Caley Orr (Personal page, Arizona State University) has an advance paper in AJPA examining convergent features in the wrists of knuckle-walking hominoids and the terrestrial giant anteater (Myrmecophaga tridactyla). Did you know that some anteaters were knuckle walkers? I certainly didn't, until I read this!
The background for this paper is the recent finding of certain features in the wrists of early hominids, specifically the distal radius of Australopithecus afarensis and Australopithecus anamensis, that appears similar to those found in chimpanzees and gorillas (Richmond et al. 2001). It has been argued that these features are primitive retentions from a knuckle-walking ancestor, a hypothesis viewed as consistent with the idea that all of the African apes descend from a single agent knuckle-walking species. The current paper gives a good synopsis of this evolutionary problem, including the alternative hypothesis that knuckle-walking evolved in parallel in the chimpanzee and gorilla lineages, so that the hominids did not descend from an age in knuckle-walker.
So why study anteaters? The idea is that different phylogenetics lineage that share a behavior ought to share convergent features to support that behavior. African apes have long fingers for suspensory climbing, which are talked out of the way for quadrupedal movement, hence walking on the knuckles. Giant anteaters have long claws for digging into insect colonies, and these long claws are talked out of the way for a quadrupedal walking. Thus the purpose of this study was:
1. To determine if the locomotion and hand postures of the giant anteater are appropriately analogous to the knuckle-walking of African apes;
2. To identify features of the Myrmecophaga hand and wrist that converge functionally with Pan and Gorilla, and that distinguish these taxa from their non knuckle-walking out groups and terrestrially digitigrade primates; and
3. Through the above analyses, to help determine the traits most likely to be adaptive to knuckle-walking, thereby suggesting which features of the early hominin and modern human wrists might be reliably indicative of a knuckle-walking ancestry (Orr 2005:3).
Did it work? The study did find some features of giant anteaters that made their wrists similar to those of chimpanzees and gorillas. But does this provide support for the idea that early hominids were descended from knuckle-walkers? Orr saves a critical piece of logic for the discussion:
A convergence study can only provide positive or equivocal evidence in testing hypotheses of adaptation for purported knuckle-walking features of the African Hominidae (or any other such study). That is, because all taxonomic groups have unique evolutionary histories, a convergence test cannot truly falsify a hypothesis of an adaptation for a particular lineage. However, convergence study can provide positive support for the hypothesis that structure X is an adaptation to function F, if X distinguishes F- performing taxa from their respective non-F-performing outgroups (Orr 2005:18, emphasis in original).
OK, so we should be wary. Comparing australopithecines to anteaters cannot falsify the hypothesis that hominids had knuckle-walking ancestors. In other words, it is not testing any hypothesis of human origins, although it may provide evidence consistent with one or more of them. Here's the summary of anteater-hominid resemblances:
Morphological features that appear in the hominin lineage shared by Myrmecophaga, Pan, and Gorilla, to the exclusion of their respective outgroups and digitigrade primates, are supported as adaptations to knuckle-walking and provide strong inference of a knuckle-walking last common ancestor (LCA) of Gorilla, Pan, and hominins. Only one such feature (proximal expansion of the non articular surface of the dorsal capitate) appears in the hominin lineage. Human capitates show the African apes state of proximal expansion, and the A. afarensis capitate (AL 333-40) shares the morphology of Gorilla, Pan, and Homo (Orr 2005:19).
Orr discusses the distal radius articular ridge that features in the arguments of Richmond and colleagues (2001), but does not designate this trait as one that necessarily reflects knuckle-walking as opposed to other kinds of vertical hand posture during locomotion, as found in cercopithecid monkeys.
If you're interested in the origins of knuckle-walking, and the question of whether early hominids were knuckle-walkers, this article is for you. As for myself, I think the issue is more likely to be settled with more fossil evidence of Miocene apes and their locomotor styles, rather than the examination of other mammalian lineages. It remains a mystery to me why early hominids should retain features useful only for a knuckle-walking, when their knuckles clearly could not have reached the ground. It seems more likely to me that there is some other function for which these characters might be adaptive related to early hominid locomotion, such as climbing, or other activities. Phylogenetic inertia is never very convincing, especially for early hominids, says they altered almost every other interface between their body and the environment in the pursuit of more perfect bipedal locomotion.
