Ardipithecus ramidus
The first Ardipithecus ramidus fossils were found in 1992-1993
in the Middle Awash valley in Ethiopia, with further fragments unearthed
in 1994. It was formally named later year. Further research in the
early years of this century unearthed further evidence. Last year, after
some years of extensive research, 11 papers were published in science
detailing the results of that field work and subsequent analyis.
Now, before I go on any further, I need to point out that Ardipithecus ramidus
is not the common ancestor since it is too young based on the currently accepted time for the divergence of
the lines leading to the Homo and Pan lineages based on molecular studies. It however is still a significant find:
This remarkably rare skeleton is not the oldest putative hominin, but it is by far the most complete of the earliest specimens. It includes most of the skull and teeth, as well as the pelvis, hands, and feet—parts that the authors say reveal an "intermediate" form of upright walking, considered a hallmark of hominins. "We thought Lucy was the find of the century but, in retrospect, it isn't," says paleoanthropologist Andrew Hill of Yale University. "It's worth the wait."
To some researchers' surprise, the female skeleton doesn't look much like a chimpanzee, gorilla, or any of our closest living primate relatives. Even though this species probably lived soon after the dawn of humankind, it was not transitional between African apes and humans. "We have seen the ancestor, and it is not a chimpanzee," says paleoanthropologist Tim White of the University of California, Berkeley, co-director of the Middle Awash research group, which discovered and analyzed the fossils.
Instead, the skeleton and pieces of at least 35 additional individuals of Ar. ramidus reveal a new type of early hominin that was neither chimpanzee nor human. Although the team suspects that Ar. ramidus may have given rise to Lucy's genus, Australopithecus, the fossils "show for the first time that there is some new evolutionary grade of hominid that is not Australopithecus, that is not Homo," says paleontologist Michel Brunet of the College de France in Paris. [1]
Artist's reconstructions show how Ardi's skeleton, muscles, and body looked and how she would have moved on top of branches.
Credits: Illustrations © 2009, J. H. Matternes
Fossil remains of Ardipithecus ramidus.
Credit: Copyright T. White, 2008
Ardipithecus ramidus and the likely post-cranial skeleton of the common ancestor
As I mentioned before, Ardipithecus ramidus is too young to
be the common ancestor, but it has a lot to say about it. In so doing,
it has invalidated hypotheses about this last common ancestor:
The picture emerging from Ar. ramidus is that this last common ancestor had limb proportions more like those of monkeys than apes. Its feet functioned only partly like those of apes and much more like those of living monkeys and early apes such as Proconsul (which lived more than 15 million years ago). Its lower back was mobile and probably had six lumbar vertebrae rather than the three to four seen in the stiff backs of African apes. Its hand was unpredictably unique: Not only was its thumb musculature robust, unlike that of an ape, but its midcarpal joint (in the wrist) allowed the wrist to bend backward to a great degree, enhancing its ability to move along tree branches on its palms. None of the changes that apes have evolved to stiffen their hands for suspension and vertical climbing were present, so its locomotion did not resemble that of any living ape.
The hominid descendant of the last common ancestor we shared with chimpanzees (the CLCA), Ardipithecus, became a biped by modifying its upper pelvis without abandoning its grasping big toe. It was therefore an unpredicted and odd mosaic. It appears, unlike Au. afarensis, to have occupied the basal adaptive plateau of hominid natural history. It is so rife with anatomical surprises that no one could have imagined it without direct fossil evidence. [2]
Suffice it to say that Ardipithecus is a transitional form:
it is a biped with limb adaptations allowing it to move in the trees.
This is not simply another ape. So, where does Ardipithecus possibly fit into the evolutionary tree? The diagram below shows where the current data place it:
Overall Summary.
Ardipithecus ramidus once again is not the common ancestor, but it gives us a very good idea of what the common ancestor may have been like. In so doing, it has negated some previous hypotheses about what that common ancestor would have been like - it would have looked more hominid-like than modern ape-like in many features. The authors note this:
Ardipithecus ramidus once again is not the common ancestor, but it gives us a very good idea of what the common ancestor may have been like. In so doing, it has negated some previous hypotheses about what that common ancestor would have been like - it would have looked more hominid-like than modern ape-like in many features. The authors note this:
"The most popular reconstructions of human evolution during the past century rested on the presumption that the behaviors of the earliest hominids were related to (or even natural amplifications of) behaviors observed in these living great apes...This will not be the final word on the subject, and details may change with further fossil evidence and/or analysis of fossil data White has unearthed. However, this is an amazing find, which does cast significant light on what the common ancestor would have been like.
