old hominid, belonging to the previously unknown species Australopithecus afarensis, referred to as A.L. 288-1 or, more commonly, as Lucy. The discovery of Lucy, at the time of her initial discovery the oldest and most complete hominid skeletons that has ever been found, has dramatically changed the way scientists thought about ancient hominids and have understood one of the basic concepts on human evolution. The find has proved that the human ancestors have been walking upright before the size of their brains has increased.
However, another early hominin skeleton, referred to as Selam (Amharic word for 'peace'), Dikika baby or DIK-1-1, which has been found at Dikika in Ethiopia and resembled the earliest, most complete juvenile ancestor ever found, has raised 'new questions about the importance of arboreal behavior in the Australopithecus afarensis repertoire' (Alemseged et al., 2006). The locomotion debate whether Australopithecus afarensis has been an effective, terrestrial biped (Lovejoy, 1988) or has retained aspects of ancestral arboreal behavior and, therefore, has been a facultative biped (Rak, 1991; Stern & Susmann, 1983; Stern, 2000), has divided paleoanthropologists into two camps, although there have been few disagreements about fossil morphology (Kimbel & Delezene, 2009).
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In addition, 'whether A.L.288-1 exhibited essentially human proportions, or whether this skeleton had comparatively longer arms and/or relatively shorter legs than modern humans, has been discussed extensively' (Hartwig-Scherer, 1991; Jungers & Stern, 1983; Wolpoff, 1983). In her article 'Interpreting the posture and locomotion of Australopithecus afarensis: Where do we stand?' Carol V. Ward (2002) pointed out that most researchers agree that individuals of the species Australopithecus afarensis have been habitual bipeds but 'disagree over the importance of arboreality for them.' She concluded that there are basic differences about how to reconstruct function from morphology and about how to relate function to behavior. The extent to which terrestrial bipedality was the committed form of locomotion is still actively debated today. (Kimbel & Delezene, 2009)
On the 30th of November, a cloudless day in 1974, the paleoanthropologist Donald Johanson and the American graduate student Tom Gray have discovered the famous fossil known as Lucy also referred to as Dinknesh (Amharic word for 'you are beautiful') or A.L.288-1. They have found the female Australopithecus afarensis in the upper part of the Hadar Formation, central Afar, Ethiopia. She has provided the best evidence for early hominin pelvic morphology (Johanson et al., 1982a; Lovejoy, 1988, 2005b). Both, Johanson and Gray were part of the International Afar Research Expedition (IARE), which was founded in 1973 by Dr. Maurice Taieb, a Tunisian born French geologist and paleoanthropologist, Donald Johanson and Yyes Coppens, a French anthropologist, after they have recognized the potential importance of the richly fossiliferous Awash valley in Ethiopia's Afar triangle to paleoanthropology (Johanson et al., 1982a; Lovejoy, 1988, 2005b).
The Afar triangle, in the Horn of Africa, is part of the East African rift, which spreads from Jordan to Mosambique. It is a large subsiding basin that is the result of tectonic plates (the Arabian Plate and the African Continental Plate) drifting apart. Several million years ago layers of sediment have been deposited and have enclosed fossils. (Sesin & Engeln, 2005)
On this day in November 1974, Tom Gray, who had come out to Hadar to study fossil animals and plants of the region and to reconstruct accurately what the climate was like in the remote past, and Donald Johanson, who was interested in hominid fossils and 'felt the strong subconscious urge to go with Gray' (Johanson & Edey, 2008), surveyed a gully at the locality #162 for several hours until the temperature became unbearable. The men decided to go back to the expedition's Land-Rover, took a small final detour before reaching the vehicle and then Donald Johanson spotted something that seemed to belong to a small hominid.
He discovered a right proximal ulna. Shortly thereafter, he saw an occipital bone, then part of a femur, a couple of vertebrae, a partial pelvis including a complete pelvic bone and the sacrum as well as pieces of a mandible. The almost 40% complete skeleton consisting of several hundred fragments, approximately 3.18 million years old, represents a female Australopithecus afarensis specimen, which was found on the edge of the small, muddy river Awash in Hadar, Ethiopia, approximately 300 miles northeast of Addis Ababa. That night there was much celebration and excitement over the discovery of the relatively complete hominid skeleton.
