D.J. Meldrum, Martin G. Lockley, Spencer G. Lucas, Charles Musiba
a Department of Biological Sciences, Idaho State University, 921 S. 8th Ave., Stop 8007, Pocatello, ID 83209-8007, United States
b Dinosaur Track Museum, CB172, University of Colorado at Denver, PO Box 173364, Denver, CO 80217-3364, United States
cNew Mexico Museum of Natural History, 1801 Mountain Road NW, Albuquerque, NM 87104-1375, United States
d Department of Anthropology, University of Colorado at Denver, PO Box 173364, Denver, CO 80217-3364, United States
At 3.6 Ma, the Laetoli Pliocene hominin trackways are the earliest direct evidence of hominin bipedalism. Three decades since their discovery, not only is the question of their attribution still discussed, but marked differences in interpretation concerning the footprints’ qualitative features and the inferred nature of the early hominin foot morphology remain. Here, we establish a novel ichnotaxon, Praehominipes laetoliensis, for these tracks and clarify the distinctions of these footprints from those of later hominins, especially modern humans. We also contrast hominin, human, and ape footprints to establish morphological features of these footprints correlated with a midtarsal break versus a stiff longitudinal arch. Original photos, including stereo photographs, and casts of footprints from the 1978 Laetoli excavation, confirm midtarsal flexibility, and repeatedly indicate an associated midfoot pressure ridge. In contrast, the modern human footprint reflects the derived arched-foot architecture, combined with a stiff-legged striding gait. Fossilized footprints of unshod modern human pedestrians in Hawaii and Nicaragua unambiguously illustrate these contrasts. Some points of comparisons with ape footprints are complicated by a variable hallucal position and the distinct manner of ape facultative bipedalism.
In contrast to the comparatively rigid platform of the modern human foot, midtarsal flexibility is present in the chimpanzee foot. In ape locomotion, flexion at the transverse tarsal joint, referred to as the ‘‘midtarsal break,’’ uncouples the respective functions of the prehensile forefoot and the propulsive hindfoot during grasp-climbing. At some point after the transition to habitual bipedalism, these grasp-climb adaptations, presumed to be present in the last common ancestor of apes and humans, were initially compromised by the loss of divergence of the hallux. An analogous trajectory is evident along an array of increasingly terrestrial extant ape species. However, a flexible midfoot was retained, presumably to spare lateral toes from bending stresses by concentrating push-off from the forefoot, beneath the metatarsals. Only later did the evolution of the longitudinal arch permit increased mechanical advantage of the plantarflexors for speed and improved economy of endurance distance walking and running.
Found in Tanzania, the Laetoli hominin trackways are dated to 3.6 Ma and offer the earliest direct evidence of hominin bipedalism (Leakey and Harris, 1987). Since their discovery and excavation in 1978 and 1979, there has been continuing discussion and debate over the interpretation and attribution of the trackways. The lack of consensus concerning the Laetoli hominin tracks clearly hinges upon the interpretation and recognition of the quantitative and qualitative features of the footprints, which are influenced, at least to some degree by conditions of preservation. These are moderately favorable but not exceptional (White and Suwa, 1987). In addition, the lack of consensus has also been influenced by differences of perspective – a glass half full vs. half empty. At the extremes, those perceiving the glass half full emphasize the human-like qualities: e.g., non-divergent hallux, relatively short lateral digits. Those perceiving the glass half empty emphasize the retention of primitive traits: e.g. lack of longitudinal arch, relatively small hindfoot. The two alternate hypotheses can be generalized as: (1) the footprints are essentially indistinguishable from those of modern humans and represent an as yet unrecognized relatively derived hominin (e.g. Day and Wickens, 1980; Chartelis et al., 1981; Alexander, 1984; Lovejoy, 1988; Tuttle, 1985, 1996; Tuttle et al., 1990, 1991; Musiba et al., 1997; Schmid, 2004; Sellers et al., 2005; Harcourt-Smith and Hilton, 2005; Kimbel and Delezene, 2009); and (2) the footprints indicate a trackmaker with a foot architecture manifestly distinct, in their mosaic and/or intermediate nature, from modern humans (Stern and Susman, 1983; Susman et al., 1984; Deloison, 1991, 1992; Clarke, 1999; Meldrum, 2000, 2002, 2004a,b; Berge et al., 2006; Meldrum and Chapman, 2007; Bennett et al., 2009).
