Sterkfontein variability
In a 2002 paper on cranial remains from Sterkfontein, Lockwood and Tobias write the following in a section called "Are there multiple hominin species from Sterkfontein Member 4?":
The Group C specimens (Stw 183 and Stw 255) arguably represent a phenon, as they deviate from the A. africanus sample in the same direction. Each exhibits some derived characters of A. aethiopicus, A. robustus, and/or A. boisei. Moreover, based on dental size and morphology, Stw 252 probably belongs in this group (Clarke, 1988).
Stw 183 is the strongest craniofacial evidence for a second species of Member 4, though it is not by itself definitive (Lockwood, 1997). Stw 183 is an immature individual, and a full interpretation of this specimen relies on a comparative framework of early hominin ontogeny that is at present incomplete. Despite its youth, it possesses characters typically found in A. robustus, such as an incipient maxillary trigon and a rounded lateral portion of the inferior orbital margin (not present in any other specimen from Sterkfontein Member 4).
The temporal bones catalogued at Stw 255 (including Stw 266a) resemble A. africanus in essentially one autapomorphic character: the prominent eustachian process. Otherwise, this individual shows traits corresponding to A. boisei, especially in the relationship of the tympanic to the postglenoid and mastoid processes. On the whole, Stw 255 suggests the appearance of the temporal bone in KNM-WT 17000 of A. aethiopicus. Moreover, Spoor (1993) showed that Stw 255 has a combination of features regarding the orientation of the posterior petrosal surface that may correspond to the external anatomy of KNM-WT 17000: an unflexed cranial base combined with a petrous axis that is relatively coronally oriented in the transverse plane. Fossils catalogued as Stw 255 may also be associated with the various specimens that make up Stw 252, but this is uncertain, as a second individual (Stw 265) of similar preservation was found in close proximity to Stw 252 (Lockwood and Tobias 2002:446-447, citations in original).
They go on to say they do not think that these specimens are sufficient evidence that another species was present, and they note details of a few other fragments that are different from the sample as a whole. They differ from Clarke (e.g. 1988), who would have divided the relatively complete cranial specimens into two samples.
Some other workers have suggested that individual specimens from Sterkfontein Member 4 might represent other species besides A. africanus. Kimbel and Rak (1993) proposed that Sts 19 probably represents Homo, and that the inclusion of the specimen into A. africanus inflates the variation within that species. This proposition was tested by Ahern (1998), who found the Sterkfontein Member 4 specimens to be quite variable without Sts 19 also. In other words, this site preserves a variable sample -- especially considering nonmetric traits observed on individual specimens.
The traits of Stw 183 and Stw 255 may fall in that category of variation, and Lockwood and Tobias (2002) suggest that possibility. I think the specimens are interesting because of whose traits they share. Studying A. africanus not as a branch of a cladogram, but as a real species with possible ancestors and descendants, the occasional presence of characters of earlier and later species is to be expected. The question is whether these characters document an ancestor-descendant relation for A. africanus and the robust taxa, or whether they might be shared by collateral taxa by virtue of common ancestry alone. The half-million years preceding the origins of Homo may have been just as interesting for the study of populations as the last half-million years.
References:
Ahern JCM. 1998. Underestimating intraspecific variation: the problem with excluding Sts 19 from Australopithecus africanus. Am J Phys Anthropol 105:461-480.
Clarke RJ. 1988. A new Australopithecus cranium from Sterkfontein and its bearing on the ancestry of Paranthropus. In (Grine FE, ed) Evolutionary history of the robust australopithecines. Aldine de Gruyter, New York. p. 285-292.
Kimbel WH and Rak Y. 1993. The importance of species taxa in paleoanthropology and an argument for the phylogenetic concept of the species category. In (Kimbel WH and Martin LB, eds) Species, species concepts, and primate evolution. Plenum Press, New York. p. 461-484.
Lockwood CA and Tobias PV. 2005. Morphology and affinities of new hominin cranial remains from Member 4 of the Sterkfontein Formation, Gauteng Province, South Africa. J Hum Evol 42:389-450.
Age of hominids from Sterkfontein
A recent spate of articles has carried on a debate about the age of the Sterkfontein hominids. Sterkfontein is a complicated site, including several distinct caverns and deposition layers, called members. The dating of these layers is a serious problem because of their complex stratigraphy and the lack of volcanics that could be subjected to radiometric dating. Until recently the only insights into the age of the fossils came from uranium-series dating and paleomagnetic analysis of calcite deposits in the caves.
The Sterkfontein deposits are divided into six members, and hominid have been recovered from Member 5, Member 4, and Member 2. Most of the hominid remains assigned to Australopithecus africanus come from Member 4, which was long thought to date to between 2.8 million and 2.6 million years. Before Member 5 was deposited, there was erosion on the top of Member 4, and the two are separated by an unknown period of time. This deposit is generally thought to be less than 2 million years in age, perhaps extending as recently as 1.4 million years (Kuman and Clarke 2000). In recent years, excavations lower in the deposit, including the Jacovec cavern and the Silberberg grotto, have produced hominid fossils attributable to Member 2. These were initially believed to be around 3.5 million years old.
The most important fossils from Member 2 belong to the specimen StW 573. The foot bones of this skeleton were initially found in a dump of breccia outside the cave (Clarke and Tobias 1995). The origin of the bones was traced to Member 2, and they were matched to the broken end of a tibia still in situ in the Silberberg grotto. The skeleton is now known to be largely complete, including a skull and mandible, forelimb and hindlimb elements, and much else. It appears to be considerably more complete than the "Lucy" skeleton from Hadar, AL 288-1, or any other australopithecine, but it is not yet fully excavated from the overlying breccia and flowstone. The idea that this skeleton might date to 3.5 million years was potentially very important. At this date, it would be a contemporary of A. afarensis from Laetoli and Maka (it would be earlier than the Hadar deposits). It is not clear yet whether StW 573 anatomically resembles A. afarensis or is more similar to later South African hominids, but this would certainly be an important question to answer from the respect of early hominid phylogeny.