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'The camp was rocking with excitement.' The team was celebrating, drinking and dancing; a tape recorder was playing the Beatles' song 'Lucy in the sky with diamonds' over and over again at full volume. At some point during the night, no one remembers when or by whom, the skeleton was given the name 'Lucy'. This incredible find was deemed significant due to the fact that it resembled the brain capacity of an ape, the bipedal upright gait of a human and provided further evidence that bipedalism occurred earlier than increased brain size in human evolution. (Johanson & Edey, 2008)
During field sessions between the end of 2000 and 2003 by the Dikika Research Project (DRP), led by Zeresenay Alemseged, a researcher at the Max Planck Institute for Evolutionary Anthropology in Leipzig, the fragmented skull and the associated partial skeleton (DIK-1-1) of a well-preserved, approximately three-year-old, individual were recovered from a block of cement-like sandstone with dentist's drills and picks in the region Dikika in Northeast Ethiopia. (Alemseged et al., 2006).
'The find comprises the whole skull with a natural brain sandstone impression in addition to previously completely unknown or very little known skeletal parts including the hyoid bone. Of the upper part of the skeleton, most of the spinal column, both shoulder blades, the ribs and both collar bones were recovered. Parts of the lower limb, including both knee caps and substantial parts of the thigh and shinbones from both legs, were also recovered in addition to an almost complete [left] foot' (Wirsing & Trepte, 2006)
Jonathan Wynn (2006), from the University of St. Andrews in Scotland, and his colleagues dated the volcanic sediments lying above and below the find to 3.3 million years, which makes Selam about 220,000 years older than Lucy (Hartwig-Scherer, 2007). 'The three-million-year old skeleton of a three-year-old child provides an outstanding resource to understand the development of a human ancestor that seems to have both walked upright and climbed through trees' (Wood, 2006).
The vertebral column
Lucy's vertebral column, represented by several partial and complete thoracic and lumbar vertebrae (Johanson et al., 1982a), shows evidence of the spinal curvature (lordosis) necessary for a permanent upright stance. This configuration is biomechanically advantageous for a biped; it aligns the support of the spine directly beneath the upper body and head and over the hips.
Owen Lovejoy (2005a) stressed that 'the morphological changes that permit increased suspension vary, but always include a change in thoracic (and scapular shape) and a significant reduction in the length of the 'free' lumbar column.' Mobility in the lumbar region in enhanced in humans by a reduction in iliac height, which reduces the lumbar entrapment present in great apes through immobilization of the lowest one or two vertebrae.
In addition, the 'hominid lumbar vertebrae exhibit a caudally progressive widening of their laminae and the space separating their articular processes (zygapophyses),' which is related to the relatively broader ilia and sacrum in Homo sapiens. (Lovejoy, 2005a)
The most complete vertebral column is represented by the partial juvenile skeleton DIK-1-1. All cervical and thoracic vertebrae as well as the first two lumbar vertebrae are preserved (Alemseged et al., 2006). LUCY VERTEBRAL COLUMN
'As is well established, the pelvis of Homo sapiens is highly derived morphologically and functionally relative to the great apes. Derived features include a relatively broad sacrum, enlarged auricular surfaces of the sacroiliac joint, a superoinferiorly short and anteroposteriorly broad iliac blade that is rotated into the sagital plane. This derived pelvic morphology affects the action of the muscles that assist in bipedal locomotion. Most importantly, the gluteus medius and minimus muscles, which originate from the lateral iliac surface, function as hip abductors' and thereby stabilize the trunk 'during stance phase of bipedalism in Homo sapiens, whereas they function as hip extensors in extant apes' (Kimbel & Delezene, 2009)
Lucy's pelvis, which provides the best evidence for early hominin pelvic morphology (Johanson et al., 1982a; Lovejoy, 1988, 2005b) exhibits a number of adaptations to bipedality. The entire structure has been remodeled to accommodate an upright stance and the need to balance the trunk on only one limb with each stride. The A.L.288-1 ilium is short, broad and sagitally rotated, which indicates the presence of the derived role for the gluteal muscles and a reduction in lumbar entrapment (Lovejoy, 2005b). Both, Owen Lovejoy and Bruce Latimer, YEARS stressed that the construction of Lucy's pelvis and the insertions of her muscles in it indicate that she would have had just as many difficulties with climbing in trees as a modern human. (Johanson, 2006)
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In the article 'Obstetric Pelvis of A.L.288-1 (Lucy)', Tague and Lovejoy (1986) illustrated that 'the innominate and pelvis of A.L.288-1 differ from human females in some of the dimorphic traits.' It is platypelloid (broad and flat), which would present obstetric complications in human females, as well as funnel-shaped, a feature which closely resembles that of human males. They pointed out that 'the platypelloid pelvis of A.L. 288-1 shows adaptations for locomotion and visceral accommodation and support.' And they thereby contradicted Wolpoff's (1978) proposal that Lucy belonged to a population characterized by large neonatal crania.