Here, we provide a comparative analysis of the Laetoli hominin footprints that begins by bringing them into the formal scope of ichnotaxonomy. In 1985, the International Commission on Zoological Nomenclature agreed to regulate formally the names of ichnotaxa based on trace fossils (e.g., Rindsberg, 1990). This formalized a confused parataxonomy of many kinds of trace fossils, especially vertebrate footprints, based on Linnean nomenclature, in which traces are assigned to ichnogenera and ichnospecies. The advantage to naming ichnotaxa is that within the parataxonomy they can be referred to unambiguously based on morphology (the basis of the parataxon) without having to make a definite statement as to the tracemaker’s identity, which is usually not known or debateable (e.g., Bertling et al., 2006).
Impetus for naming the Laetoli hominin tracks arises in part from growing interest in hominin tracks (e.g., D’Aout et al., 2010; Raichlen et al., 2010; Bennett et al., 2009; Kim et al., 2008a; Lockley et al., 2007, 2008; Meldrum, 2004a,b, 2007a,b) and the recognition that none had ever been formally named, until Kim et al. (2008b) proposed the name Hominipes modernus, for well-preserved tracks unequivocally attributed to Homo sapiens. That ichnotaxon is based on the type footprints at the Acahualinca Footprint Museum, Managua, Nicaragua (Lockley et al., 2008). There is no systematic reason to exempt fossil hominin footprints from the same ICZN-approved methods of ichnotaxonomic scrutiny applied to other mammal and tetrapod footprints from the fossil record. Indeed, to do so would be to create an unjustified exception. Thus, it is logical to infer that any hominin tracks that are morphologically distinct from H. modernus, regardless of the trackmaker(s) that made them, could potentially form the basis of a new ichnotaxon. As aforementioned debates indicate, several authors have already intimated that older hominin tracks appear to be morphologically distinct from tracks assigned to the ichnospecies H. modernus, and although trackmaker identity is not the prime criterion in making such judgments, there has been considerable debate about trackmaker identity. Moreover, by definition pre-sapiens species are morphologically distinct from H. sapiens in terms of various diagnostic skeletal features. Thus, the question arises as to whether the trackmaker foot morphology is distinct, and represented by morphologically distinct and distinguishable footprints.
We argue that this is the case for the Laetoli footprints and that naming them brings them into a growing parataxonomy of Cenozoic mammal footprints (McDonald et al., 2007), in particular hominin footprints (Lockley et al., 2007, 2008), and allows their distinctive morphology to be readily recognized and systematically evaluated.
2. Systematic ichnology
2.1. Praehominipes ichnogen. nov
Diagnosis: Plantigrade, entaxonic, elongate footprints of a hominin biped, that differ from Hominipes in their lack of a consistent fixed longitudinal arch, evidence of midfoot flexibility, under-differentiated ball, tapered heel, and indistinct lateral digit (II–V) impressions, which generally extend distal to hallux (see Fig. 1).
Included ichnospecies: Known only from the type ichnospecies.
Distribution: Pliocene of East Africa.
Discussion: Praehominipes is the second ichnogenus of fossil hominin footprint named. It differs from the first described ichnogenus, Hominipes (Kim et al., 2008a,b), foremost by lacking the preeminent feature of the modern human foot, the fixed longitudinal arch. Its flat sole imprint also lacks the correlated features associated with the points of plantar support of the arch, i.e., a well-differentiated ball, and distinctly wide rounded heel. Praehominipes is further distinguished from Hominipes by exhibiting longer lateral digits (II–V), extending distal to the hallux.
2.2. Praehominipes laetoliensis ichnosp. nov
Derivation of the name: (L. prae: before; homi: human; pes:foot) Prehuman footprints from Laetoli (see Fig. 2).
Diagnosis: Same as for ichnogenus.
Type material: Holotype, trackway G1 and paratypes, trackways G2 and G3, remain interred at the Laetoli site. Replicas are accessible at the Kenya National Museum, Tanzania National Museum and House of Culture, and the British Museum of Natural History.
Type locality: Laetoli, Tanzania. Near Lake Eyasi, about 50 km. south of Olduvai Gorge. The Pliocene-aged Laetolil Beds are volcanic in origin. Tuff 7, Site G, bears the hominin footprints and was radiometrically dated to 3.6 Ma (Drake and Curtis, 1987; Hay, 1987; Manega, 1993).
