Ethiopia

Manuel Dominguez-Rodrigo, writing with my University of Wisconsin colleagues Travis Pickering and Henry Bunn, has challenged the interpretation that two bovid bones from Dikika bear cutmarks made by hominins [1]. I wrote about the Dikika cutmark claims earlier this year (Australopithecus afarensis used stone tools). The new paper is a strong critique of that earlier work.

Our taphonomic configurational approach to assess the claims of A. afarensis butchery at Dikika suggests the claims of unexpectedly early butchering at the site are not warranted. The Dikika research group focused its analysis on the morphology of the marks in question but failed to demonstrate, through recovery of similarly marked in situ fossils, the exact provenience of the pub- lished fossils, and failed to note occurrences of random striae on the cortices of the published fossils (incurred through incidental move- ment of the defleshed specimens across and/or within their abrasive encasing sediments). The occurrence of such random striae (some- times called collectively “trampling” damage) on the two fossils provide the configurational context for rejection of the claimed butchery marks. The earliest best evidence for hominin butchery thus remains at 2.6 to 2.5 Ma, presumably associated with more derived species than A. afarensis.

These authors are experts on cutmarks, both from their work on Oldowan faunal assemblages and from experimental work where they have controlled the actual circumstances of cutmarking, trampling and weathering. Their critique of the two Dikika bones takes two main paths:

1. The surfaces of the bones themselves are relatively poorly preserved, with evidence of "trampling" modification and subadult status for one specimen and evidence of "moderate weathering" on the other. The matrix containing the bones was highly abrasive, making spurious marks more likely. This would make it difficult to get clear results even in an experimental context.

2. The purported cutmarks themselves are similar to marks that occur in bones subject to trampling damage. Dominguez-Rodrigo and colleagues argue that some of these marks are more diagnostic of trampling than of cutting or hammerstone damage.

The authors do not say they have disproven the hypothesis that A. afarensis cut on these bones with naturally-occurring stones, but they clearly question whether such a hypothesis is credible:

The Dikika “butchery mark” evidence does not, however, withstand peer scrutiny undertaken from an actualistic perspective and with a configurational approach. Our approach in assessing the Dikika claims was intentionally conservative: the claims are extraordinary because of their singularity and because of the inferred age of the fossils. Thus, natural processes of bone modification need to be eliminated before precluding nonanthropogenic origin(s) for the surficial marks on DIK-55–2 and DIK-55–3. High probability trampling damage on both specimens does not allow for this elimination and, again, taking our contextualized, maximally conservative position, forces us to reject even marks A1 and A2, the two morphologically strongest claims of cutmarks on DIK-55–2.

Their discussion emphasizes that, in their view, a hypothesis that an unusual tool type was responsible for cutmarks should be accompanied by experimental or actualistic evidence concerning the effects of that tool type. I think that for discoveries as potentially important as this, it is very reasonable for reviewers to expect such evidence will be provided. Also, a full statistical workup of other faunal bones from the site would be worthwhile. If the matrix really is abrasive and readily gives rise to trampling scratches, these should be evident in a wider distribution of bone from the site.

But for the moment, it looks like we should continue to treat cautiously claims of very early stone tool use. Possibly further comparisons will back up the hypothesis of cutmarks with more evidence. Since it took only three months from the initial publication of the Dikika evidence to this response, maybe we won't have to wait long for more comparisons!

References

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

In the fossil record, a species is a hypothesis. We can't test that hypothesis in the way we do with living animals. Even in the dark, after all the paleontologists have left, the fossil bones just won't get it on. No reproduction, no test.

So, sometimes we have to live with hypotheses that we can't immediately test. Because many hypotheses are wrong, we have to keep juggling in our minds the names of more species than probably existed.

All the juggling frequently leads to confusion. One may reasonably wonder how we know that species X evolved into species Y, when half the field rejects the hypothesis that X was a real species. Often we don't disagree about the "evolving into", but we do disagree about the boundaries and other relationships of the populations -- which we can understand only indirectly from the fossils. The fossils don't change, but our hypotheses about them do.

That brings us back to A. anamensis. Here's a hypothesis about an ancient lineage of hominins, based on a certain number of differences from later fossil samples assigned to A. afarensis. That's a precarious place for a hypothesis, because from the beginning, the definition of A. afarensis has encompassed geographic and temporal variability. How hard would it be to recognize a little more temporal variability? Not very.

So, as I described last week, Haile-Selassie and colleagues (2010) propose sinking A. anamensis ("Woranso-Mille: A ladder not a bush"). I should mention, if I haven't already, that I have great sympathy for this viewpoint. Absent some compelling evidence that the lineage includes a speciation event, I prefer slow gradual anagenesis to be categorized into one evolving species, not an arbitrary set of chronospecies.

In the same post, I described the work of Kimbel and colleagues (2006), who had argued for anagenesis in the same sample of A. anamensis and A. afarensis-referred fossils, but retained the two distinct names for them. One thing stands out as a mystery to me in that paper. Why didn't they let Kenyanthropus make the argument for them?

If you want to establish that A. anamensis is taxonomically valid, the simplest way to answer all critics is if it has more than one descendant. You don't have to demonstrate the phylogeny beyond all doubt, I would say, you just have to take the hypothesis seriously.