Hartwig-Scherer and Martin (1991) compared A.L.288-1, the partial skeleton attributed to Australopithecus afarensis (dated 3.18 million years ago; Johanson et al.; 1982a), and OH62, referred to as `Lucy's child' and attributed to Homo habilis (dated 1.8 m.y.a.; Johanson & Shreeve, 1989). The relationships between various dimensions of upper and lower limb bone remains compared to those of modern African apes and humans. 'For the comparative sample, measurements were conducted on associated adult skeletons with humerus, radius, femur and tibia of 54 male and female African apes and 40 Homo sapiens.' The dimensions of OH62 were taken on the originals, and the measurements of A.L.288-1 were based on casts. The results of the comparison indicated that OH62, with longer and more robust upper limb bones but smaller and more gracile legs than A.L. 288-1, displays closer similarity to African apes, although Australopithecus afarensis is supposedly the ancestor of Homo habilis.
Drapeau and Ward (2007) assessed the relative length of the arm, forearm, wrist and palm of the partial skeleton A.L.438-1 and the more complete skeleton A.L.288-1 of which a nearly complete humerus, partial radii and ulnae, and a capitate are preserved (Johanson et al., 1982). They compared the scaling relationships between pairs of forelimb bones to extant hominoids and Australopithecus afarensis, and length of individual forelimb elements to estimated body size. Their conclusion was that 'A. afarensis has no forelimb bones are significantly longer than those of humans for its size. It falls within the range of variation seen in modern humans for all comparisons relative to size, but appears to differ from the typical human brachial index due to a slightly shorter humerus and/or slightly longer ulna.' The brachial index of Lucy (term sometimes used interchangeably with the species name) has been estimated to be about 91 to 92.5%, an intermediate value between Pan troglodytes (95%) and Homo sapiens (80%) (Drapeau & Ward, 2007; Kimbel & Delezene, 2009).
The scapulae in the juvenile specimen DIK-1-1 are virtually complete and more like that of a juvenile and an adult gorilla than that of a modern human, as Bernard Wood (2006) and Zeresenay Alemseged (2006) remarked. The superiorly facing glenoid fossa may indicate a superiorly positioned shoulder girdle, 'and possibly the absence of effective decoupling of head and trunk movements, typical of modern humans and their capability for endurance running.' The orientation of the glenoid fossa may also indicate frequent use of the arms above the head, which would be consistent with climbing in trees (Alemseged et al., 2006).
The scapula associated with the adult specimen A.L.288-1 is represented by the complete and undamaged glenoid fossa, the partial lateral border and the portion of the base of the acromion. (Johanson et al., 1982a). The deltoid muscle insertion site is prominent; the concoid tubercle is small and located on the inferior surface of the clavicle, as in Homo sapiens (Johanson et al., 1982a; Lovejoy et al., 1982a; Drapeau et al., 2005).