Description: Plantigrade, entaxonic, elongate footprints of a hominin biped. Footprint is often flat, lacking a fixed longitudinal arch typical of modern human footprints. Frequently, indication of a transverse axis of flexion at midfoot present, occasionally producing a midtarsal pressure release ridge. Ball is poorly differentiated from surrounding forefoot. Deepest point of forefoot impression often beneath inferred position of lateral metatarsal heads. Widest part of foot lies at inferred position of metatarsal heads. Heel is narrow and tapers proximally. Hallux is larger and separated from lateral digits, allowing extrusion of substrate at first interdigital space (=hypex between digits I and II). Hallucal pad impression often not sharply demarcated from forefoot. Lateral digits generally extend distal to hallux, but individual digit impressions indistinct. Step length generally greater than 2.5 times foot length.
The Laetoli site comprises a stratified sequence of sediments of middle and late Pliocene, early Pleistocene, and late Pleistocene age, ranging between 4.3 and 0.1 Ma (Drake and Curtis, 1987; Hay, 1987; Manega, 1993). Abundant vertebrate fossils and fragmentary hominin skeletal fossils come from the Upper Laetoli Beds (3.7–3.5 Ma). The hominin skeletal fossils are all attributed to Australopithecus afarensis. The footprints were discovered in 1978 by Paul Abell, and excavations led by Mary Leakey during 1978 and 1979 revealed a 27-m-long trackway, comprising 70 footprints of three individual hominins, each indicated by the prefixes G1–3 (Leakey and Harris, 1987). Other associated footprints include those of monkeys, antelopes, elephants, rhinos, three-toed horses, cat, hyaenas, giraffes, guinea fowl, lagomorphs, and francolins (Hooijer, 1987; Leakey and Harris, 1987; Musiba et al., 2008).
Meldrum (2000) first drew attention to a feature evident in a number of the Laetoli hominin footprints (e.g. G1–25, G1–26) that suggests a midtarsal pressure ridge and the retention of midfoot flexion at the midtarsal (transverse tarsal) joint. This footprint artifact is the result of a combination of local substrate character, forefoot plantar pressure, and forward propulsion of the trackmaker, so it is inconsistently expressed in individual footprints of a trackway. In spite of its variable presence, its consistent relative placement within the footprint when present is indicative of midtarsal flexibility, the ‘‘midtarsal break’’ absent in modern human feet (Elftamn and Manter, 1935). In a depiction of a reconstruction of the A. afarensis foot skeleton superimposed upon a Laetoli hominin footprint, this feature lies immediately proximal to the reconstructed position of the midtarsal joint (Fig. 3) (White and Suwa, 1987). Some have suggested this ridge may be the result of termite burrowing or an excavation artifact, but the repeated and consistent position of the feature, combined with other indications of a transverse axis of foot flexion evident in many of the footprints (Deloison, 1992; Meldrum, 2007a,b), justify its interpretation as a pressure ridge.
Among the Laetoli hominin footprints, an exceptional example of a midtarsal pressure release is clearly evident in the G1–26 footprint, indicated by the plastic flow of wet ash proximal to the midtarsus (Fig.3). This is certainly not the result of bioperturbation as has been suggested for the apparent midtarsal ridge (White, personal communication). The possibility of the feature in this instance resulting from laminar fracture and exfoliation or being another excavation artifact is reasonably excluded upon close examination. The proximal or leading edge of the flow is continuously rounded and has the same appearance as obvious ash extrusion features in other footprints (such as the ash extrusion through the hypex between the first and second digits of the G1–36 footprint).
Indeed, a careful reexamination of the qualitative features of the Laetoli hominin tracks, often neglected by rote reliance on linear and angular metrics alone, reveals a consistent constellation of correlated features that betray an archless flexible midfoot, lacking the differential development of the heel pad and ball; a hallux lacking an enlarged toe pad; and relatively long, curled lateral digits. This suggests that the reduced divergence of the hallux was not correlated with the development of a stiff medial longitudinal arch, but rather is an extension of a trend for reduced divergence of the medial digit correlated with increased terrestriality – an analogous trend evident among extant apes. Instead, the derived arch is an innovation of the foot found in much later hominins.