References:
Orr CM. 2005. Knuckle-walking anteater: a convergence test of adaptation for purported knuckle-walking features of African Hominidae. Am J Phys Anthropol (advance before print).
Richmond BG, Begun DR, Strait DS. 2001. Origin of human bipedalism: The knuckle-walking hypothesis revisited. Yrbk Phys Anthropol 44:70-105.
Pickering and colleagues (2004) examine the fauna from Sterkfontein Member 2, coming to the following conclusion:
In summary, the mammalian fauna from Member 2 indicates a paleohabitat that was probably typified by rolling, rock-littered and brush- and scrub-covered hills (suitable for caracals and Makapania, and also commonly exploited by papionins). The valley bottom might have retained standing water year-round, and perhaps supported a tree line or restricted riverine forest, fringed by open woodland or grassland -- a setting appropriate for Alcelaphini, the abundant monkeys, and ambush predators, such as leopards (292).
The authors find this paleoenvironmental reconstruction to be basically similar to other contemporary hominid sites, such as Kanapoi (Wynn 2000), as well as the fauna from the Jacovec cavern. All of these contrast with earlier hominid sites, which were predominantly closed woodlands (WoldeGabriel et al. 2001; Pickford and Senut 2001). Indeed, WoldeGabriel and colleagues (2001:177) conclude that:
The demonstration that the earliest hominids consistently derive from strata bearing indicators of wooded environments may explain their rarity at some sites. It therefore seems increasingly likely that early hominids did not frequent open habitats until after 4.4 Myr. Before that, they may have been confined to woodland and forest habitats.
The final conclusion of Pickering and colleagues (2004) is about the relative abundance of hominid fossils, which are much rarer in the overall composition of the fauna than at sites like Kanapoi. They consider that this relative absence of hominids may either result from a relative scarcity of hominids in the environment, or instead from taphonomic biases that may have led hominids to be underrepresented in Member 2 in particular. They point out that the Member 2 hominids are relatively unaffected by carnivores, with an absence of toothmarks or other indicators of predation. This contrasts with the hominid fossils from the open-air sites in East Africa, where marks from carnivores and other predators are common In their view, the hominids mainly got into the deposit by walking in and dying. This is not as common as carnivores carrying in prey to eat it, but both recent papionins and hominids are known to enter caves -- in the case of the baboons, apparently because caves are cool places to escape the sun. They do not evaluate whether predation may have been higher in Member 4, but are apparently open to the possibility that differences in the taphonomy are mainly consequences of differences in hominid behavior.
Barrett L, Gaynor D, Rendall D, Mitchell D, Henzi SP. 2004. Habitual cave use and thermoregulation in chacma baboons (Papio hamadryas ursinus). J Hum Evol 46:215-222.
Leakey MG, Feibel CS, McDougall I, Ward C, Walker A. 1998. New specimens and confirmation of an early age for Australopithecus anamensis. Nature 393:62-66.
Pickering TR, Clarke RJ, Heaton JL. 2004. The context of Stw 573, an early hominid skull and skeleton from Sterkfontein Member 2: Taphonomy and paleoenvironment. J Hum Evol 46:279-297.
Pickford M, Senut B. 2001. The geological and faunal context of Late Miocene hominid remains from Lukeino, Kenya. C R Acad Sci Paris Sciences de la Terre et des planetes 332:145-152.
Ward CV, Leakey MG, Walker A. 2001. Morphology of Australopithecus anamensis from Kanapoi and Allia Bay, Kenya. J Hum Evol 41:255-368.
WoldeGabriel G, Haile-Selassie Y, Renne PR, Hart WK, Ambrose SH, Asfaw B, Heisken G, White TD. 2001. Geology and paleontology of the late Miocene Middle Awash Valley, Afar Rift, Ethiopia. Nature 412:175-178.
Wynn JG. 2000. Paleosols, stable carbon isotopes, and paleoenvironmental interpretation of Kanapoi, northern Kenya. J Hum Evol 39:411-432.
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.
Paleoanthropologist Sileshi Semaw has a new paper in Nature describing fossils of Ardipithecus from the Gona research area in Ethiopia. Semaw is a researcher at the CRAFT Stone Age Institute at the University of Indiana. (I would just like to enter a note here that the web pages of the CRAFT Institute and the Department of Anthropology at Indiana are extraordinarily difficult to find and use. Can you imagine a university department with a Flash-only website? Compared to this, the CRAFT page is easy to use, but hard to find. The most important problem is that the word "CRAFT" appears nowhere on the site or its address (www.stoneageinstitute.org), and so searches don't find it! Maybe it will move up the Google rankings soon....)
