pelvis

The other day, I started writing about the Sarmiento-White exchange on Ardipithecus, by describing how they disagree about the implications of the molecular clock.

What really prompted me to break up my discussion into three posts was that it takes quite a lot of space to explicate the features of the pelvis. I've taken care to reference the description by Lovejoy and colleagues (2009c), the general discussion of Ardi's locomotor anatomy in Lovejoy et al. (2009a, 2009b), and the discussion of early hominin pelvic evolution by Lovejoy and colleagues (1999).

I have a major hesitation that keeps me from writing anything about the Ardipithecus pelvis beyond those descriptions: Independent investigators at present cannot verify or replicate any comparisons made in Lovejoy and colleagues' analyses. Most of the measurements and many quantitative observations depend on a 3-d model. That model is not available for inspection, and the published description does not provide enough detail about the model to independently assess its accuracy. Worse, as I discussed last fall, the model appears to have been derived from the a priori expectations about pelvis evolution that Lovejoy and colleagues published in 1999.

As a result, I don't think any independent reader, including me, can tell how much of the model is real.

Given my problems understanding their pelvis 3-d model, I've decided to limit myself to the narrow points considered by Sarmiento's (2010) comment and White and colleagues' (2010) reply. Lovejoy and colleagues (2009b, 2009c) claimed that most of the pelvic anatomy of Ardipithecus is primitive for great apes, and that many of the pelvic features shared by chimpanzees and gorillas evolved in parallel in those two lineages. But they listed a few features that they considered to be derived in Ardipithecus and shared with Australopithecus. Sarmiento lists these, together with two features of the foot, and argues that they are not compelling evidence that Ardipithecus is a cladistic hominin:

Of the remaining characters listed as common to Ardipithecus and Australopithecus, none of the eight postcranial characters (sagittal iliac/isthmus orientation, slightly broadened iliac breadth, strong anterior inferior iliac spine formed by separate ossification center, robust second metatarsal base and shaft, dorsally domed second to fifth metatarsal heads, upwardly canted proximal foot phalanges, and short iliac isthmus and pubic symphysis outline), nor the other four craniodental characters [anterior basion position (14), advanced cranial flexion, and broad lower molars and mandibular corpus] are shown by systematic comparisons to be exclusive to humans or share-derived with humans. Nearly all are quantitative characters that appear in early hominoids (i.e., Oreopithecus and Dryopithecus) and have appeared independently in other primate lineages, and character simplicity is such that parallelisms or reversals in polarity cannot be demonstrated (12, 15).

I think Sarmiento's argument is entirely reasonable. Lovejoy and colleagues (2009a, 2009b) claimed a long series of parallelisms between chimpanzees and gorillas. Despite some reservations, I tend to agree -- Ardipithecus is primitive in its postcranial anatomy, and living apes are convergently derived. But take the argument to its logical end, and it becomes Sarmiento's. Ardi shares some postcranial features with hominins that living apes lack, but how do we know that any of them are derived? Or if they are derived, how do we know that they aren't trivially simple to evolve in parallel?

In their published reply to Sarmento, White and colleagues do not mention the long series of great ape postcranial features that they previously argued to be cases of parallel evolution (Lovejoy et al. 2009b, 2009c). Instead, they claim that three features of the pelvis are so convincingly like Australopithecus that Ardi must be a hominin:

Although isolated aspects of pelvic morphology of Oreopithecus may partially mimic those of Ar. ramidus [such as a projecting anterior inferior iliac spine (AIIS)], crucial postcranial elements of the latter (9, 10) are unambiguously derived toward the Australopithecus condition, to the exclusion of Oreopithecus. Some of these derivations probably stem from shared changes in pattern formation exhibited by both Ar. ramidus and Australopithecus. In the pelvis, these include (i) superoinferior approximation of the sacroiliac and acetabular joints by iliac isthmus shortening and (ii) a sagittally oriented and greatly broadened lower iliac isthmus accompanied by (iii) an exaggerated anterior margin, itself the product of a unique physis for the AIIS, shared only with phyletic hominids.

I find this reply very strange. The "shared changes in pattern formation" hypothesis actually supports Sarmiento's argument. If White and colleagues are correct about the morphogenetic basis of the Ardipithecus pelvic anatomy, that makes it more likely to have evolved convergently with Australopithecus, not less likely. Lovejoy and colleagues (1999) emphasized this point -- the pelvic features of hominins were likely to have evolved due to selection for a shorter pelvis, principally for biomechanical reasons, with other characters of the pelvis and femur changing entirely due to their genetic correlation with this major target of selection.