"Ardipithecus ramidus nullifies these presumptions as it shows that the anatomy of living African apes is not primitive but instead has evolved specifically within extant ape lineages. The anatomy and behavior of early hominids are therefore unlikely to represent simple amplifications of those shared with modern apes. Instead, Ar. ramidus preserves some of the ancestral characteristics of the last common ancestor with much greater fidelity than do living African apes. Two obvious exceptions are its ability to walk upright and the absence of the large projecting canine tooth in males, derived features that Ardipithecus shares with all later hominids.
"Ar. ramidus illuminates our own origins because it clarifies our relationship to Australopithecus. For example, the enlarged rear teeth of Australopithecus have long been viewed as adaptations to a rough, abrasive diet. This has led to speculation that canine teeth might have become smaller simply to accommodate the emergence of these other enlarged teeth, or that the importance of canine teeth in displays of male-to-male aggression waned with the development of weapons.
"Ar. ramidus negates such hypotheses because it demonstrates that small canines occurred in hominids long before any of the dental modifications of Australopithecus or the use of stone tools. The loss of large canine teeth in males must have occurred within the context of a generalized, nonspecialized diet. Comparisons of the Ar. ramidusdentition with those of all other higher primates indicate that the species retained virtually no anatomical correlates of male-to-male conflict. Consistent with a diminished role of such agonism, the body size of Ar. ramidus males was only slightly larger than that of females.
"The discovery of Ar. ramidus also requires rejection of theories that presume a chimpanzee- or gorilla-like ancestor to explain habitual upright walking. Ar. ramidus was fully capable of bipedality and had evolved a substantially modified pelvis and foot with which to walk upright. At the same time, it preserved the ability to maneuver in trees, because it maintained a grasping big toe and a powerful hip and thigh musculature. Because upright walking provided no energy advantage for Ar. ramidus (it lacked many of the adaptations evolved in later hominids such as Australopithecus), reproductive success must have been central to its evolution in early hominids." [3]
Appendix
The following posts (edited quotes from the references) will cover Ardipithecus ramidus in more detail, but an executive summary of these points will follow for those not wishing to follow up the details.
Details of Hand Anatomy
"The thumb metacarpal of ARA-VP-6/500 was more aptly proportioned for manual grasping than are those of extant apes...
"Ar. ramidus greatly illuminates the natural history of the thumb in higher primates. Its robusticity in hominids, while certainly enhanced during the past 3 million years, is nevertheless at least partially primitive. In contrast, in taxa adapted to vertical climbing and suspension, lengthening of the palm has become so dominant as to eclipse some of the thumb’s function, a condition that has reached its apogee in Ateles and, to a lesser extent, large-bodied extant apes...
"Additional relevant hand anatomy leads to the same conclusions. Ar. ramidus is the only hominid fossil thus far recovered with a metacarpal head reminiscent of the metacarpophalangeal (MP) joint structure seen in many Miocene hominoids [such as Equatorius, Proconsul, Dryopithecus, and Pierolapithecus]. The collateral ligament facets in these taxa colocate with deep symmetric invaginations of the metacarpal head’s dorsum. This morphology is typical of Old World monkeys and is thereby associable with substantial dorsiflexion of the MP joint, an obvious manifestation of their palmigrady. The trait is only moderately expressed in Oreopithecus. Modern human and orangutan MP joints are substantially less constricted, and neither taxon exhibits appreciable locomotor-related MP dorsiflexion. [4]
Significance of hands for common ancestor locomotion
"The primitive metacarpal head morphology within the overall primitive hand anatomy of Ar. ramidus carries obvious implications for reconstruction of GLCA/CLCA locomotion. The unique combination of marked midcarpal mobility, ulnar withdrawal, and moderate MP dorsiflexion in Ar. ramidus, probably mostly primitive retentions, implies that the GLCA/CLCA locomotor pattern was also characterized by some form of arboreal palmigrade quadrupedality, unlike that in any extant descendant great ape." [5]
Implications of the feet of Ardipithecus ramidus for the common ancestor
"In summary, a comparison of the pedal proportions of Ar. ramidus and the extant African apes suggests that the GLCA/CLCA hindlimb remained dominant for body mass support during bridging and arboreal clambering, to the extent that it later proved permissive to bipedality in transitionally terrestrial hominids."[6]
Details of the pelvis and implications for the common ancestor
"Pelvic structure indicates that Ar. ramidus retained a primitive spine. Its iliac and acetabular regions establish not only that it was habitually bipedal when terrestrial, but also that this was achieved by combining situational anterior pelvic tilt to accentuate substantial lordosis during upright walking . Such rotation placed the still partially primitive anterior gluteal musculature into a position of functional abduction for single support stabilization. In contrast to Ar. ramidus, Au. afarensis is known to have exhibited highly evolved mechanisms of hip abduction, confirmed by the distinctly stereotypic trabecular profile of its femoral neck.
