Earliest hominids :: thoughts and roundup

Today I lectured on the earliest hominid samples for my graduate course on australopithecines. This is the first time I have been able to give a full lecture on the Late Miocene hominids up to A. anamensis since their discovery. The thing that struck me is that even if you study a set of fossils and literature for research, there are things that don't really strike you until you are standing in front of a full-screen slide projection talking about them.

One of the interesting things was to see the juxtaposition of the Lothagam and Tabarin mandibles with the new Ardipithecus and other early hominid samples. It's worthwhile remembering that not that long a time ago, Lothagam was the earliest hominid, and was argued to fit within the range of variation of A. afarensis (Hill et al. 1992). Tabarin, likewise, was not specially distinguishable from the A. afarensis sample (Ward and Hill 1987). They undoubtedly remain so, but chronologically they make sense as part of the Ardipithecus sample, which raises the anatomical question: are they Ardipithecus or something very like it? The answers are out there, I set the graduate students on the problem, and I have every confidence they will come up with an answer in the next couple of weeks.

Another observation is the very distinctive mandibular anatomy of A. anamensis The well-preserved mandibles all have very long postcanine tooth rows, certainly compared to the relatively narrow breadth of the dentition. Moreover, the mandibular symphysis is very long and slopes posteriorly to a greater extent than in later hominids (all this reviewed in Ward et al. 2001). This has a couple of interesting consequences. The first is that the mandible begins curving medially toward the symphysis relatively distally--around the first molars. This is exactly the morphology that was said to be hominid-like about Ramapithecus, and indeed the curvature itself is similar to later hominids. What is different about A. anamensis is the extent of the anterior dentition. This all tends to say that A. afarensis was substantially more orthognathic than earlier A. anamensis. A question is whether the other early hominids were similar to A. anamensis in this respect.

As yet, none of the earlier hominid mandibles are sufficiently preserved to evaluate the symphyseal morphology or the shape of the anterior dentition. The maxilla of Sahelanthropus is sufficiently preserved in the Toumai specimen, but it badly needs reconstruction to say for sure what its shape is (Brunet et al. 2002). From the basal view, it appears more apelike in shape than in A. afarensis, but the lower maxilla appears rather less prognathic than in AL 444-2 or other A. afarensis remains, raising doubt as to whether the front of the face was really more projecting than in later hominids.

The question left to answer with these missing observations is whether A. anamensis was intermediate between the earliest hominids and A. africanus in toothrow shape and its anterior dentition, or whether it diverges from both these samples. Its configuration appears unique at the moment, and seems to provide a distinctive combination of posterior tooth expansion, mandibular strengthening and buttressing, and the retention of a large anterior dentition.

How much of a dental difference is there between A. anamensis and Ardipithecus? I ask this because one of the most distinctive differences between A. afarensis and Ardipithecus is the dm1, which is very apelike in the Aramis ARA-VP-1/129 mandibular fragment (White et al. 1994). The apelike morphology is a mesiodistally long tooth, without buccolingual expansion, and without the elaboration of occlusal topology such as a marked talonid basin. Early hominids have expanded deciduous molars, and the dm1 in particular is quite molariform (as in the Taung mandible). But the morphology in A. anamensis is close to that described for Ardipithecus. The tooth is best preserved in the KNM-KP 34725 dentition (and is less well preserved but consistent with this form in KNM-KP 31712). The tooth is much longer mesiodistally than buccolingually, and it is narrower than any in the A. afarensis sample (Ward et al. 2001). The only notable difference between this tooth and the Aramis specimen is its larger size, being nearly 2 mm larger in both length and breadth, which makes it more similar in size to an A. afarensis tooth, though not in shape.