In this instance, we seem to have two good candidates for a non-A. afarensis descendant of A. anamensis. The more obvious of them is Kenyanthropus. Why didn't they advance the hypothesis of a A. afarensis-Kenyanthropus clade? Here's what Kimbel and colleagues wrote about the latest Leakey find:

A more significant concern is the presence of Kenyanthropus at 3.5 Ma. Kenyanthropus may demonstrate cladogenesis prior to this time, but this taxon is only directly relevant to the analysis if any of the samples share derived character-states with it. At present, the Kenyanthropus hypodigm does not match the others in the availability or quality of character data for the mandible and anterior teeth, while the evidence that exists (from the maxilla, for example) does not suggest a close relationship of Kenyanthropus to any of the phena considered here (Leakey et al., 2001).

So, they took the hypothesis off the table. And again, they've refocused the question solely upon dental and mandibular evidence.

This is a very large hole. At 3.5 million years, KNM-WT 40000 is earlier than any other comparably complete hominin skull, except for Ardi and Toumaï. It doesn't look like them, that's for sure. Not the same phenum at all. If you're going to insist that KNM-WT 40000 isn't A. afarensis, it's hard to see a better hypothesis than that it's descended from A. anamensis.

If it doesn't have a close relationship to either A. afarensis or A. anamensis, then I'm at a loss to figure out what they think it is related to!

I'm willing to believe White's (2003) argument that it just is a member of the A. anamensis-A. afarensis lineage, but I don't see the contrary argument that it's so different from this lineage that it must be derived from some as-yet-undiscovered hominin. Keep in mind that the argument was formed by people, many of whom already knew basically what Ardi's skull was going to look like. I just can't feature why this phylogenetic problem didn't raise itself to a higher profile.

As I wrote above, we juggle more hypotheses than can be true. In this instance, the null hypothesis is that all these hominins belong to a single evolving species, which would be called A. afarensis. But one alternative, in which A. anamensis existed as the ancestor of A. afarensis and at least one other species, has some utility. It lets us refer clearly to phylogenies with late-diverging sister taxa. As Yoda might have said, there is another possible sister taxon for A. afarensis: A. africanus might be derived from, or might itself be, a South African contemporary of the Hadar-Maka-Laetoli sample. The earliest Sterkfontein dates go up and down; one or more of the remains might be contemporaries of Laetoli or even earlier East African localities.

Anyway, I'm not so interested in this question of bushes versus ladders, or "Pliocene diversity". I'm more focused on the curiously non-hominin-like features of Ardipithecus. If we suppose a late molecular divergence of hominins and chimpanzees (say after 4.4 million years ago), then Ardipithecus might be an ape or ancestral (stem) hominid, not a hominin. If so, then samples now referred to A. anamensis, including later specimens from Aramis, Asa Issie, and Kanapoi, are in fact the earliest-known members of the human lineage.

A. anamensis is no afterthought in that case, it may be the stem hominin.

That is, if it hasn't been sunk into A. afarensis.

UPDATE (2009-12-10): A reader writes to remind me about A. garhi, which presents itself as very much like A. africanus and is quite a lot later than any known A. afarensis samples. One alternative is that A. africanus is simply the latest element of the single A. anamensis-A. afarensis-A. africanus anagenetic sequence.

I don't think that's very easy to reject, considering the lack of good cranial remains between 3 and 2.5 million years ago (or for that matter, even later) in East Africa.

Some of the Sterkfontein specimens, particularly those from Jakovec cavern and from Member 2 (including Little Foot) have been suggested to be older than 3 million years. Partridge and colleagues (2003) put them at older than 4 million years ago, which would make them rivals of A. anamensis as the earliest hominins. But Berger and colleagues (2002) argued (apparently preemptively) that these early dates are not necessary on faunal or magnetostratigraphic grounds.

A second reader wonders why I didn't mention A. bahrelghazali as a possible sister to A. afarensis. Well, nothing's impossible, but I'd say the case for the Bahr el Ghazal mandible being distinct from the Hadar-Maka-Laetoli sample isn't very strong. Still, we're only talking about hypotheses here, I suppose.

References:

Berger LR, Lacruz R, de Ruiter DJ. 2002. Revised age estimates of Australoipithecus-bearing deposits at Sterkfontein, South Africa. Am J Phys Anthropol 119:192-197. doi:10.1002/ajpa.10156

Haile-Selassie Y, Saylor BZ, Deino A, Alene M, Latimer BM. 2010. New hominid fossils from Woranso-Mille (Central Afar, Ethiopia) and taxonomy of early Australopithecus. Am J Phys Anthropol (in press) doi:10.1002/ajpa.21159

Kimbel WH, Lockwood CA, Ward CV, Leakey MG, Rak Y, Johanson DC. 2006. Was Australopithecus anamensis ancestral to A. afarensis? A case of anagenesis in the hominin fossil record. J Hum Evol 51:134-152. doi:10.1016/j.jhevol.2006.02.003

White T. 2003. Early hominids -- diversity or distortion? Science 299:1994-1996.

Suwa G, Asfaw B, Kono RT, Kubo D, Lovejoy CO, White TD. 2009. The Ardipithecus ramidus skull and its implications for hominid origins. Science 326:68e1-68e7. doi:10.1126/science.1175825

Leakey MG. Spoor F, Brown FH, Gathogo PN, Kiarie C, Leakey LN, McDougall I. 2001. New hominin genus from eastern Africa shows diverse middle Pliocene lineages. Nature 410:433-440.

Partridge TC, Granger DE, Caffee MW, Clarke RJ. 2003. Lower Pliocene hominid remains from Sterkfontein. Science 300:607-612.