"The Ar. ramidus pelvis retained other elements in common with extant African apes (and presumably the GLCA/CLCA). These include a long, expansive and rugose ischial region and shorter pubic rami (but not a long pubic corpus). The species’ highly flexible lower lumbar column, coupled with its narrower interacetabular distance, still must have provided a moderately reflexive hindlimb for arboreal climbing. Not until hominids became habitually terrestrial bipeds with broad interacetabular distances, reduced and angulated ischial tuberosities (possibly indicating hamstring deceleration of the hindlimb at heel strike), and extremely shortened, flared, and broadened ilia did they then exchange such flexibility for the much more rigid constraints of lower-limb stabilization that characterize Australopithecus."[7]
Implications of postcranial data for the common ancestor
"Therefore, we hypothesize that Ar. ramidus retains much of the ancestral GLCA and CLCA character states, i.e., those that relate to above branch quadrupedality. In particular, contra Gorilla and Pan, the GLCA carpometacarpal, midcarpal, radiocarpal, and ulnotrochlear joints must have lacked notable adaptations to suspension and/or vertical climbing. The GLCA foot seems to have been only partially modified for manual-like grasping. Its hindlimb remained fully propulsive at its midtarsal and tarsometatarsal joints. Although its shoulder joint must have been fully lateralized, its lumbar column nevertheless was still long. Its limb proportions were still primitive (see earlier). If body size was as large as in Ar. ramidus, it may have been too large for habitual, unrestricted above-branch quadrupedality, but this remains uncertain. Assuming considerable reliance on arboreal subsistence, it is likely that body mass did not exceed 35 to 60 kg [i.e., combined probable range of Ar. ramidus and 6 Ma Orrorin].
"The GLCA picture that emerges, therefore, is one of generalized, deliberate bridging with quadrupedal palmigrady and preference for large diameter substrates. This may have involved either suspension or vertical climbing, but without sufficient frequency to elicit morphological adaptations specific to these behaviors. It is likely that these hominoids ranged mostly in the lower canopy, and perhaps were even partially terrestrial. However, their mode of terrestrial locomotion remains unknown.
"The GLCA therefore represents a foundation for two adaptive paths. Gorilla and Pan independently specialized for both suspension and vertical climbing (and eventually knucklewalking). Gorillas might have acquired larger body size in relation to mixing higher-canopy frugivory with a more terrestrial herbaceous or folivorous dietary component. Lacking definitive fossil evidence, it is currently impossible to determine when the large body mass of Gorilla evolved, but it probably occurred in concert with its more herbaceous diet. The 10 Ma Chororapithecus, which shows incipient signs of Gorilla-like molar morphology, may be an early representative of the Gorilla clade. If so, then this clade’s shift toward increased body mass and terrestriality must have occurred early in its phyletic history.
"The other adaptive pathway retained palmar flexibility, with a short metacarpus that lacked notable syndesmotic restriction. This was combined with retention of an essentially rigid midtarsal joint that was insufficiently flexible to perform vertical climbing, but was fully satisfactory for less specialized careful climbing, clambering, and bridging. This is the hypothesized structure of the CLCA, from which Pan would have evolved a greater reliance on vertical climbing and suspension than occurred in the Gorilla clade, never reaching as large a body size.