Stern and Susman (1983) discussed the bar-glenoid angle of the scapula associated with A.L.288-1 and concluded that Australopithecus afarensis possessed anatomic characteristics that indicate a significant adaptation for movement in the trees. However, Inouye and Shea (1997), using the ontogenetic allometric approach, examined the bar-glenoid angle in several ape species, in humans and in A.L.288-1. They demonstrated that the glenoid orientation in humans and apes overlaps at small body sizes and concluded that the feature, which was thought to indicate arboreal tendencies, is not tightly correlated with function and, therefore, cannot be used as morphological evidence for arboreal behavior. However, other parts of the forelimbs in Australopithecus afarensis may indicate suspensory behavior.
STERN 2000 for contradiction of Inouye & Shea (1997)
The humerus of Australopithecus afarensis is represented by several adult specimens from Hadar as well as by the juvenile specimen DIK-1-1 (Alemseged et al., 2006; Johanson et al., 1982a; Lovejoy et al., 1982b). As Hartwig-Scherer and Martin (1991) confirmed in their study, the humerus of A.L.288-1 is shorter than that observed in extant apes and in this is similar to the modern human. DESCRIPTION (head elliptical as opposed to spherical condition in great apes, etc.) LOVEJOY et al., 1982b
'The humerus bears strong muscle scars, including a well-developed lateral supracondylar crest (the origin of the brachioradialis muscle) and a prominent insertion for the pectoralis major along the lateral crest of the bicipital groove (Lovejoy et al.; 1982b; Drapeau et al., 2005), which suggests that the A. afarensis arm was heavily muscled' (Kimbel & Delezene, 2009).
The olecranon process of the ulna in A.L.288-1 projects proximally, which resembles the condition in Homo sapiens. To Drapeau et al. (2005) 'hominin olecranon morphology signals a reduction of the use of the arm in extended postures, as occurs in suspensory contexts, as well as in knuckle-walking. Drapeau et al. argued that the hominin configuration might be associated with manipulative behaviors that occur when the forearm is flexed
at ~ 90'' (Kimbel & Delezene, 2009). The morphology of the ulna supports the hypothesis of Australopithecus afarensis being a fully terrestrial biped rather than a facultative biped.
The femur of Australopithecus afarensis is present in A.L.288-1 (Johanson et al., 1982a; Lovejoy et al., 1982c) as well as in DIK-1-1 (Alemseged et al., 2006). DESCRIPTION
Several features of the femur may indicate bipedality in its possessor. For example, in Homo sapiens, the attachment site of the gluteus maximus (gluteal tuberosity sometimes referred to as third trochanter) is located on the back of the femoral shaft rather than on its side, as it is the case in the great apes. In chimpanzees, there is no gluteal tuberosity; there is a lateral spiral pilaster. (Pickford et al., 2002) As pointed out by Lovejoy et al. (2002) 'the variations in muscle attachments in this area of the femur reveals a fundamental difference between hip and thigh musculature in apes and human, which almost certainly relate to the major postural and locomotor repertoire of the hominoids involved ' quadrupedalism and semi-orthograde posture in African apes, and bipedalism and fully orthograde posture in humans and australopithecines.'
Different hip and thigh muscles in Homo sapiens help to balance the trunk on the lower limbs. The gluteus maximus keeps the trunk from rotation forward. The gluteus medius and the gluteus minimus, which are positioned lateral in Homo sapiens, keep the trunk from falling sideways upon shifting of the centre of mass during one-legged stance and prevent hipdrop during locomotion. And ' the obturator externus is an adductor, flexor and external rotator of the thigh. One of its functions is to shorten the distance between the pelvis and the femur thereby steadying the hip joint' (Pickford et al., 2002).
It has been suggested that the obturator externus groove, the attachment site of the obturator externus muscle which is located on the posterior neck of the femur, is a landmark indicative of hyper-extended femora and, thus, bipedal locomotion. It is thought to be caused by pressure of the obturator externus tendon against the femoral neck during extension of the hip. An obturator externus groove, which is usually absent in apes due to little or no pressure on the femoral neck, does exist in A.L.288-1 (Lovejoy et al., 1982c, 2002; Pickford et al., 2002). However, Stern and Susman (1983) considered that the presence of a small tubercle at the base of the femoral head of A.L.288-1 gives the illusion of an obturator externus groove.