As was previously mentioned, qualitative features have often been underappreciated by some researchers, citing a difficulty in replication. We suggest that careful anatomical description and illustration are quite replicable and appreciated by those sufficiently familiar with the structures addressed. Furthermore, modern techniques of illustration, including stereophotography and photogrammetry provide quantitative dimensions to qualitative aspects of footprints, which do not otherwise lend themselves to linear or angular assessment. Granted, the description and interpretation of the morphology and kinematic signatures of a footprint involve a different task than describing the dimensions and features of a static structure, such as a fossil bone. However, we maintain that said task is tenable when based upon extensive familiarity with footprints and the correlated foot anatomy of the trackmaker, and that it provides critical information essential to a meaningful interpretation of the fundamental aspects of the functional morphology of the foot of, in this case, an extinct species.
Replicas of Laetoli G1–26, 35, 36 and G2/3–26, 27 were examined, as were two series of stereophotographs of the better-preserved southern section of the trackway, made during the original 1978 and 1979 excavations (courtesy of Tim White). The photos were taken under both am and pm lighting conditions, providing contrasting illumination of the south-to-north-oriented hominin trackways. These proved critical for interpreting detailed features of the original individual footprints, often obscured in strong shadow in one set of photos, or the other. The condition of the initially exposed tracks was noticeably superior to that documented by the subsequent re-excavation project of the Getty Conservation Institute (Demas, 1996, 2000; Agnew and Demas, 1998).
A replica of the G1–36 Laetoli footprint, which is a particularly clear imprint from the few available replicas of the G1 series, was three-dimensionally scanned at the Idaho Virtualization Laboratory, and a positive image (equivalent to a natural cast) was generated to better illustrate the contour of the imprint and the inferred features of the correlated plantar surface of the foot of the trackmaker (Fig. 4). This reversal procedure largely circumvents the aforementioned difficulties associated with interpreting the original trackway (or replicas) in variable conditions of illumination. To this end a 3-D scan was similarly carried out on a cast acquired from the Kenya National Museum, which included both the G1–35–36, as well as the G2/3–26–27. Also referenced were colorized photogrammetric images of the larger available cast that includes G1–35–37, as well as the G2/3–26–28 (Fig. 1) (Kim et al., 2008a,b).
Comparisons were made with several hominins leaving tracks in various substrates. Habitually-shod human footprints in fine damp beach sand were examined for a variety of locomotor speeds and directional changes. Two subjects in particular were studied at length, an adult male and a juvenile male. Footprints made by a large sample of university students progressing at a normal walk in a track box filled with fine damp loess were photographed. Fossilized footprints of habitually unshod humans were examined in Hawaii (Meldrum, 2004a,b). These are footprints left by native Hawaiians in historic volcanic ash deposits on Kilaeua, ca. 200 and 400 yBP (Moniz Nakamura, 2009). Additional examples of modern human footprints in a pyroclastic substrate were examined at the Acahualinca site, in Nicaragua where multiple trackways were preserved in lithified deposits of a phreatic eruption of Masaya volcano ca. 2.1 ka (Houck et al., 2009; Schmincke et al., 2009). These later two examples, in particular, provide a natural experiment to contrast early hominin footprints with modern human footprints, both laid down in a substrate of very similar consistency (Mastin et al., 1999).
Representing non-human hominids, the footprints of a captive chimpanzee made in a sand track box during bipedal progression were documented and correlated with simultaneous video recordings of the kinematics of his bipedal walking (primate facility, SUNY at Stony Brook, New York). Fresh gorilla footprints of an adult male and female in light snow were photographed and cast (Hogle Zoo, Salt Lake City, Utah).
The results of the reversed rendering of the 3-D scan of one of the most distinct examples of a Laetoli hominin footprint clearly demonstrate the intermediate nature of the trackmaker’s foot with respect to the pes in modern humans: tapered heel, midfoot transverse axis of flexion, concave forefoot, lack of well-developed ball, long lateral toes (Fig. 4). When the Laetoli hominin footprints are examined collectively, there are clearly repeated features that distinguish them from modern human footprints.