"In contrast to Pan, the forebears of Ar. ramidus early in the hominid clade must have relied increasingly on lower arboreal resources and terrestrial zones, without being dependent on higher-canopy resources (such as ripe fruits). From the comparative evidence now available from Ar. ramidus and Pan dental anatomy and isotopes, we posit that the chimpanzee clade increasingly developed a preference for (or dependency on) ripe fruit frugivory, whereas hominids retained a more primitive dental complex adequate for the range of transitional arboreal/ terrestrial resources. [8]
The Foot of Ardipithecus ramidus
The feet of modern apes and humans are different. While ours is rigid, and can act quite well as a lever during walking and running, the ape feet are less rigid and more effective for grasping - useful if you are mobilising through trees. It is not unreasonable to ask what the common ancestor's foot would have looked like - more like the chimpanzee foot, or more like ours. We now have an answer:
Hominid morphology has often been presumed to have evolved from ancestral morphotypes like those of extant African apes. Ar. ramidus now establishes that this was not the case. The hominid foot was instead derived from one substantially less specialized. [9]
In short, the foot of Ardipithecus ramidus had a grasping
great toe which would have helped it climb in trees, while its four
remaining toes were modified to make it suitable for upright walking -
it was a 'generalist' foot. A digitally rendered composite foot is shown
below:
After a significant amount of technical detail, the authors interpret the significance of what they have found:
Ar. ramidus is the only known hominid with an abducent great toe. Its foot, along with other postcranial elements, indicates that the Late Miocene hominid precursors of Ar. ramidus practiced mixed arboreal and terrestrial locomotion during which the lateral forefoot became extensively adapted to upright walking, even as the medial forefoot retained adaptations for arboreal exploitation. [10]
In other words, it is quite likely that the common ancestor (the Late
Miocene hominid precursors) had a foot whose lateral four toes were
modified to permit upright walking, while the medial forefoot allowed
grasping. To date, it is the only hominid with a great toe that allows
grasping. This makes sense for a common ancestor, since this means after
the divergence of the Homo and Pan lines, the line
leading to us would become more and more suited to walking, and lose the
need for a grasping toe. Conversely, the line leading to chimpanzees
and bonobos would retain the grasping toe as they maintained a largely
arboreal environment.
We do see evidence of this evolution towards more advanced bipedality in later hominids such as Australopithecus afarensis:
Indeed, by the time of emergence of Au. afarensis, hominids had evolved substantially more advanced adaptations to bipedality than were present in Ardipithecus. In the former, the knee had become tibial dominant with accentuated valgus (exceeding even that of modern humans). Hip abduction had been established with a human-like distribution of proximal femoral cortical and trabecular bone. Moreover, in all known subsequent hominids, the more posterior location and elevation of the os peroneum facet on the cuboid [direct selection acting on morphogenetic fields; type 1] signals the presence of longitudinal and transverse arches, and thereby the addition of the transverse axis of fulcrumation (fig. S12). The facet’s position in the OH-8 cuboid is virtually human, as is the length of its calcaneal process. Doming and the simpler unnotched dorsal surface of the Mt1 head characterize both Au. afarensis (A.L. 333-21) and Au. africanus, confirming an immobile first ray with fundamentally human-like propulsion during toe-off. [11]
Having such a generalised foot was critical for our evolution as
bipeds. As the authors point out in their summary, "If our last common
ancestor with the chimpanzee had not retained such an unspecialized
foot, perhaps upright walking might never have evolved in the first
place." [12]
The Pelvis and Femur of Ardipithecus Ramidus - Adaptations for upright walking
Our pelvis is of great significance in our bipedal gait - it helps up keep our balance on one lower limb while walking. If you look at chimps walking, the side of the pelvis to which the leg that is swinging forward is attached slumps down. That does not make for a very useful gait. Chimps and bonobos are our closest relatives, but they don't have a pelvic girdle like ours. The fossil record to date has not shed much light on the evolution of the early hominid pelvis. Ardipithecus ramidus may change this. From the author's summary:
Until now, the fossil record has told us little about when and how the early hominid pelvis evolved. Even 3 to 4 million years ago (when our brains were still only slightly larger than those of chimpanzees), it had already undergone radical transformation. One of the oldest hominid pelves, that of Australopithecus afarensis (A.L. 288-1; “Lucy”), shows that her species had already evolved virtually all of the fundamental adaptations to bipedality. Even the kinetics of her hip joint were similar to ours. Although the human pelvis was later further reshaped, this was largely the result of our much enlarged birth canal.
Ardipithecus ramidus now unveils how our skeleton became progressively modified for bipedality. Although the foot anatomy of Ar. ramidus shows that it was still climbing trees, on the ground it walked upright. Its pelvis is a mosaic that, although far from being chimpanzee-like, is still much more primitive than that of Australopithecus...
