Technical Comments

Response to Comment on the Paleobiology and Classification of Ardipithecus ramidus

Science  28 May 2010:
Vol. 328, Issue 5982, pp. 1105
DOI: 10.1126/science.1185462

Abstract

We assigned Ardipithecus to the Hominidae based on numerous dental, cranial, and postcranial characters. Sarmiento argues that these characters are not exclusive to hominids, contending that Ardipithecus is too old to be cladistically hominid. His alternative phylogeny, however, is unlikely because it requires tortuous, nonparsimonious evolutionary pathways.

Sarmiento (1) doubts that Ardipithecus ramidus represents a cladistic hominid (phylogenetically on the human side of our divergence with chimpanzees). He argues that biomolecular studies accurately converge on a divergence date of approximately 3 to 5 million years ago, concluding that Ar. ramidus “probably predates the human and African ape divergence.” However, his cited estimates vary widely and all rely on inadequate calibration. Indeed, the strongest calibration is now from hominids themselves: Late Miocene fossils from Chad, Kenya, and Ethiopia whose derived characters effectively falsify late divergence estimates (2).

Initial diagnosis of Ar. ramidus (3) identified features of the C/P3 complex and basicranium shared exclusively with later hominids. These were based on comprehensive comparisons with extant and Miocene ape genera, including Sivapithecus, Kenyapithecus, Ouranopithecus, Lufengpithecus, and Dryopithecus. Included were multiple characters of the C/P3 complex (canines more incisiform, less projecting, with relatively higher crown shoulders; relatively smaller P3, with weaker mesiobuccal projection of the crown base and without functional honing facet, etc.), as well as the anteriorly placed foramen magnum relative to lateral basicranial structures. In the 15 years that followed, the hominid status of Ardipithecus became widely accepted and strengthened when independent numerical cladistic analyses (that also included the more recently established taxa Au. anamensis, Ar. kadabba, Orrorin tugenensis, and Sahelanthropus tchadensis) firmly nested Ar. ramidus within the hominid clade [reviewed in (4), supplementary discussion for (5)].

The greatly expanded Ar. ramidus dental sample now further obviates Sarmiento’s assertions by establishing a metrically and morphologically refined Ar. kadabbaAr. ramidusAu. anamensisAu. afarensis morphocline (57). It now seems clear that not all recovered Ar. ramidus canines can be female (7). Feminization of the male Ardipithecus C/P3 complex is robustly documented [detailed in the supporting online material in (7)] and is incompatible with Sarmiento’s argument that Ar. ramidus represents the stem taxon for both African apes and humans (1). If that were the case, a hominid-like C/P3 complex with lack of honing would need to have evolved in Ar. ramidus, only to have independently reverted to the honing complexes in each African ape clade.

The level of detail involved in the uniquely derived C/P3 complex morphologies shared exclusively by Ar. ramidus and later hominids—and not by any known fossil or extant ape—is notably unambiguous. Its direct association with equally unique character complexes in the postcranium (see discussion below) is also clear. We have always considered Oreopithecus and all other fossil apes when establishing morphocline polarities. The “atomized” list of the C/P3 complex features provided in table 1 of (4) was intended as a simplified guide to the stratophenetic continuity that we see from ~6-million-year-old hominids through Ar. ramidus to Au. afarensis. If Sarmiento wishes to modify that table to generate phylogenetic conclusions different from ours, then he needs to be explicit as to where, why, and how our assessments of these characters were in error.

Sarmiento also dismisses our basicranial and pelvic results, but again without offering alternative anatomical assessments or analyses. We provided extensive relevant information in our main texts, figures, and supporting online material. We showed in (8) that Ar. ramidus shares the derived Australopithecus condition of a short cranial base even relative to the bonobo (which most closely approximates the hominid condition among all known apes, both extant and fossil). Again, as with the C/P3 complex, a complicated and nonparsimonious sequence of cranial transformations would be required for Ar. ramidus to represent a stem taxon of African apes and humans.

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.

Sarmiento states that pelvic and femoral morphologies are “open to interpretation” but offers no alternative interpretations. In the meantime, there is no evidence that characters exhibited by Oreopithecus are phyletically or developmentally shared with the ilia of Ar. ramidus and Australopithecus. 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.

The character distributions we noted in the pelvis, C/P3 complex, and basicranium are consistently indicative of a sister relationship of Ar. ramidus with Australopithecus (and later hominids). For Ar. ramidus to be a stem species of the African ape and human clade as Sarmiento advocates, its highly derived C/P3 complex morphology, basicranial shortening, and iliac structure must have first emerged in some yet-unidentified Miocene ancestor before then reverting to an African ape–like condition. Such multiple, nonparsimonious character reversals are highly unlikely.

References and Notes

  1. The responding authors note that the comment by Sarmiento (1) was directed at multiple papers, each with a different authorship, published in the 2 October 2009 issue of Science. Due to space and time allowances, this smaller set of authors has taken responsibility for the response, while at the same time acknowledging the wider authorship of our original papers under comment here.
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