As many recent studies have relied heavily upon the photogrammetric map of Leakey and Harris (1987, Fig. D-3), the results of the systematic comparison of contours depicted there with White’s exceptionally clear stereo photos was noteworthy. This comparison revealed significant distinguishing features recognizable in the photos, but not in the map, discrepancies in contour depictions, or unresolved details of footprints that do not preserve footprint contact surfaces, which cannot be discerned on the maps. These observations are summarized for the two trails in Meldrum (2007a,b). Not only did this systematic review of the footprints corroborate the interpretations of the reverse image renderings discussed above, it also affirmed that the shape of the Laetoli hominin footprints is quite distinct from those of modern unshod human pedestrians (e.g. Hoffman, 1905; James, 1939; Stewart, 1970; Meldrum and Chapman, 2007), lacking the stereotypical ‘‘waisted-’’ or ‘‘hourglass-shape’’ of the arched foot, which is quite evident, even in unshod modern human pedestrians walking in conditions very similar to those encountered by the Laetoli hominins (Meldrum, 2004a,b; Swanson and Christiansen, 1973) (Fig. 5). Similarity is evident in the artifacts of impression produced by the Nicaraguan footprints left in basaltic cinders and viscous mudflows or lahars (Lockley et al., 2009). The fossilized Hawaiian human footprints provide a most dramatic demonstration of the distinctions between the shape of the Laetoli hominin tracks and a habitually unshod modern human foot (Meldrum (2007a,b)) (Fig. 6). This point of distinction is further echoed in the results of the analysis of the Ileret hominin footprints (Bennett et al., 2009). Even under varied conditions of depth of the substrate and degree of weathering, the distinctly modern features of the Hawaiian footprints are clearly evident: broad rounded heel pad, waisted medial arch outline, well-differentiated ball, occasional pressure ridge proximal to inferred position of metatarsal heads, greatest distal extent of sole pad delineated behind second digit, distinct toe pad impressions and short, well-impressed lateral toes.
In isolated instances, individual Hawaiian footprints may on first impression resemble the Laetoli hominin tracks. For example, the footprint labeled 28 in Fig. 6 bears superficial similarity to the Laetoli footprint depicted in Fig. 5, such as in the degree of hallucal abduction. However, a closer comparison reveals that even this particular Hawaiian footprint exhibits the distinguishing characteristics of a modern human footprint enumerated above. The same holds for the modern human footprints from the Acahualinca site in Nicaragua (Fig. 7).
In contrast, examples of Laetoli hominin tracks cited in the past as displaying a medial arch typically either have none, or the relatively raised medial border bears little resemblance to the distinctive shape and contour of the medial arch of the human foot – a longitudinal arch spanning a well-developed heel pad and ball centered on the hallucal metatarsophalangeal joint. Transient elevation of the medial border of an archless foot due to supination can be observed in extant chimpanzee footprints. Just as in chimp trackways, the occasional expressions of supination in the Laetoli hominin footprints are interspersed with counter examples of a pronated flatfoot, or midfoot flexion about a transverse axis (see also Deloison, 1992). Furthermore, a number of footprints exhibit signs of a midfoot, or midtarsal pressure ridge. Comparing the G1–26 cast to the superimposition of the skeletal reconstruction of White and Suwa (1987) over G1–25 demonstrates this (Fig. 3). Notice that the inferred position of the transverse tarsal joint is immediately distal to the apparent pressure ridge in both tracks.
Further evidence of this midtarsal flexibility comes in the form of a ‘‘half-track.’’ A half-track occurs when weight is applied through the midfoot and forefoot while the hind foot is elevated. Plantar pressure is concentrated beneath the midtarsus, i.e. the navicular and the cuboid. The G2/3–19 footprint, in particular the hindermost footprint, lacks an imprint of the heel, yet there is a pronounced impression beneath the reconstructed position of the navicular. It is not possible for a foot possessing a stiff longitudinal arch to leave a footprint like this (Meldrum (2007a,b)).
The consistently vague and often shallow impressions left by the lateral toes suggest very little compaction of the substrate beneath those digits. On the rare instances where impressions of toe tips are discernable, their position and extent of intervening ridged substrate suggest relatively long, curled digits. Some variation in the degree of hallucal abduction is evident in the Laetoli tracks. There is a consistent correlation between the degree of hallucal abduction and the extent of medial bulging of the outline of the inferred abductor hallucis muscle.
An interesting note is that a degree of idiosyncrasy was evident in hallucal position among the extant apes studied. One chimp and one gorilla in particular tended to adduct the hallux preferentially and orient it to the line of travel consistently (Meldrum, 2004b). A pair of female gorilla tracks cast in snow displays some notable similarities to the Laetoli tracks in the position of the hallux (Fig. 8 ). Also apparent is a differential depth of impression of the hindfoot as compared with the forefoot, likewise evident in a number of Laetoli footprints (e.g. G1–36).