The gluteal muscles had been repositioned so that Ar. ramidus could walk without shifting its center of mass from side to side. This is made clear not only by the shape of its ilium, but by the appearance of a special growth site unique to hominids among all primates (the anterior inferior iliac spine). However, its lower pelvis was still almost entirely ape-like, presumably because it still had massive hindlimb muscles for active climbing.
Changes made in the upper pelvis rendered Ar. ramidus an effective upright walker. It could also run, but probably with less speed and efficiency than humans. Running would also have exposed it to injury because it lacked advanced mechanisms such as those that would allow it to decelerate its limbs or modulate collision forces at its heel. Australopithecus, which had given up its grasping foot and abandoned active climbing, had evolved a lower pelvis that allowed it to run and walk for considerable distances.
Ar. ramidus thus illuminates two critical adaptive transitions in human evolution. In the first, from the human-chimp last common ancestor to Ardipithecus, modifications produced a mosaic pelvis that was useful for both climbing and upright walking. In the second, from Ardipithecus to Australopithecus, modifications produced a pelvis and lower limb that facilitated more effective upright walking and running but that were no longer useful for climbing. Because climbing to feed, nest, and escape predators is vital to all nonhuman primates, both of these transitions would likely have been a response to intense natural selection. [13]
The mosaic nature of the pelvis is shown in the reconstruction below:
The Ar. ramidus pelvis has a mosaic of characters for both bipedality and climbing. Left to right: Human, Au. afarensis
(“Lucy”), Ar. ramidus, Pan (chimpanzee). The ischial surface is angled
near its midpoint to face upward in Lucy and the human (blue double
arrows), showing that their hamstrings have undergone transformation for
advanced bipedality, whereas they are primitive in the chimpanzee and
Ar. ramidus (blue arrows). All three hominid ilia are vertically short
and horizontally broad, forming a greater sciatic notch (white arrows)
that is absent in Pan. A novel growth site [the anterior inferior iliac
spine (yellow arrows)] is also lacking in Pan.
The pelvic bones and their reconstructions are shown below:
Fig. 1 Original and reconstructed os coxa of ARA-VP-6/500. (Left) Anterolateral and anteromedial views of the original, with a close-up of the AIIS. (Middle) CT scans of same views, except the bottom view is a close-up of the pubic symphyseal face, preserved in its entirety but damaged at its inferior extremity. It is like later hominids in its dorsoventral height and ovoid outline. (Right) Anterolateral and anteromedial views of the reconstructed os coxa (11th permutation) and of the entire pelvis using mirror reconstruction and a conjectural sacrum. Various permutations were attempted with respect to sacral breadth; the solution shown provides presumed functional minimal inlet and outlet dimensions for a hominoid of this individual’s body size (Fig. 2 and figs. S1 to S3). (Middle) Arrows in the CT images indicate major areas of distortion corrected by reconstruction. The entire iliac blade was bent anteromedially (single yellow arrow) (additional and corrective data were provided by right iliac fragment). Two green arrows indicate primary foci of subduction of three largely intact segments of the superior pubic ramus; obvious overlapping allowed accurate restoration of original length (compare with reconstruction). The acetabulum was separated into two halves (total height of the exploded acetabulum is indicated by white arrows), with substantial intervening matrix infill. This separation greatly elongates its unrestored appearance. The two halves were recompressed on the basis of a calculated rim circumference obtained through the summation of individual segments (a range of probable values is presented in discussions of metric parameters). Two red arrows mark the inferior edge of the intact acetabular rim and the superior edge of the (missing) surface of the ischial tuberosity. These provide a minimum ischial length. Various additional dimensions were corrected by means of surface metrics. The breadth of the iliac fossa was intact from the AIIS to the lateral edge of the auricular surface; the degree of individual fragment separation was assessable from surface observation and CT scan data as indicated. Areas of particular anatomical importance include the protuberant sigmoid AIIS and sagittally oriented iliac isthmus, typical of later hominids, and the notably short, ovoid, pubic symphyseal face. Major metrics and angles are provided in Fig. 3. CT scans were taken at 300-µm voxel resolution on the University Museum, the University of Tokyo, micro-CT system [TX-225 Actis (Tesco, Tokyo)] and processed with the software Analyze 6.0 (Mayo Clinic, Rochester, MN) and Rapidform 2004/2006 (Inus Technology, Seoul).