Bacon (1997) called the functional significance of the obturator externus groove into question. She claimed to have observed the presence of such groove on the posterior femoral neck in primates that do not use frequent hip extension and pointed out that no evidence for the constant presence of an obturator groove in fossil hominids could be found. And, again, it is Pickford et al. (2002) who stated in their concise review paper that 'the occurrence of a groove in a few catarrhine and platyrrhine primates is due to convergence and not to phyletic relationship, and thus it does not weaken the value of the character for determining functional anatomy within the Hominoidae.' They concluded that within Hominidae 'the presence of a clear obturator externus groove is a derived character related to orthograde posture and bipedal locomotion.'
Another important bipedal feature is a low bicondylar angle between the shaft of the femur and the femoral neck. A higher bicondylar angle than that observed on average in Homo sapiens, is present in the femora of both australopithecines, the adult specimen A.L.288-1 and the juvenile specimen DIK-1-1 (Lovejoy et al., 1982c, Alemseged et al., 2006). In other words, the shaft is angled relative to the condyles, which allows bipeds to balance on one leg at a time during locomotion. In addition, Lucy's femoral condyles are large, flattened and ellipsoid; they are adapted to handling the added weight which results from shifting from four limbs to two.
Ultimately, the femoral neck is put under tension superiorly and under compression distally as the body weight is transferred to the ground through the hip joint. Therefore, cortical bone distribution in the femoral neck would be expected to be thick superiorly and inferiorly in order to withstand the forces due to loading with body weight. However, as Ohman et al. (1997) showed in their study of 35 hominoid femora, 'throughout the femoral neck H. sapiens displays thin superior cortical bone and inferior cortical bone that thickens distally. In marked contrast, cortical bone in the femoral neck of African apes is more uniformly thick in all directions, with even greater thickening of the superior cortical bone distally. Because the femoral neck acts as cantilevered beam, its anchorage at the neck-shaft junction is subjected to the highest bending stresses and is the most biomechanically relevant region to inspect for response to stress. Cortical distribution in the African ape indicates much greater variation in loading conditions consistent with their more varied locomotor repertoire. Cortical distribution in hominids is a response to the more stereotypic loading pattern imposed by habitual bipedality.'
JIM'S ARTICLE 1997
' Lovejoy (1988, 2005b) argued that the femoral neck acts as the cantilevered beam during bipedal locomotion and that the combined action of body weight and hip abductor muscle force during single-limb support phase subjects the inferior aspect of the femoral neck to very high compressive loads, resulting in the development of an especially thickened band of cortical bone in this part of the neck' (Kimbel & Delezene, 2009). Therefore, the cortical bone distribution in the femoral neck may be classed as another important implication of bipedalism. And in Australopithecus afarensis the cortical bone distribution in the femoral neck resembles the asymmetric pattern found in humans (Ohman et al., 1997; Lovejoy et al., 2002). However, the importance of the cortical bone distribution in the proximal femur as an indicator of bipedalism has been questioned (Stern, 2000).
Another bipedalism feature is an elongated femoral neck, which acts as a cantilevered beam, as previously stated (Lovejoy et al., 1988, 2002, 2005b). LOVEJOY 1988 + 2005b
The valgus knee was fully extendable. The knee joint in the species Australopithecus afarensis has a high carrying angle of 15' as opposed to an angle of 9' in humans. There is also a prominent patellar lip present, to keep the patella from dislocating due to this angle, (Lovejoy, 2007). LOVEJOY 2007
The tibiae of DIK-1-1 with their transversely expanded shafts below the tibial plateau resemble those of Homo sapiens. (Alemseged et al., 2006). And the right distal fibula, in association with the distal tibia and talus of A.L.288-1 (Johanson et al., 1982a) permitted a reconstruction of the talocrural joint. LATIMER et al. 1987
In contrast, Stern and Susman (1983) claimed that the distal fibula of Australopithecus afarensis reflects increased plantarflexion of the talocrural joint, as opposed to the neutral position observed in Homo sapiens. CONTRADICTION ->DORSIFLEXION
The foot is particularly specialized for obligate bipedalism in both its anatomy and its function. Some of the features resembling adaptations to obligate bipedalism include pronounced heel strike, which leaves a deep impression on soft ground (LAETOLI?), lateral transmission of force from the heel to the base of the lateral metatarsal, a well-developed medial longitudinal arch and a converged, unopposable hallux positioned in front of the ball of the foot and parallel to the remaining metatarsals which functions to transfer weight efficiently during toe off (Harcourt-Smith & Aiello, 2004). Langdon et al. (1991) stated that the human foot is involved in weight-bearing as well as in shock absorption; and it can act as an extended lever arm for propulsion. 'The creation of a rigid foot and its longitudinal arch is a critical adaptation for efficient bipedalism.'