As most researchers have not been afforded the opportunity to examine the Laetoli trackways firsthand, but must rely instead on the few published photos, photogrammetric maps, and a limited number of replicas, several observations should be emphasized. The differences between the details apparent in the tracks when lit from the east versus the west can be dramatic. As has been noted elsewhere, but which cannot be appreciated on the photogrammetric mapsalone, there are large areas of exfoliation of the original contact surface of some individual footprints (see further discussion of the taphonomy and preservation of the trackway in White and Suwa, 1987). Therefore, surface irregularities apparent in published photos are difficult to interpret without varied lighting and stereo-imaging. Likewise, undue reliance on individual photogrametric images can be misleading. For example, a photogrammetric image of G1–37, in Day and Wickens (1980, their Fig. 3) has often been used to support the suggestion of a well-developed medial arch, but the apparent raised medial border is the result of an artifactual irregularity on the medial border of the print. No arch can be justifiably inferred from it. Therefore, care should be taken when drawing comparisons between the Laetoli footprints and contemporary unshod pedestrian footprints based on these, and some other, published sources (e.g. Leakey and Harris, 1987).
Occasionally, a selective comparison between a modern human and a Laetoli hominin footprint may suggest a superficial similarity to one another. However, features in the Laetoli hominin footprints, such as the supposed appearance of a medial arch, a relatively fixed feature due to the underlying osseoligamentous framework, must be present consistently, conditions permitting, and exhibit comparable shape to support assertions of modernity in form and function.
Raichlen et al. (2010) have suggested that their experimentally derived sample of modern human footprints frequently do not exhibit an arch. They offer their Fig. S2 as an example to illustrate their point. This assertion is inexplicable since the photogrammetric contour lines of the selected footprint clearly and incontestably depict the typical shape of a modern human foot with a well-developed medial longitudinal arch.
6.1. The hominid foot
One of the hallmarks of the hominid locomotor system is the grasping foot. A robust medial digit, the hallux, functions in opposition to the relatively long lateral digits in a pincher-like grip. This foot posture is especially evident when the ape is climbing on vertical or inclined supports. The forefoot functions as a prehensile organ, maintaining a secure grasp during the contact phase, while the hindfoot serves as a propulsive organ. The plantarflexors of the ankle elevate the heel as the power arm of a lever with its fulcrum at the midtarsal, or transverse tarsal joint. This joint is actually a compound of the articulations between the talus and navicular on the medial column of the foot and the calcaneus and cuboid on the lateral column.
Elftamn and Manter (1935) first drew attention to the flexibility at the midfoot in chimpanzees. They referred to the coordinated flexion/rotation of the talonavicular and calcaneocuboid joints as the ‘‘midtarsal break.’’ This kinematic has received little attention since (Bojsen-Moller, 1979; Susman, 1983; Meldrum and Wunderlich, 1998; Meldrum, 2002; D’Aout et al., 2002; DeSilva, 2010). The midtarsal break permits the corresponding, but regionally specialized functions of the fore- and hind-foot, prehension and propulsion, respectively.
6.2. The terrestrial foot
During terrestrial locomotion, the midtarsal break is also evident in the chimpanzee foot. In plantar pressure studies the elevation of the heel coincides with a shift of the center of pressure to the tarsus distal to the midtarsal joint, especially beneath the cuboid (Meldrum and Wunderlich, 1998) (Fig. 9). It has been suggested that contributions to midfoot dorsiflexion in the lateral column of the foot occur at the cuboid–metatarsal joint (DeSilva, 2010). The human foot is a comparatively rigid platform, built upon a relatively fixed longitudinal arch. This adaptation unites the hindfoot and midfoot into a lengthened power arm of the foot lever. Elevation of the heel in the latter part of the stance phase during human walking shifts the center of pressure beneath the metatarsal heads, especially that of the hallucal metatarsal, which now serves as the primary fulcrum of the foot lever. The primitive midfoot flexibility was presumably present in the last common ancestor of chimpanzees and humans. At some point in human evolutionary history, the hominoid legacy of midfoot flexibility was relinquished in favor of a striding gait on much stiffer, arched feet. Selection increased the mechanical advantage of plantarflexors of the ankle, combined with extended legs for increased stride length, thereby improving economy of distance walking and running.