Ilium, Ischium and pubis
Examination of these bones show clear evidence of transitional features:
"The protuberant and anteriorly positioned AIIS is associated with a broad, short, and sagittally disposed iliac isthmus. We define the isthmus as the constricted inferior portion of the iliac blade immediately superior to the acetabulum. These features are shared with later hominids, but in both modern and Early Miocene apes and monkeys the isthmus is markedly elongate, and the greater sciatic notch (GSN) angle is more obtuse. The Ar. ramidus GSN is intermediate between its counterpart in modern apes and those of later fossil hominids...
"In stark contrast to its distinctly hominid ilium, the preserved Ar. ramidus ischium is like that of African apes. Although the ischial tuberosity is not preserved, the ischium is intact from its typical concave surface flare just superior to the tuberosity to the inferior border of the acetabulum. Even this minimum preserved length of the superior ischial ramus is substantially longer than any known Australopithecus example. The pubis preserves an intact, superoinferiorly elongate body. However, the outline of the pubic symphyseal face is similar to that of Homo and A.L. 288.-1 and unlike its extreme dorsoventral elongation in African apes. [14]
"In stark contrast to its distinctly hominid ilium, the preserved Ar. ramidus ischium is like that of African apes. Although the ischial tuberosity is not preserved, the ischium is intact from its typical concave surface flare just superior to the tuberosity to the inferior border of the acetabulum (Fig. 1). Even this minimum preserved length of the superior ischial ramus is substantially longer than any known Australopithecus example. The pubis preserves an intact, superoinferiorly elongate body. However, the outline of the pubic symphyseal face is similar to that of Homo and A.L. 288.-1 and unlike its extreme dorsoventral elongation in African apes (Fig. 3A). This more ovoid shape may be a collateral pleiotropic manifestation...of the shortened iliac isthmus and elongated superior pubic ramus, especially in A.L. 288-1 (which exhibits both an unusually broad pubic face and greatly elongated pubic rami)."[15]Pelvis form and function
Examination of the pelvis of Ardipithecus ramidus shows evidence of change
"The form and size of the AIIS in ARA-VP- 6/500, as well as its projection anterior to the acetabular margin, indicate that this structure had already begun to appear and mature via a novel physis. Isolation of the AIIS as a separate growth center, which is unique to hominids, was probably a consequence of its increased separation from the original iliac portion of the acetabular chondroepiphysis. This is because much of iliac broadening must occur at the triradial epiphysis (as well as the posterior iliac crest). An analogous phenomenon occurs in mammals with elongate femoral necks, in which accelerated growth in the region separating the presumptive greater trochanter and femoral head isolates these structures before separate ossification of each epiphysis.
"The emergence of a novel AIIS center of ossification, as seen in ARA-VP-6/500, attests to global modification of the entire pelvis. This is also demonstrated by the abbreviated craniocaudal length of the pubic symphyseal face. The anteroventral pubic surface is surmounted by a rugose pectineal line that is continuous with the nonperiosteal attachments of abductor brevis and gracilis muscles. This is a derived character in human females... It is only rarely present in great apes. There is no lateral displacement of this feature in Ar. ramidus as there is in human females (and partially in A.L. 288-1)...
"Given retention by Ar. ramidus of multiple primitive skeletal characters...its exceptionally derived ilium is striking. It implies an early adaptation to habitual terrestrial bipedality before any increase in the lumbar entrapment seen in the African apes, but after the lateral iliac expansion shared with them." [16]
Conclusion
We see in the feet of Ardipithecus adaptations both for tree climbing and walking. What we see in the pelvis are clear signs of modification of a pelvis for walking:
The pelvis, femur, and preserved thoracic elements of Ar. ramidus establish that adaptations to upright walking in these regions were well established by 4.4 Ma, despite retention of a capacity for substantial arboreal locomotion. Ar. ramidus thus now provides evidence on the long-sought locomotor transition from arboreal life to habitual terrestrial bipedality. This evidence suggests that the transition took place in the absence of any of the characters that today substantially restrict upright walking in extant apes (particularly lumbar column abbreviation, trans-iliac space narrowing, and approximation of iliac crest and thorax, and the muscles that traverse this gap). As a consequence, explications of the emergence of bipedality based on observations made of African ape locomotion no longer constitute a useful paradigm. [17]
The forelimbs of Ardipithecus ramidus and their comparison to human forelimbs
One defining feature of primates are highly mobile upper limbs with hands that are capable of grasping and manipulating objects. While chimps and gorillas can walk upright, they also knuckle walk - a trait we don't have since we no longer use our arms for propulsion. This raises the question: did the common ancestor knuckle walk (which meant that this trait was lost by the line leading to us after divergence from the common ancestor) or was its anatomy more generalised?