The fossils come from a locality called As Duma, and likely date to between 4.3 and 4.5 million years ago, based on stratigraphic and paleomagnetic considerations. The remains are mostly dental, with three substantially preserved dentitions and several isolcated teeth. There is also a fragment of mandibular corpus and a mandibular ramus matched with one of the mandibular series. Otherwise, there are three hand bones (phalanges) and one toe bone.
Is there evidence for bipedality?
As is the case for other remains from Ardipithecus, here it is argued that the toe bone represents a biped, because the proximal articular facet is oriented upward (dorsally). This means that the toes were characteristically dorsiflexed rather than plantarflexed, and therefore were used for walking but not substantially for gripping objects.
Why are the fossils assigned to Ardipithecus?
The teeth resemble known dental remains of Ardipithecus. General similarities include the size of the postcanine teeth, the height of the upper and lower canines, and the diamond-like shape of the upper canine in labial view (the broadest mesiodistal point is about halfway between the apex and base of the crown). These features are not entirely compelling because of the notable variability among early australopithecines, including A. anamensis and A. afarensis. According to Semaw and colleagues,
The P4 crown is mesiodistally compressed with an oval plan, a simple cusp pattern and a single root matching the Aramis (Ethiopia) specimens of Ardipithecus ramidus and distinct from the triangular/quadrangular cervical cross-section of Australopithecus multi-rooted P4.
In other words, the fossils "fit" well with Ardipithecus by virtue of their anatomy and date, and have at least one tooth crown that is fairly distinctive.
Is there any other Ardipithecus insight?
The evolving story of Ardipithecus is that it is a likely ancestor of later hominids. Contrast this with the situation in 1995, shortly after the initial announcement of both Ardipithecus ramidus and Australopithecus anamensis. These two hominid species overlapped in time, but had clear anatomical differences. Although Ardipithecus had an earlier range of dates (4.1 million to 4.4 million years ago according to White et al. 1994), it was viewed to be morphologically distinct from all later hominids, in particular having molars that were described as smaller, with thinner enamel and more chimpanzee-like morphology than any known australopithecine.
But being described as "chimpanzee-like" has obfuscated much about the anatomy of Ardipithecus. Consider the dental features of A. afarensis compared to living apes and humans: it has thick molar enamel, large molars, molars that increase in size posteriorly, canines that are variable in height, ranging on the small end to incisor-sized, and on the large end still much shorter than females of living ape species. Australopithecines evolved from apes with large projecting canines. It is unclear whether the Miocene apes that were ancestral to later hominids had thick or thin enamel, or large or small molars, because these traits are variable among Miocene hominoids. But they certainly had smaller molars and premolars than the australopithecines, and could not have matched their enamel thickness. Therefore, it is exceedingly likely that the early ancestors of the australopithecines had smaller molar sizes, thinner molar enamel, larger canines, and cutting P3's. Compared to A. afarensis these ancestors would be chimpanzee-like in these details.