It has been hypothesized that midfoot flexion associated with the midtarsal break under the appropriate substrate conditions, produces a distinct pressure release as weight is transferred distal to the midtarsus (Meldrum, 2004b). This pressure release artifact in a footprint is a characteristic deformation of the compressed substrate as a result of the action of the foot during the terminal stance phase of the step. A human foot frequently produces a typical pressure release proximal to the ball of the foot, i.e., behind the hallucal metatarsophalangeal joint (Fig. 10). By comparison, the situation in the chimpanzee footprint is somewhat confounded by the variable position of the divergent hallux, elongated lateral toes, and frequent high angle of gait (i.e., toeing-out) associated with a facultative form of bipedalism, Chimpanzee footprints in sand do occasionally demonstrate a pressure release associated with the midtarsal break, as indicated by a primary pressure disk illustrated in Fig. 11.
It has been asserted by others that occasionally human footprints exhibit a midtarsal pressure ridge and therefore its presence in some Laetoli tracks does not distinguish them from modern human footprints. Raichlen et al. (2010) offer their Fig. S1 as evidence of their contention. This figure depicts a photogrammetric image of a human footprint displaying the trailing edge of a pressure disc (similar to that seen in Fig. 10 herein) running obliquely across the heel imprint. They fail to distinguish between the feature repeatedly present and consistently placed in some Laetoli footprints, i.e. a narrow ridge just proximal to the reconstructed position
of the midtarsal joint, and the pressure disc in the footprint illustrated in their Fig. S1. It is quite evident that the point of origin of displacement of the pressure disc in the human footprint is at the hallucal metatarsophalangeal joint. An entire disc of compacted sand has been elevated and translated posteriorly at push-off through the ball of the foot. There is no suggestion of midfoot flexibility present in this footprint. The pressure ridge we have described for the some Laetoli footprints, and the example of a pressure disc offered by Raichlen et al., are plainly not equivalent.
After the initial transition to habitual bipedalism, the hominoid grasp-climb adaptation was compromised by shortening of the lateral toes and reduction in the range of abduction, or divergence, of the hallux. This was a trend already evident among hominoid feet, as can be seen in a series of feet from chimpanzee to lowland gorilla, and onto the mountain gorilla, going from smaller body mass with more arboreal propensities to larger body mass and greater commitment to terrestriality. The Laetoli hominin footprints indicate a foot that possessed these modifications to the prehensile portion of the foot, but retained the flexible midfoot. Analyses of early hominin foot skeletons also indicate midfoot flexibility in australopithecine, and early hominin feet, such as that represented by the OH8 foot skeleton and Homo floresiensis (Gomberg and Latimer, 1984; Kidd et al., 1996; Harcourt-Smith et al., 2002; Berillon, 2004; Harcourt-Smith and Hilton, 2005; Klenerman and Wood, 2006; Jungers et al., 2009). In a correlated fashion, it has been demonstrated that the metatarsophalangeal joints of the australopithecines are oriented in an intermediate fashion to that condition in apes and humans, and are not capable of the range of dorsiflexion associated with a modern toe-off during terminal stance (Duncan et al., 1994).
At some later point in the evolution of modern human foot morphology, changes occurred to stabilize the foot platform, increase the mechanical advantage of ankle plantarflexors and improve efficiency and economy in long distance, endurance walking and running (Hilton and Meldrum, 2004; Bramble and Lieberman, 2004). Determining the timing and pattern of the evolution of these characteristics has remained a challenge due to the paucity of fossilized footprints or foot skeletons from the period spanning 2.0–0.5 Ma (Meldrum, 2004a,b). The head of the hallucal metatarsal in Homo erectus (Homo ergaster) lacks the derived morphology of the modern human counterpart, associated with peak pressures at terminal stance concentrated beneath the hallucal metatarsophalangeal joint (Meldrum and Chapman, 2008; Meldrum et al., 2010).
The proposition that H. floresiensis represents a novel hominin species derived from either H. erectus, or an even earlier hominin, is significant to this point. The foot skeleton exhibits features indicating bipedalism on flat flexible feet, lacking stable longitudinal arches (Jungers et al., 2009). This confirms that the arched foot had not emerged by the time H. floresiensis diverged, whether from the Homo lineage, or possibly a late australopithecine ancestor.