We now have an answer. In their summary, the authors state that:
"Ardipithecus hands were very different from those of African apes. Its wrist joints were not as stiff as those of apes, and the joints between their palms and fingers were much more flexible. Moreover, a large joint in the middle of the wrist (the midcarpal joint) was especially flexible, being even more mobile than our own. This would have allowed Ardipithecus to support nearly all of its body weight on its palms when moving along tree branches, so that it could move well forward of a supporting forelimb without first releasing its grip on a branch.
"This discovery ends years of speculation about the course of human evolution. Our ancestors’ hands differed profoundly from those of living great apes, and therefore the two must have substantially differed in the ways they climbed, fed, and nested. It is African apes who have evolved so extensively since we shared our last common ancestor, not humans or our immediate hominid ancestors. Hands of the earliest hominids were less ape-like than ours and quite different from those of any living form." [18]
Here's a picture of the hand model:
Analysis of the hand bones does not show the traits you would expect to see in knuckle walking:
"The medial metacarpals (Mc2 to Mc5) of Ar. ramidus are short compared with those of African apes. Their heads lack any traits associated with knuckle-walking, such as prominent ridges and/or grooves. Ar. ramidus metacarpal heads exhibit marked, proximally located, dorsal invaginations that are associated with their metacarpophalangeal (MP) joint’s collateral ligaments (reflecting routine hyperdorsiflexion at this joint), as do those of some Old and New World monkeys and some Miocene hominoids, including Proconsul...Such constrictions have become minimal in all extant hominoids but for different functional reasons... Ar. ramidus also lacks the expansion of the metacarpal heads that is typical of knuckle-walking apes...
"The first ray’s terminal phalanx exhibits a symmetrically constructed, rugose insertion gable for the flexor pollicis longus, in contrast to African apes in which this muscle’s tendon is reduced or absent."[19]
The hands of apes are actually specialised for things like knuckle walking and tree climbing. Ardipithecus had a more generalised hand which we have generally retained (though with somewhat shorter fingers!)
"In summary, the hand of Ar. ramidus appears almost entirely primitive relative to the anatomical specializations seen in extant apes (for example, metacarpal elongation, elaboration of CJC articulations and ligaments, novel capitate geometry, reorientation of the scaphoid’s radial surface, enlargement of the radioscaphoid ligament, relative diminution of the first ray, etc.). Ar. ramidus establishes that these changes in the ape hand are independent specializations for arboreal access and terrestrial travel (vertical climbing, forelimb suspension, knuckle-walking) and were apparently never established in hominids, which retained a more generalized, substrate-conforming, grasping hand."[20]
Finally, Ardipithecus allows us to postulate that our hominid ancestors likely never were knuckle walkers.
"The forelimb has played a definitive role in most chronicles of human evolution since Huxley’s and Keith’s original accounts. Most recent narratives of its anatomical and behavioral evolution have emphasized a heritage of suspensory locomotion, vertical climbing, and knuckle-walking in the common ancestors that humans shared with extant African apes.
"Encouraged by human and chimpanzee genetic similarity and cladistic analyses, such views have come to dominate recent explications of early hominid evolution, although alternative interpretations based on classical comparative anatomy have long differed. Ar. ramidus now permits resolution of these controversies. It indicates that, although cranially, dentally, and postcranially substantially more primitive than Australopithecus, these known Late Miocene to Early Pliocene hominids probably all lacked the numerous, apparently derived, forelimb features of extant African apes. The most probable hypothesis to explain these observations is that hominids never passed through adaptive stages that relied on suspension, vertical climbing, or knuckle-walking. Further fossil remains from the Late Miocene, including those before and after the African ape–hominid phyletic divergences, will test this hypothesis derived from our analysis of Ar. radius." [21]
Ardipithecus ramidus Skull
It is somewhat uncommon to have complete hominid skulls, which is why complete skulls are quite prized. Obviously, the more complete skulls you have, the better you can track the evolution of brain, face and skull. Furthermore, one can infer details of diet and behaviour from skulls.
A reassonably complete - though crushed - skull was found, which meant that the bones needed by be scanned and digitally reassembled to reconstruct the skull. The key findings, to quote from the author summary: [22]
- It had a small brain (300-350 cc), smaller than the Australopithecines (400 -550 cc), and around the size of bonobos and female chimpanzees
- Its face was small, and did not have the large cheek bones that we see in Australopithecines that were capable of grinding tough food
- It had a projecting muzzle, similar to Sahelanthropus, giving it an ape-like feel
- However, its skull does not resemble that of chimps closely.