The reply omits the most persuasive of the derived features in hominins -- the short ilium -- which was at the center of Lovejoy and colleagues' (1999) account of hominin pelvic evolution. Here's a comparison of 3-d models:

Ardi looks very obviously like the human and Lucy, and very different from the chimpanzee, right? But I think that the chimpanzee model in this picture is larger than it should be, as the acetabulum looks much larger than Ardi even though Lovejoy and colleagues (2009c) report Ardi's acetabulum as right in the middle of the chimpanzee range. Maybe they chose a large chimpanzee, or built the Ardi 3-d model using the smaller end of their range of possible acetabular diameter. You see the problem of using a model instead of the actual fossil?

In any event, the differences between Ardi's os coxa and the chimpanzee's are obvious. Ardi has a much shorter ilium. The chimpanzee has an iliac blade that comes right out of the picture toward us, because it is oriented along a coronal axis. Ardi's angles forward, or anteriorly, like the hominins.

In fact, if we look at the model in superior view superimposed on Lucy's pelvis, you can see that Ardi's iliac blades angle even more anteriorly than Lucy's:

The three features White and colleagues (2010) list, as quoted above, are morphological side effects of the shorter, more sagitally angled ilia. Lovejoy and colleagues (1999) paper would likely have described these features as side effects of selection for a shorter pelvis with an anteriorly directed origin for the rectus femoris muscle.

The question is: How much of the functional similarity between Ardi and hominins is homology, and how much is convergence? Similarity may not reflect homology -- descent of the feature from the same ancestor.

That point is especially notable when White and colleagues (2010) discuss Oreopithecus -- an extinct ape whose pelvis shares some features with hominins, and other features with apes. Oreopithecus is not a hominin, but it may have had some adaptations to a bipedal stance. Yet it also shares features that Lovejoy and colleagues (2009b) have argued must have evolved convergently in orangutans, chimpanzees and gorillas. That seems like a real problem for the idea that Ardipithecus represents the primitive condition for such traits.

Here's the Oreopithecus paragraph from White et al. (2010), the first time that Ardipithecus and Oreopithecus pelvic features have been compared (other than here on the blog):

Indeed, Oreopithecus diverges from hominids remarkably in features ranging from limb proportions to dental anatomy. In the pelvis, it features bi-iliac entrapment of at least one lumbar vertebra and general immobilization of the lumbar column (including transformation of lumbar somites into its six-segment sacrum). Such changes stand in stark contrast to the six lumbar, four-segment sacrum of Au. afarensis, a character adumbrated by the precipitous reduction in iliac height (and extensive broadening) of the Ar. ramidus ilium (10). African apes have entirely rigidified lumbar columns that differ radically from those of hominids.

I think this comparison is very important. Oreopithecus is not a member of the orangutan clade, and Lovejoy and colleagues' (2009b) scenario implies that if Oreopithecus is a member of the African ape clade, it -- like chimpanzees and gorillas -- must have evolved these features convergently.

Can it be that orangutans, chimpanzees, gorillas, and Oreopithecus all acquired the distinctive "bi-iliac entrapment" of the lower lumbar vertebrae in four separate instances of evolutionary convergence? Put those together with the elongation of the arms, reduction in the length of the lumbar column, and sacralization of lumbar vertebrae. Far from a simple change, it a series of complicated, correlated changes. Lovejoy and colleagues (2009b) defended the hypothesis that these traits are parallelisms shared by all the lineages of living great apes. Now, White and colleagues (2010) are forced to posit a fourth independent evolution of many of these traits in Oreopithecus.

Despite those similarities to living great apes, Oreopithecus shares with hominins the development of a relatively prominent anterior inferior iliac spine. This implies an adaptation to hip flexion or knee extension with a more extended leg. Bipedal stance is one possible explanation for this anatomy, and is the explanation that Lovejoy and colleagues (2009c) offer for its presence in Ardipithecus. White and colleagues (2010) include this as their feature (iii), the "unique physis for the AIIS, shared only with phyletic hominids." But this description seems exaggerated, when we consider what Lovejoy and colleagues (2009c:71e3) actually wrote:

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.