The discovery of additional fossilized footprints attributed to H. ergaster posed a potential resolution to this issue of timing (Bennett et al., 2009). Although the authors inferred a modern-type of foot form for the trackmaker, including foremost a longitudinal arch, the evidence is not present to bear that conclusion out. For example, the cover illustration, which depicted three of the best footprints reported upon, provided no evidence of a medial longitudinal arch. In two instances, the apparent raised medial border of the footprints was the result of extrusion artifacts introduced secondarily by the over imprinting of ungulate tracks, distorting or obscuring any details of that area of the hominin footprint. The medial area of the third footprint is not well-preserved, displaying an irregular topography that is not the footprint contact surface. None of the reported footprints provide sufficient detail to determine whether an arch was present in the foot or not (Meldrum et al., 2009). In this instance there is insufficient evidence to trump the inferences drawn from the other footprint (Terra Amata) and skeletal elements, especially the morphology of the hallucal metatarsal as discussed above.
Taken in context, there is ample evidence to corroborate the interpretation that the Laetoli hominin trackmakers had not evolved the modern human foot form, but rather their tracks display features consistent with a terrestrial biped possessing a mosaic morphology characterized by reduced abduction of the hallux, intermediately shortened lateral toes, and a flat, flexible midfoot, characteristic of the australopithecine grade of hominin present during the Plio-Pleistocene. These distinctions are the basis for recognizing a new ichnotaxon, Praehominipes laetoliensis.
This research was supported in part by Grant no. 829 from the Idaho State University Faculty Research Committee, by the LSB Leakey Foundation, and supporters of the Idaho Virtualization Laboratory Footprints Project: John Green, Thom and Susan Stepp, and Regal Ridge; the University of Colorado at Denver Center for Faculty Development. Cooperation and assistance were received from: Jill Rivolli, for bringing the Hawaiian footprints to our attention, and the staff of the Office of Cultural Resources Management and Hawaiian Volcanoes Observatory; Susan Larson and the Stony Brook animal care facility staff (supported by NSF BCS980629); The Hogle Zoo, Salt Lake City, Utah; the hospitality and shared insights of Tim White; Martha Demas for providing unpublished reports of the Laetoli Project; and The Idaho Virtualization Laboratory and staff; Neffra Matthews for the photogrammetric images. We also thank the editor and two anonymous reviewers.
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Fig. 1. Color rendering of a photogrammetric image derived from a replica of a portion of the Laetoli hominin trackway (on the left from top are G1–34, G1–35, and G1–36; on the right from the top are G2/3–25, G2/3–26, and G2/3–27) [image courtesy of Neffra Matthews].
Fig. 2. Rendering of a 3-D scan of a replica of a portion of the Laetoli hominin trackway (on the left from top are G1–35 and G1–36; on the right from the top are G2/3–26 and G2/3–27) [replica courtesy of Kenya National Museum].
Fig. 3. Comparison of the G1–26 Laetoli hominin cast to the superimposition of the skeletal reconstruction of White and Suwa (1987) over G1–25. They are depicted at the same scale with corresponding landmarks aligned horizontally. Arrow indicates the common level of the pressure ridges.
Fig. 4. Multiple views of a positive rendering of a 3-D scanned Laetoli hominin track replica (G1–36).
Fig. 5. A comparative series (l–r) depicting footprints of a chimpanzee, Laetoli hominin, habitually unshod Hawaiian pedestrian, habitually shod modern human.
Fig. 6. A representative selection of variation in the appearance and preservation of the unshod Hawaiian pedestrian footprints.
Fig. 7. Multiple views of a positive rendering of a 3-D scanned track replica (230–134) from the Acahualinca site, Managua, Nicaragua.
Fig. 8. Footprint casts of a female lowland gorilla left imprinted in shallow snow fall and made using snow-print wax method exhibiting adducted halluces (indicated by arrows).
Fig. 9. Video frame and corresponding plantar pressure trace of the step of a chimpanzee. Lighter shades indicate higher pressures. Elevation of the heel coincides with a shift of the center of pressure to the tarsus distal to the midtarsal joint, especially beneath the cuboid (indicated by arrow).
Fig. 10. A slightly oblique perspective of a human footprint illustrating a pressure release (indicated by the arrow) proximal to the ball of the foot, behind the hallucal metatarsophalangeal joint.
Fig. 11. Chimpanzee footprint in sand demonstrating a pressure release disk (indicated by arrows) associated with the midtarsal break.
Please cite this article in press as: Meldrum, D.J., et al. Ichnotaxonomy of the Laetoli trackways: The earliest hominin footprints. J. Afr. Earth Sci. (2011), doi:10.1016/j.jafrearsci.2011.01.003