There are interesting comparisons between its skull and that of Sahelanthropus:
Like Ar. ramidus, S. tchadensis had a brain that was less than 400 cm3 in size. It also resembled Ar. ramidus in having small nonsharpened canines. Details of the bottom of the skull show that both Ar. ramidus and Sahelanthropus had a short cranial base, a feature also shared with Australopithecus. Furthermore, we infer that the rear of the Ar. ramidus skull was downturned like that suggested for Sahelanthropus. These similarities confirm that Sahelanthropus was indeed a hominid, not an extinct ape. [23]
One of course recalls Wolpoff et al and their caution about the status of Sahelanthropus. Ardipithecus may well tip the basis in favour of Sahelanthropus being a hominid - time will tell. Their conclusion:
These and an additional feature of the skull hint that, despite its small size, the brain of Ar. ramidus may have already begun to develop some aspects of later hominid-like form and function. The steep orientation of the bone on which the brain stem rests suggests that the base of the Ar. ramidus brain might have been more flexed than in apes. In Australopithecus, a flexed cranial base occurs together with expansion of the posterior parietal cortex, a part of the modern human brain involved in aspects of visual and spatial perception. [24]
The fragmented skull bones are shown below:
Fig. 1 The fragmented skull of ARA-VP-6/500. (Upper panel) Identifiable pieces of the skull after limited refitting for digital and physical molding. (Lower panel) (A) ARA-VP-6/500-032, (B) micro-CT rendered image of the same, with cross-sectional locations of (C) and (D) indicated. Arrowheads in (A) denote the positions of (C) and (D).
Fig. 1 The fragmented skull of ARA-VP-6/500. (Upper panel) Identifiable pieces of the skull after limited refitting for digital and physical molding. (Lower panel) (A) ARA-VP-6/500-032, (B) micro-CT rendered image of the same, with cross-sectional locations of (C) and (D) indicated. Arrowheads in (A) denote the positions of (C) and (D).
Comparisons between chimps and the Ardipithecus reconstruction:
(Right) Oblique and side views of a female chimpanzee (right) and the Ar. ramidus female reconstruction (left; the oblique view includes a separate mandible). (Left) Comparison of brain and tooth sizes (arrows) of chimps (Pan, blue), Ar. ramidus (red), and Australopithecus (green). Means are plotted except for individual Ar. ramidus and Au. afarensis cranial capacities. Canine unworn heights (bottom) are based on small samples, Ar. ramidus (females, n = 1; males, n = 3), Au. afarensis (n = 2), Pan (females, n = 19; males n = 11
References
1. Gibbons A "A New Kind of Ancestor: Ardipithecus Unveiled" Science (2009) 326:36 - 40
2. C. O. Lovejoy et al. "The Great Divides: Ardipithecus ramidus Reveals the Postcrania of Our Last Common Ancestors with African Apes" Science (2009) 326:73. Author Summary)3. Lovejoy CO "Reexamining Human Origins in Light of Ardipithecus ramidus" Science (2009) 326:74
4. C. O. Lovejoy et al. "The Great Divides: Ardipithecus ramidus Reveals the Postcrania of Our Last Common Ancestors with African Apes" Science (2009) 326:73. Author Summary)
ibid, p 102
5. loc cit
6. loc cit
7. Lovejoy, p 103
8. loc cit
9. Lovejoy CO et al "Combining Prehension and Propulsion: The foot of Ardipithecus ramidus" Science (2009) 326:72
10. ibid, p 72e5
11. ibid, p 72e6
12. ibid, p 7213. C. O. Lovejoy et al "The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking" Science (2009) 326:71, 71e1-71e6
14. ibid, p 71e1
15. loc cit
16. ibid, p 71e3
17. ibid, p 71e5
18. Lovejoy CO et al "Careful Climbing in the Miocene: The Forelimbs of Ardipithecus ramidus and Humans Are Primitive" Science (2009) 326:70
19. ibid, p 70e1
20. ibid, p 70e5
21. ibid, p 70e6-7
22. Suwa G et al. "The Ardipithecus ramidus Skull and Its Implications for Hominid Origins" Science (2009) 326:68
23. loc cit
24. loc cit