Is this what the initial description of the Aramis hominids described? Let's consider what White and colleagues (1994) wrote:
Morphology of the known Aramis canines, however, diverges from that of known apes. The upper canines are slightly less incisiform than homologues of A. afarensis but more incisiform than any ape counter part, with occlusally placed terminations of the mesial and distal apical crests. The visual result of apically placed crown shoulders is a low, blunt canine tooth relative to more projecting ape canines, a morphological condition which may have important evolutionary implications. The Aramis upper canine is large buccolingually, formaing a further contrast with mesiodistally elongate African ape canines. Wear pattern also differs significantly from the ape condition. Mandibular canine wear does not show the pattern typical of great apes. (p. 308)
The broken canines and lower P3 in ARA-VP-1/128 and -6/1 exhibit thin enamel distinct from previously known hominid conditions. Canine enamel thickness approximates the chimpanzee condition, with a lack of apical thickening we observe in other hominids. ... The relatively thin enamel and large size of the Aramis canine, together with its primitive P3 morphology, suggest a C/P3 complex morphologically and functionally only slightly removed from the presumed ancestral ape condition. (p. 308)
The ARA-VP-6/1 P3 is markedly more apelike than any A. afarensis homologue in its high protoconid with extensive buccal face and steep, distolingually directed transverse crest. In these features it is indistinguishable from ape homologues. (p. 308)
Molar morphology resembles the A. afarensis condition, but lacks the extreme buccolingual breadth relative to mesiodistal length common in that species. The 'serrate' root pattern and deep dentine wear on the buccal cusps described in A. afarensis, Tabarin, and Lothagam also occur in Aramis specimens. All molars lack the extensive crenulation and broad occlusal foveae characteristic of modern chimpanzees, or the high cusp topography of gorillas. (p. 309)
On these dental characters, the Aramis hominids do correspond to what we might expect of an ancestor of later hominids. The "chimpanzee-like" character is canine enamel thickness. Other characters are either near or within the range of later hominids, or more apelike. In this context, apelike means that these characters are not specifically similar to any living ape, and indeed the living apes are substantially different from the Aramis remains in several characters. In particular, the chimpanzee-like dm1 is not specifically like chimpanzees, but instead is in the range of all living and fossil apes. This is a plesiomorphy, not a derived similarity with chimpanzees.
Aramis enamel thickness is similar to chimpanzees not only in the canines but also in the molars. But enamel thickness itself is a problematic character. White and colleagues (1994) say:
A distinct difference from known hominids occurs in molar enamel thickness. Maximum radial enamel thickness of crown faces can be measured in three fractured Aramis specimens and it ranges from 1.1 to 1.2 mm buccally, at or near the unworn cusp apex, perpendicular to the enamel-dentine junction. These values are comparable to the uppermost range of our homologous enamel thickness values measured on broken P. troglodytes molars (n=22; M1 through to M3). Equivalent measures in A. afarensis range from 1.4 to 2.0 mm (n=5). (p. 309)
Of course, this places the Aramis specimens intermediate between chimpanzees and other early hominids. White and colleagues (1994) did not report the range of the chimpanzee comparative sample, so it is hard to say what the import of the enamel thickness of Aramis really is. Haile-Selassie (2001, 180) writes:
Another candidate for hominid ancestry is the recently described Orrorin tugenensis. The authors report thick molar enamel and suggest that Ardipithecus and African apes are commonly derived in having 'thin' enamel. However, enamel thickness is a complex character and intraspecifically variable, and its within-tooth three-dimensional patterning is characteristically expressed both serially and taxonomically. Therefore the simplistic dichotomous characterization of enamel as either 'thick' or 'thin' on the basis of unspecified measurements of naturally broken sections (as was done in the Orrorin report) is problematic.
Of course we can compare this quote with the report of Senut and colleagues (2001, p. 4), "Enamel thickness at the apex on the paraconid [of the BAR 1000'00 left M2] is 3.1 mm. This is comparable to other hominids, Ardipithecus excluded." There certainly does seem to be a difference between the Middle Awash sample and Lukeino in enamel thickness, based on the limited number of examples. It is a shame that the range of variation in living hominoids has not been better reported in the context of these early hominids. But even if everything rises or falls based on enamel thickness alone, we have one sample (Middle Awash) with an intermediate morphology between australopithecines and chimpanzees, and another sample (Lukeino) with an australopithecine-like morphology, which is probably also more similar to earlier Miocene apes like Ouranopithecus.
White and colleagues have clearly moved toward an interpretation of the early hominids in which the taxonomic diversity was minimal. For example, Haile-Selassie, Suwa and White (2004, p. 1505) wrote:
Metric and morphological variation within available small samples of late Miocene teeth attributed to A. kadabba, O. tugenensis, and S. tchadensis is no greater in degree than that seen within extant ape genera. Despite claims of molar enamel thickness differences among these late Miocene fossils, we question the interpretation that these taxa represent three separate genera or even lineages. Given the limited data currently available, it is possible that all of these remains represent specific or subspecific variation within a single genus.
One may point out that if the Asa Koma and Lukeino hominids actually do represent a single species, they are properly referred to Orrorin (or Ardipithecus) tugenensis, rather than A. kadabba. One may speculate that it is for this reason that Haile-Selassie, Suwa and White (2004) focus on the genus level in their argument, since it is at this level that Ardipithecus has priority. It is worth mentioning that the enamel thickness in the A. anamensis molars is more similar to later australopithecines than to Ardipithecus, with measurements around 1.9 mm on the protocone of KNM-ER 30748 (Ward et al. 2001). Again, the measurements here emphasize the problems with comparison of such values, since measurements on fossils are taken where breaks naturally occur, and with the possibility of attrition on the teeth.