A "novel physis" refers to a separate growth plate for the anterior inferior iliac spine. Ardi was an adult, and her pelvis was fully developed. So there's no observing whether the anterior inferior iliac spine had its own growth plate. Lovejoy and colleagues (2009c, 2010) are just claiming there must have been one. What basis could there be for such a model, other than an allometric analysis of the anterior inferior iliac spine in humans and other primates where it is present -- such as Oreopithecus? Remember that Ardi is more than twice the body size of Oreopithecus, yet Rook and colleagues (1999) showed that the cancellous structure within the anterior inferior iliac spine of Oreopithecus is a close match to Homo. That anatomical similarity may imply a common developmental pathway in Oreopithecus and hominins.

Is the anterior inferior iliac spine homologous in Oreopithecus and Ardipithecus? If so, it is probably primitive for great apes, not derived in hominins. Does it have another functional role besides bipedal stance? If so, that functional role might well have occurred in Ardipithecus, another arboreal quadruped.

Could other features of Ardi's pelvis be consequences of arboreal quadrupedal locomotion in an ape with a long lumbar spine? The sagittal orientation of the iliac blades and isthmus is not like living great apes, but it is like living Old World monkeys. Ardi's ilia are shorter than monkey ilia, but the question deserves some serious allometric study. Also deserving of study is whether isthmus orientation in monkeys matches that of the iliac blades, and if not, why not? One hypothesis would be the morphogenetic effects of selection for a shorter ilium length, the scenario published by Lovejoy and colleagues (1999).

I don't think there's any question that the evolutionary scenario outlined by Lovejoy and colleagues (2009b) is highly non-parsimonious with respect to the postcrania. It requires the convergent evolution of a long suite of characters within all the living great apes in at least three separate evolutionary histories. Add in fossil apes -- at least Oreopithecus, and possibly Morotopithecus and Dryopithecus -- and the number of parallelisms is extreme. The chimpanzee-gorilla convergences go even further beyond those shared with orangutans to include the knuckle-walking features of the wrist and hand, and several dental characters.

White and colleagues (2010), as I'll describe in the next post, argue that the shared dental characters of Ardipithecus and Australopithecus necessitate their close relationship. Once this is assumed, the many postcranial convergences become necessary. In that perspective, it helps to "soften the blow" somewhat by identifying those postcranial features shared by Ardipithecus and the hominins.

From the perspective of the pelvis, I'll return to one feature of Ardipithecus that seems independent, shared with hominins, and lacking in Oreopithecus: the "precipitous reduction in iliac height," so obvious in the picture above. But Ardi's os coxa is badly crushed at the superior border of the ilium. My post from last fall includes photos of both Ardi's os coxa and the pelvis of Oreopithecus. Ardi's is relatively shorter, no question, and it lacks the great height on its medial aspect, that creates the "entrapment" of the last lumbar vertebra of Oreopithecus. But the crushing seems to obscure this anatomy, so that it's not possible to be sure from the photos.

I wish we had better than a cartoon model to compare. During the seven months since I first detailed what I see as weak points in the pelvic description, I've become less and less persuaded that the pelvic features reflect any hominin-like locomotor adaptations in Ardipithecus. There are many unresolved functional issues, which obscure the phylogenetic relations between living and fossil apes. Ardi makes every tree less parsimonious, no matter which branch we put her on. Shoe-horning her into the hominins doesn't solve many problems, and creates some intractable ones.

I find myself calling her an ape.

References:

   Abitbol MM. 1995. Reconstruction of the sts 14 (Australopithecus africanus) pelvis. Am J Phys Anthropol 96:143–158.

   Harrison T. 1986. A reassessment of the phylogenetic relationships of Oreopithecus bambolii. J Hum Evol 15:541–584.

   Harrison T. 1991. The implications of Oreopithecus bambolii for the origins of bipedalism. In: Coppens Y, Senut B, editors, Origine(s) de la bipédie chez les hominidés, Cahiers de Paléoanthropologie. Paris: Editions du CNRS. p 235–244.

   Köhler M, Moyà-Solà S. 1997. Ape-like or hominid-like? the positional behavior of Oreopithecus bambolii reconsidered. Proc Natl Acad Sci U S A 94:11,747–11,750.

   Lovejoy CO, Cohn MJ, White TD. 1999. Morphological analysis of the mammalian postcranium: A developmental perspective. Proc Natl Acad Sci U S A 96:13,247–13,252.