What is lacking now is an appreciation of the probable level of variation among the dentitions within a single sample. Here, we face the problem of comparing multiple samples separated by hundreds of thousands of years, without even the possibility of a test of significance between them. For example, the mandibular premolars of A. anamensis are described by Ward and colleagues (2001) as being similar to Ardipithecus in having a unicuspid P3 and a less expanded talonid on the P4 compared to A. afarensis. But the P3 form is highly variable at the single site of Hadar, as the form of the upper canine shows substantial variation in only a few specimens at Laetoli and Hadar. If the earlier hominids are all fairly similar, is this to be interpreted as an important degree of similarity compared to later hominids? Is there any substantial evidence of multiple species here at all? Tough question (Ward et al. 2001 duck it by noting that descriptions of Ardipithecus and Orrorin are lacking).

It is probably necessary to look into the quantification of nonmetric variation within dental samples of early hominids. This will be tough since there are really not enough teeth to create good seriations to examine character variation. But the diagnosis of A. kadabba (Haile-Selassie 2001) is an interesting case study in the delineation of minor morphological details. In this instance, only one specimen of each potential subspecies (now species, and one specimen published, although surely the author saw a broader sample of unpublished remains) were compared with each other. Each difference was tabulated as part of the subspecies diagnosis (without direct consideration of whether the traits might vary in earlier or later species or samples). How likely is it that two fossils within a sample will differ in the presence of a shallow mesial fovea on P3? Or in the diagnosis of the species (Haile-Selassie et al. 2004), how likely is it that two specimens will differ as A. kadabba and O. tugenensis evidently do by a more circular canine shape in occlusal view? This is the important kind of question for testing the validity of the diagnosis, but it is as yet unanswered.

References:

Brunet M, Guy F, Pilbeam D, Mackaye HT, Likius A, Ahounta D, Beauvillain A, Blondel C, Bocherens H, Boisserie JR, De Bonis L, Coppens Y, Dejax J, Denys C, Duringer P, Eisenmann V, Fanone G, Fronty P, Geraads D, Lehmann T, Lihoreau F, Louchart A, Mahamat A, Merceron G, Mouchelin G, Otero O, Campomanes PP, Ponce de Leon M, Rage JC, Sapanet M, Schuster M, Sudre J, Tassy P, Valentin X, Vignaud P, Viriot L, Zazzo A, Zollikofer C. 2002. A new hominid from the Upper Miocene of Chad, Central Africa. Nature 418:145-151.

Haile-Selassie Y. 2001. Late Miocene hominids from the Middle Awash, Ethiopia. Nature 412:178-181.

Haile-Selassie Y, Suwa G, White TD. 2004. Late Miocene teeth from Middle Awash, Ethiopia, and early hominid dental evolution. Science 303:1503-1505.

Hill AH, Ward S, Brown B. 1992. Anatomy and age of the Lothagam mandible. J Hum Evol 22:439-451.

Leakey MG, Feibel CS, MacDougall I, Walker A. 1995. New four-million-year-old hominid species from Kanapoi and Allia Bay, Kenya. Nature 376:565-571.

Semaw S, Simpson SW, Quade J, Renne PR, Butler RF, McIntosh WC, Levin N, Dominguez-Rodrigo M, Rogers MJ. 2005. Early Pliocene hominids from Gona, Ethiopia. Nature 433:301-305.
Review on this site

Senut B, Pickford M, Gommery D, Mein P, Cheboi K, Coppens Y. 2001. First hominid from the Miocene (Lukeino formation, Kenya). C R Acad Sci Paris Sciences de la Terre et des planetes 332:137-144.

Ward CV, Leakey MG, Walker A. 2001. Morphology of Australopithecus anamensis from Kanapoi and Allia Bay, Kenya. J Hum Evol 41:255-368.

Ward SC, Hill A. 1987. Pliocene hominid partial mandible from Tabarin, Baringo, Kenya. Am J Phys Anthropol 72:21-33. PubMed

White T, Suwa G, Asfaw B. 1994. Australopithecus ramidus, a new species of early hominid from Aramis, Ethiopia. Nature 371:306-312.

WoldeGabriel G, White TD, Suwa G, Renne P, deHeinzelin J, Hart WK, Helken G. 1994. Ecological and temporal placement of early Pliocene hominids at Aramis, Ethiopia. Nature 371:330-333.