At any rate, this argument continues with the discovery of the Gona Ardipithecus remains. According to the BBC, Tim White
agreed it was becoming apparent A. ramidus was an important species that was a very plausible ancestor to later hominids. "It's already clear that we're seeing the basic grade from which Australopithecus evolved,"he told the BBC News website.
One feature of these hominids points toward that conclusion--even at the expense of A. anamensis. The reconstruction of the GWM-3/P1 mandible has a shape similar to AL 288-1 (Lucy) with diverging (V-shaped) tooth rows. Semaw and colleagues contrast this morphology with A. anamensis, where the tooth rows are oriented parallel to each other.
Is it possible that the Kanapoi and Allia Bay hominids were a side-branch of an Ardipithecus--A. afarensis lineage? Sure, there is actually little to preclude this scenario beyond the relatively thicker enamel and larger molars of the sample assigned to A. anamensis. It would be helpful to have a full characterization of the variability of these samples as they existed over time, to judge the magnitude of change necessary under different evolutionary scenarios.
But there is one more point on which White and colleagues (1994) described Aramis as chimpanzee-like: the basicranium.
The ARA-VP-1/125 and -1/500 specimens represent adult temporal and occipital regions. Both are smaller than their A. afarensis counterparts, but no female temporal is known for that species. The Aramis cranial fossils evince a strikingly chimpanzee-like morphology that includes marked pneumatization of the temporal squama which even invades the root of the zygoma. (p. 310)
And the postcranial remains are described as displaying "a mosaic of characters usually attributed to hominids and/or great apes" (p. 311). In fact, the known postcrania are an area of resemblance between Ardipithecus and A. anamensis, since both these samples appear to have relatively long forelimb bones compared to later hominids (this comparison is imperfect since body size is not known from other evidence). And the basicranium of A. anamensis was probably similar to Aramis, considering that the KNM-KP 29281 temporal appears to have been pneumatized in the same areas as its Aramis counterpart where it is preserved (Ward et al. 2001). Much appears to have been similar between these two samples, just as much is similar among the earlier hominid samples.
Is this a single lineage changing over time? That would be an appropriate null hypothesis to pursue, and there is little evidence tending to refute it now.
References
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. Full text at Nature
Early hominid teeth changed substantially over time. A number of fossil apes of the Middle and Late Miocene had a dental pattern featuring low-cusped, grinding molars with relatively thick enamel. In females of some species such as Ouranopithecus, Kenyapithecus wickeri, and Gigantopithecus, the canine teeth were small in size compared to living apes like chimpanzees. Living chimpanzees, bonobos, and gorillas differ from the pattern of these fossil apes, as they all share molar teeth with relatively thin enamel and high crowns, and large canines that project well beyond the incisors and premolars even in females. These substantial differences between living African apes and fossil Miocene apes make it unclear which pattern may be the ancestral condition for early hominids. But this pattern of diversity does suggest that the dental characteristics of hominoids tend to evolve readily in response to dietary changes.
By the time of their earliest known fossil representatives, hominids had established their own, unique dental adaptation. This pattern is present at the earliest clear hominid site, Lukeino (Senut et al. 2001), as well as at a number of Middle Awash localities including Asa Koma, dating to between 5.2 and 5.8 million years (Haile-Selassie 2001, Haile-Selassie et al. 2004) and in isolated mandibles from Lothagam and Tabarin, both dating to between 5 and 6 million years ago. The pattern includes molars that are similar in size and morphology to the teeth of late Miocene apes like Ouranopithecus. There has been some suggestion that these teeth may have varied in enamel thickness (Senut et al. 2001), but systematic comparisons have yet to be performed. The main distinguishing feature of early hominids is a reduction in the size and projection of the canine teeth, in both sexes. Although these canine teeth were reduced in size compared to apes, they still projected beyond the crowns of the neighboring teeth and interlocked with each other (Haile-Selassie et al. 2004). Ape upper canines, like those in living chimpanzees and fossil Ouranopithecus, have a sharpened edge resulting from wear against the lower P3. This pattern of wear is called honing. The earliest hominid canines are not only smaller in size, but tend to lack this kind of honing wear. Some of the canines were worn not on their back edge but instead on their tips, showing that they functioned more like incisors than like ape canines. This pattern of canine size and wear is also found at Toros-Menalla, and is the major piece of evidence that Sahelanthropus may be a hominid. The last fossils with dental characteristics similar to the earliest hominids come from Aramis, also from the Middle Awash region dating to between 4.5 and 4.3 million years ago (White et al. 1994, WoldeGabriel et al. 1994).