   Lovejoy CO, Simpson SW, White TD, Asfaw B, Suwa G. 2009a. Careful climbing in the Miocene: The forelimbs of Ardipithecus ramidus and humans are primitive. Science 326:70e1–70e7.

   Lovejoy CO, Suwa G, Simpson SW, Matternes JH, White TD. 2009b. The great divides: Ardipithecus ramidus reveals the postcrania of our last common ancestors with African apes. Science 326:100–106.

   Lovejoy CO, Suwa G, Spurlock L, Asfaw B, White TD. 2009c. The pelvis and femur of Ardipithecus ramidus: The emergence of upright walking. Science 326.

   Robinson JT. 1964. Adaptive radiation in the australopithecines and the origin of man. In: Howell FC, Bourlière F, editors, African ecology and human evolution. London: Methuen and Company, Limited. p 385–416.

   Rook L, Bondioli L, Köhler M, Moyà-Solà S, Macchiarelli R. 1999. Oreopithecus was a bipedal ape after all: Evidence from the iliac cancellous architecture. Proc Natl Acad Sci U S A 96:8795–8799.

Sarich VM. 1971. A molecular approach to the question of human origins. In (P. Dohlinow & V.M. Sarich, Eds.) Background for Man: Readings in Physical Anthropology, pp. 60‐81. Boston: Little, Brown.

Sarmiento EE. 2010. Comment on the paleobiology and classification of Ardipithecus ramidus. Science 328:1105. doi:10.1126/science.1184148

   White TD, Asfaw B, Beyene Y, Haile-Selassie Y, Lovejoy CO, Suwa G, WoldeGabriel G. 2009. Ardipithecus ramidus and the paleobiology of early hominids. Science 326:75–86.

White TD, Suwa G, Lovejoy CO. 2010. Response to Comment on the paleobiology and classification of Ardipithecus ramidus. Science 328:1105. doi:10.1126/science.1185462

One of the grottiest, most severely crushed parts of the Ardipithecus ARA-VP-6/500 skeleton is the pelvis. The left os coxa is nearly complete but badly distorted, part of the right ilium is preserved along with a bit of the sacrum. The research team had to correct for the distortion and breakage of the os coxa to interpret its form. The resulting paper, by Lovejoy and colleagues 2009c, describes the anatomy as present in the distorted fossil, the CT-assisted reconstruction of the fossil, and a whole-pelvis model based on the reconstruction.

I've written some thoughts on the specimen and its reconstruction -- not nearly comprehensive, but enough to introduce a bit of the complexity of the locomotor inference from this odd pelvis. Also, I've pointed to the one Miocene ape notably absent from the published comparisons, Oreopithecus.

The pelvic model

I want to spend some time considering their 3-d pelvic model. I find it to be a provocative application of visualization technology. On the one hand, a model greatly facilitates the description and comparison of the specimen with other key fossils. But on the other hand, any model comes with a load of assumptions, which may not be made explicit when anatomical comparisons are made. It’s our job to approach models skeptically, to try to identify the implicit assumptions and bring them out to be examined.

In this case, much thought has gone into the virtual model. For astute Ardipithecus-watchers, a paper published ten years ago has played a central role in understanding the probable anatomy of the specimen. Here’s the first figure from that paper, published in 1999 by Lovejoy, Martin Cohn and Tim White:

Very interesting indeed. The top left image is indeed very close to the Ardipithecus model that Lovejoy and colleagues present in the current paper. The figure (Figure 1 in Lovejoy et al. 1999) had the following, very long caption:

OK, so the top right image is a squashed chimpanzee pelvis; the top left is the product of “morphing” the chimpanzee pelvis with Lucy’s pelvis. Lovejoy et al. (1999) were making a simple point: you can get almost all the features of the hominid pelvis by a very simple process of morphological change. The model of the pelvis was an illustration for a developmental model – the idea was that many morphological features of the hominins might result from a few changes to gene regulation early in development.

In the current paper, the model is no longer a mere thought experiment. In 1999, readers of the paper could hypothesize that Lovejoy and colleagues already knew the morphology of the Ardipithecus pelvis — Tim White’s “Star Wars” comment was a clue. But in the current paper (Lovejoy et al., 2009c), it seems clear that the 3-d digital version of the model also shaped the reconstruction of the highly crushed pelvic remains. That means we have to carefully read Lovejoy and colleagues’ (2009) paper to determine which parts of the model are fully supported by the specimens’ preserved (non-crushed) anatomy, and which are more speculative.