They have examined '. LANGDON et al. 1991
Sesin and Engelin (2005) claimed that paleoanthropologists assume that Lucy spent a considerable amount of time in the trees because the species had an abducted big toe. However, they failed to explain how they came to this conclusion and whether or not studies were conducted to support this statement. Harcourt-Smith and Aiello (2004), for example, reviewed the evolution of human bipedal locomotion with a specific focus on the evolution of the foot. They did not state anything about an opposable hallux in Australopithecus afarensis though.
They discussed and assessed the varying interpretations based on fossil material such as the A.L.288-1 skeleton along with shifting theoretical perceptions and analytical approaches in combination with new three-dimensional morphometric analyses of australopithecine foot bones. They pointed out that 'there may have been greater diversity in human bipedalism in the earlier phases of our evolutionary history than previously suspected', and divided the debate about the development of obligate bipedal locomotion into three distinct parts. According to them the first part of the debate focuses on the likely locomotor repertoire that preceded bipedalism, the second part discusses the ecological and behavioral reasons for bipedalism, and the third part considers 'the degree to which certain hominin taxa were obligate bipeds (Stern & Susmann, 1983; Latimer et al., 1987).'
The foot bones of Australopithecus afarensis have been subjected to controversies. 'They are decribed by some as being compliant with full bipedal locomotion (Latimer & Lovejoy, 1982; Latimer et al., 1987), whereas others have suggested that the same fossils show traits that indicate a mosaic of terrestrial and arboreal locomotion (Stern & Susman, 1983)' (Harcourt-Smith & Aiello, 2004).
The species Australopithecus afarensis was clearly adapted to bipedal locomotion, though biomechanically less efficient than Homo sapiens, and thereby giving the impression of a 'partly arboreal, funny-walking biped' (Boaz, 1988; Stern, 2000). The more human-like, derived post-cranial morphology of Australopithecus afarensis includes a broad, short, sagitally rotated ilium and a wide sacrum, lumbar column lordosis, a human-like bicondylar angle of the femur, a valgus and fully extendable knee, and a human-like talocrual joint. All of these derived features can clearly be seen in A.L.288-1 in combination with primitive features such as short femora. Some paleoanthropologists emphasized the clear evidence in the A.L.288-1 skeleton and others fossil hominids supporting the theory of selection for terrestrial bipedality, which in their view occurred at the expense of arboreal efficiency (Latimer et al., 1987). However, there may also be some indications of well-developed climbing abilities.
Several studies concluded that the upper body of Australopithecus afarensis is, in many ways, more like that of apes rather than that of humans. These primitive traits may have been have been of adaptive significance in Australopithecus afarensis and may have been preserved by selection as well. Primitive features in the upper body of Australopithecus afarensis include but are not limited to a more cranially orientated scapular glenoid fossae, long forearms, a prominent brachioradialis origin, and a funnel-shaped thorax, which may imply an at least partial ability for arboreal climbing. Other studies concluded that some features in Australopithecus afarensis resemble those found in humans rather than those of apes (Wolpoff, 1983).
'The functional interpretation of these features is highly debated, with some arguing that the upper limb features are non-functional retentions from a common ancestor only, whereas others propose that they were preserved because A. afarensis maintained, to some degree, an arboreal component in its locomotor repertoire.' (Alemseged et al., 2006). Carol V. Ward (2002), for example, suggested that climbing behavior cannot be confidently predicted from primitive traits and can only be assumed if based on derived traits that reflect selection for arboreal locomotion.