After the Aramis hominids there appears to have been a fairly strong change in the hominid dentition. The fossil samples from Kanapoi and Allia Bay, at the southern end of Lake Turkana, are slightly more recent than the Aramis hominids at between 4.1 and 3.8 million years ago. The important changes are in the molar teeth and the size and robusticity of the mandible. Compared to earlier hominids, the molar teeth are larger and have thicker enamel (Ward et al. 2000). The mandible, represented by KNM-KP 29281, is tall--well over twice the height of the molar roots inside the mandible--and like later hominids has a strong reinforcing bar behind its symphysis. However, unlike later hominids, the molar tooth rows are long and parallel, giving the mandibular and maxillary dentitions a very U-shaped occlusal configuration. The canine teeth are similar to those of earlier hominids in size and projection. Like earlier hominids, these canines did not have strong honing wear, but the adaptation to cutting against the lower third premolar was not entirely gone, as evidenced by the single-cusped P3 in the KNM-KP 29281 mandible (Ward et al. 2000).
The teeth from Laetoli, Maka, and Hadar appear to form a single series of continuous morphology spanning from 3.7 million to slightly less than 3 million years ago. The basic elements of the dental morphology of these hominids make up the core adaptation of one of the most successful and long-lasting hominid lineages. Over a dozen well-preserved mandibular pieces are preserved, including complete or near-complete mandibles from each of these three sites (White 1977, White et al. 2000, Kimbel et al. 1982). These mandibles are large and thick. They have a distinct buttress along the posterior side of the mandibular symphysis--at the center of the mandible--which is clearly visible in several of the mandibles that are broken at the midline.
The canine teeth are reduced in this sample compared to earlier hominids. There are still large single canines--especially at the earlier sites of Laetoli and Maka--but these increasingly exhibit wear on the tip and project less beyond the other teeth than in earlier remains. In this sample there is rarely a gap, or diastema, between the canine and the incisors (White et al. 2000), and the canine often takes on an incisor-like function. Most other anthropoids have large canine teeth, and these teeth are often strongly sexually dimorphic. They are apparently sexually dimorphic in these early hominids as well, with strong differences in canine size between the larger and smaller mandibles. The large canines of most primates are not principally a dietary adaptation, but reflect the social aspects of directly fighting or communicating threats. The reduction of the canine teeth in early hominids likely indicates that these social interactions had changed.
One possibility is that social competition, particularly among males, may have reduced in intensity. Such a reduction in male competition is consistent with models of the evolution of bipedalism that involve greater parental investment and provisioning of offspring. On the other hand, competition may have remained strong but may have taken a form for which large canines were useless. For example, the development of weapons such as clubs or accurately thrown rocks would reduce the advantages of large canines. Likewise, the development of more effective vocal communication might reduce the impact of visual signals like the canine teeth. Amid these possibilities, the reasons for smaller canines in hominids remain uncertain, but are clearly linked to the evolution of other features such as bipedality and social complexity.
The most distinctive dental feature of these early hominids is the large size of their molar teeth. The earliest hominids had larger molars than chimpanzees or most Miocene apes. The molars of the Hadar hominids average nearly twice the occlusal area of the earliest hominid teeth. Unlike living humans or chimpanzees, these molars increase in size from the front of the mouth to the back, so that the entire tooth row is elongated. And their large size combined with the smaller size of the canines lead the tooth rows to have a more parabolic shape, diverging from each other further back in the mouth.
The premolars are large as well. The third mandibular premolars are sexually dimorphic. Males lack any trace of honing morphology in the P3, with the tooth more similar in orientation to the P4 and having two distinct cusps. Female specimens tend to have a single-cusp P3 that has a higher angle of rotation from the tooth row. Especially the fourth premolars are larger and more molar-like in function than in earlier hominids. In this way, the area of the postcanine teeth has been increased both by increasing the size of each tooth and by changing the function and form of the premolars.