The ilia

One aspect of the digital model got my attention more than the others, because it was not evident in the 1999 pelvis model. It’s not the length or breadth of the ilium — based on the preserved bone, they have reconstructed both as intermediate between the chimpanzee and australopithecine morphology. Nor is it the chimpanzee-like long ischium, without which it’s hard to see how Ardipithecus could have taken much advantage of those grasping feet. No, to me the surprising thing is that they’ve given the iliac blade a pronounced anterior cant, far more so than in Australopithecus.

The best view of this aspect of the reconstruction is the supplementary figure S1 from Lovejoy and colleagues 2009c, which presents the Ardi reconstruction superimposed in space on the reconstructed pelvis of Lucy.

You can see that the grey Ardi pelvis has ilia that angle far more anteriorly than Lucy’s. This is the opposite direction from any of the living great apes, which have iliac blades aligned more parallel to the back of the trunk.

It is unclear to what extent this position is necessary based on the preserved morphology. The os coxae are crushed, and the paper does not illustrate the parts in sufficient detail to for me to judge the reconstruction. It appears to me that the position is determined by two assumptions. First, they provide the specimen with a wide sacrum. In the comparison between the Ardi model and Lucy’s pelvic reconstruction, it is clear that they’ve fitted Ardi with the same proportionate size sacrum as Lucy, which — since the comparison is with a Lucy blown up to 115% life-size — is 15 percent wider than Lucy’s. Second, they key the position of the anterior superior iliac spine off that of the anterior inferior iliac spine. Since this anterior inferior spine is especially prominent on Ardi’s os coxa, keeping the two spines in their usual conformation would require an anteriorly placed anterior superior iliac spine.

Of course, the only extant hominoids to assess the relation between these spines are highly suspensory apes with stiff, short lumbar spines, and us: short-pelvised obligate bipeds. If Ardipithecus was neither of these — and it wasn’t — then maybe it had an unexpected relation between these anatomical features.

At any rate, the shape reconstructed for Ardipithecus goes along with Lovejoy and colleagues hypothesis about ilium and lumbar spine evolution. They propose that the ilia have flattened and extended in length independently in at least the three lineages of chimpanzees, gorillas, and orangutans (and we may add gibbons) as a consequence of sacralizing the lower lumbar vertebrae and reducing the flexibility of the lower back, thereby creating a short, stiff trunk.

Given that scenario, we can’t consider the ilia apart from the lumbar spine and some assumptions about the requirements of different locomotor patterns. Unfortunately, Ardipithecus doesn’t have any lumbar vertebrae — or indeed any sacrum worth reconstructing.

But there is another Miocene ape that does.

The relevance of Oreopithecus

The best-preserved pelvic remains from any Miocene ape are those of Oreopithecus bambolii. This species has been found in lignite deposits of Tuscany and Sardinia, dating to between 10 and 7 million years ago. At that time, these regions were joined as a single landmass, an island not connected to the European mainland, making it credible (though not certain) that Oreopithecus faced lower predation than mainland primates.

I’m not going to do any kind of detailed description here of the Oreopithecus pelvic remains. A review of the Oreopithecus phylogenetic status by Terry Harrison 1986 gives a literature review to that point, more recent papers by Rook and colleagues 1999 and Kohler and Moyà-Solà 1997 took up the problem of possible habitual bipedality.

It has been evident for a long time that several features of the Oreopithecus pelvis are shared with hominins and not with living great apes. Harrison 1991 gives a list of some of these features, focusing on the importance of the anterior inferior iliac spine:

We can add to Harrison’s list other features recognized by earlier and later workers, including a short symphysis, a relatively short sacral articulation, increased breadth of the lateral ilium relative to the medial (sacral) portion, and a network of cancellous bone similar in some ways to humans.

Some argue that the biped-like anatomy of Oreopithecus is there because Oreopithecus was, in fact, a terrestrial biped. The pelvic features combine with others across the skeleton, including a lordotic lumbar spine and an extremely adducted hallux, which is proposed to give a tripod-like stability to the foot in bipedal stance (Köhler and Moyà-Solà, 1997).