With low cusps and thick enamel, the large postcanine teeth clearly are used for grinding. These teeth and the powerful jaws that contain them reflect a dietary concentration on lower-energy plant materials, at least during part of the year. The postcanine teeth of the Hadar hominids are perhaps three times as large relative to their body size than most humans, and over twice as large as in chimpanzees. Chimpanzees and humans both eat rather high-energy foods, such as fruits and meat. The large molars of early hominids indicate that such foods were probably eaten more rarely or were unavailable for large parts of the year.
Finally, the incisors are relatively large, possibly with a role in stripping plant material as in living apes.
Two samples from between 3.4 and 3.5 million years ago deviate from the pattern established by the Laetoli--Maka--Hadar sequence. One, from Bahr el Ghazal in central Chad, is not well dated but is likely around 3.5 million years old. The fossil is a partial mandible, preserving the front of the mandible anterior to the first molars, and including canines and premolars on both sides. Unlike other early hominid premolars, which typically have one or two roots, both the P3 and P4 of this specimen have three roots. This unusual feature, as well as the relatively vertical symphysis and relatively thin premolar enamel make this central African specimen stand out somewhat compared to contemporary fossils (Brunet et al. 1995). The other sample is the dental sample from Lomekwi. The teeth from this site, including those in the KNM-WT 40000 skull, have similar morphology and enamel thickness to teeth from other sites, but the sizes of the teeth are at or below the minimum size observed at Hadar or Laetoli (Leakey et al. 2001). Both of these sites have been suggested to represent separate species from the Laetoli--Maka--Hadar sequence as discussed below.
References:
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The sample from Kanapoi, along with other smaller samples predating Laetoli, at 3.7 million years, exhibit several differences from the later A. afarensis sites. Since jaws and teeth are best known for these early hominids, these dental features have been used extensively to define both samples and species. The major differences involve morphology of the mandible, including:
long mandibular symphysis, angled posteriorly
long mandibular surface behind the incisors, called a postincisive plane
long parallel tooth rows
There are also several dental differences, including:
more projecting canines
more asymmetric P3
small P4
Postcranially, the Kanapoi sample and other samples from the same time exhibit no specific morphological differences from the much later Hadar hominids. However, the three postcranial fragments from which body mass can be estimated, including a tibia, a distal humerus, and a radius from Sibilot Hill, all result in a body mass estimate larger than the mean size for male remains at Hadar. These findings may imply a larger body size for these earlier hominids, or they may merely represent the chance discovery of three large specimens from these early sites (Ward et al., 2001).
The dental and mandibular differences in the earlier sample support to some extent the hypothesis that the earlier sites represent a distinct species. This species was named Australopithecus anamensis by Leakey and colleagues, where ``anam'' means ``lake'' in the local Turkana language. Despite the differences, however, these samples represent a hominid that is clearly similar in dental adaptation to the later A. afarensis sample, including:
large molars with thick enamel
tall mandible, over twice the height of the molar roots
relatively small canines compared to Miocene apes, with no diastema
Most dental differences between the Kanapoi dental sample and later samples are differences of degree, with the Kanapoi hominids exhibiting a form that is clearly like later hominids, but slightly in the direction of Miocene apes, or in some cases in the direction of the Aramis sample. The overall appearance of the Kanapoi dental sample is that it is a primitive form of the later Laetoli, Maka, and Hadar samples. No significant postcranial differences---other than possibly body size---separate the samples. It is therefore reasonable to support the hypothesis that the major sites of Kanapoi, Laetoli, Maka, and Hadar represent a single evolving lineage, developing an adaptation to greater chewing over time. This hypothesis has been supported by analysis of temporal change among the A. afarensis samples (Lockwood et al., 1999), and would imply that the hominids from around 4.0 million years ago in fact represent an early segment of the evolutionary species A. afarensis. Naturally, if it were shown that a significant speciation had taken place between the early and later samples in this lineage, perhaps involving the appearance of hominids in South Africa or Central Africa, then the species name A. anamensis would be justified for Kanapoi and other contemporary samples.
Just outside Johannesburg, the Malapa site is producing some of the most exciting finds in human evolution. This site is the headquarters of the Malapa Soft Tissue Project.