I don’t necessarily share that conclusion, nor do I think that Oreopithecus was the ancestor of hominins. It evolved on an island in the middle of the ancient Mediterranean, and apparently became extinct once a land bridge connected the island with Europe. Other aspects of the skeleton, including the skull and limb proportions, show that Oreopithecus was very different from Ardipithecus.

Still, two Late Miocene hominoids with short, broad ilia and prominent anterior inferior iliac spines, is one too many. Here's a picture of the most complete Oreopithecus pelvis:

And here's the unreconstructed Ardipithecus os coxa in medial view:

We can hypothesize a generalized ape ancestor, from which the specializations of today’s great apes and those of bipeds might arise with high likelihood under alternative selection regimes. This idea is far from new — before the early 1960’s, the human-chimpanzee clade was a minority viewpoint, and many scientists were looking for quadrupeds that might show that some of the “bipedal” features of our pelves were primitive instead of derived.

Here’s John Robinson, writing in 1964:

For Robinson, Oreopithecus was no more bipedal than many living apes, but lacked their extreme specializations. This made it a possible representative of the ancestral condition, somewhat more hominin-like than any of the living great apes.

I am inclined to think that’s exactly what Ardipithecus is showing. The similarity of Oreopithecus, Ardipithecus, and ultimately Australopithecus in these pelvic features suggests that this pelvic anatomy may have been the generalized form, with chimpanzees, gorillas, and orangutans separately derived toward a longer and flatter pelvis. These features would have given the large suspensory apes more stability in the lumbar region, short in living great apes, but long in both Oreopithecus and hominins.

The small acetabulum

Lovejoy and colleagues 2009c report upper and lower bounds on acetabulum height for Ardi’s pelvis. These bounds more or less bracket those for Lucy’s pelvis, the upper bound for Ardi is the same value as the acetabulum height of Sts 14.

Both these australopithecine pelves belonged to individuals on the order of 30–35 kg in mass. White and colleagues (White et al., 2009) report an estimated mass for Ardi of 51 kg. Ardi’s acetabulum was at most the size of an average-sized chimpanzee’s despite the fact her mass as larger than most male chimps, and as much as 75 percent larger than female australopithecines. The low bound on acetabulum height is essentially the same as for the Proconsul os coxa, KNM-MW 13142 D.

If we’re looking for evidence of significant habitual weight transfer through the hindlimb, it just isn’t there.

I wouldn’t make overly much out of that, especially without a better estimate of acetabulum area. But it is one aspect of pelvic anatomy that we can observe that is more or less independent of the reconstruction of the ilia. It points away from the hypothesis that Ardipithecus was habitually bipedal.

Looking for a locomotor reconstruction

Notwithstanding any lingering uncertainty about the pelvic anatomy, I pretty much accept the arguments of Lovejoy and colleagues (Lovejoy et al., 2009a,b): Ardipithecus was a generalized arboreal quadruped. With the pelvis, the lingering questions are how much the anatomy suggests upright bipedality or carrying (Lovejoy’s “provisioning” model). My initial reaction was that Ardi may have used an arm-assisted bipedality in the trees. But having played with that idea a little, I just don’t see much support for it in the skeletal remains.

To test the importance of facultative bipedality in Ardipithecus, I think we’ll have to have a better model for how quadrupedality functions in an ape pelvis that is nonetheless very different from those of living great apes, including orangutans. The reconstructed limb proportions are within the range of Old World monkeys, suggesting that Ardipithecus would have been capable of palmigrade quadrupedalism. Lovejoy and colleagues 2009a argue that the proximal ulna morphology is consistent with this form of locomotion also, like earlier Miocene apes and monkeys. Ardi’s long apelike ischia retain the long hamstrings lever arm necessary for powerful hindlimb extension in a flexed, quadrupedal position. And although apes do not have anteriorly-flaring ilia, monkeys do.

This is not to say that the locomotor pattern was entirely monkey-like, but merely that the ancestral, generalized quadrupedal ancestor did not have great ape locomotor specializations, including rapid vertical climbing and long-armed quadrumanous movement (Lovejoy et al., 2009a). These adaptations mainly make sense in a canopy forest habitat, where there are tall trunks devoid of lower-story branches and a continuous canopy. The grassy woodland habitat occupied by Ardipithecus would not have selected for the below-branch specializations of the forelimb and hand, and thereby would have given no reason for knuckle-walking as a terrestrial locomotor strategy.

References