Making Sterkfontein later
McKee (1996) suggested that Member 2 was likely immediately earlier than Member 4. His argument was that the fauna of Member 2 were all found in Member 4, but several species were absent from Makapansgat Member 3 and 4, which date to between 3.2 and 2.9 million years. He proposed that this could be explained by the chance lack of these species at Makapansgat, but viewed that possibility as less likely than the hypothesis that the species appeared after Makapansgat Member 4, to be found in the later Sterkfontein deposits.
Clarke and Tobias (1996) responded to this argument by noting the long stratigraphy of Member 3 between the Member 2 and 4 sequences, with several flowstones that must have taken a long time to deposit. They note that although Makapansgat does not preserve all the Member 2 fauna, the species that are absent are known from other African sites prior to 3.5 million years, and therefore are not of use in dating the deposits. The exception is one baboon species, Papio izodi, which is known only from Member 4 and Taung, and may therefore be rare enough to be absent from other sites.
Berger and colleagues (2002) argued that the entire Sterkfontein sequence is substantially later than had previously been thought. They base their argument on biostratigraphic and paleomagnetic considerations. They have a number of reasons for this:
- The presence of Equus in the deposit, which is not radiometrically dated in Africa earlier than 2.36 million years ago.
- In addition to Equus, several other taxa are found in Member 4 that do not have secure radiometric dates above 2.5 million years anywhere in Africa.
- A later date for Member 4 would suggest that the sequence of magnetic samples from the site should be displaced earlier by a reversal cycle. This would place the top of Member 2 within the Olduvai subchron, and the StW 573 hominid would then date to between 2.15 and 3.04 million years ago. If this is displaced by another cycle more recently, StW 573 would date to as recently as 1.07 to 1.95 million years.
As far as Equus, Kuman and Clarke (2000) are at pains to show that it actually may not occur in Member 4. According to them, only one equine tooth has been excavated from Member 4 in situ, with the remaining bones taken from fill that may derive from Member 5. They argue that the one tooth is insufficient evidence of the presence of the genus, considering the possibility of erosion from later deposits.
Making Sterkfontein earlier
Partridge and colleagues (2003) dated the Sterkfontein Member 2 deposits by using the radioactive decay of cosmogenic isotopes. These are created when cosmic rays from outer space interact with the elements in quartz grains near the earth's surface. In particular, aluminum-26 and beryllium-10 accumulate in quartz grains at a predictable ratio. These two isotopes have different half-lifes (26Al = 1.02 million years, 10Be = 1.93 million years), which means that once the quartz grain is buried and no longer exposed to cosmic rays, the ratio of the two isotopes changes.
Sediments near the StW 573 specimen gave a date estimate of 4.17 million years, while the orange breccia in the Jacovec Cavern gave an estimate of around 4.02 million years. These date estimates are substantially earlier than were previously estimated for these localities at the site.
It was not possible to date Member 4 in this way, because it is shallow enough that cosmic rays can still affect the quartz grains used for dating.
Partridge et al. (2003) do not present a response to Berger et al. (2002), except to note that their earlier dating "is unsustainable on stratigraphic and faunal as well as on paleomagnetic grounds" (612, note 12). In any event, there seems to be no strong biostratigraphic reason to place Member 2 at either an earlier or later date; the preserved fauna is not specific as to age.
Member 5 stratigraphy
Kuman and Clarke (2000) review the stratigraphy of Member 5. The most important hominid specimen that has been attributed to Member 5 is StW 53, a nearly complete skull that has been variably attributed to A. africanus or Homo habilis. Kuman and Clarke (2000) show that the skull derives from an area that likely is intermediate in age between Members 4 and 5 proper. They call this area the "StW 53 Infill." No artifacts derive from this area. The authors argue that the infill is likely more recent than Member 4 because of the presence in the deposit of Theropithecus oswaldi, a species found in the later Swartkrans Members 1--3, and associated with drier open grassland habitats. On this basis, they place the StW 53 Infill between 2 million years ago and 2.4 million years, which marks the earliest appearance of T. oswaldi in East Africa.
According to Kuman and Clarke (2000), Member 5 can be divided by the presence of two distinct tool industries. The Oldowan Infill dates to between around 2 million and 1.7 million years ago, and preserves 3245 excavated artifacts (Field 1999). The paleoenvironment seems to indicate a grassland. The later phase is referred to the Acheulean because of the presence of bifaces, and is placed between 1.7 and 1.4 million years ago. Like the earlier Oldowan infill, the Acheulean infill represents a predominantly grassland fauna, similar to Swartkrans.
Kuman and Clarke (2000) provide a list of hominid fossils with their probable associations in the stratigraphy. They also discuss the taxonomy of the fossils and their resemblances with elements of the earlier Member 4 and Swartkrans remains.
More on Sterkfontein
More on Makapansgat
References:
Berger LR, Lacruz R, de Ruiter DJ. 2002. Brief communication: Revised age estimates of Australopithecus-bearing deposits at Sterkfontein, South Africa. Am J Phys Anthropol 119:192-197.
Clarke RJ, Tobias PV. 1995. Sterkfontein Member 2 foot bones of the oldest South African hominid. Science 269:521-524.
Clarke RJ, Tobias PV. 1996. Faunal evidence and Sterkfontein Member 2 foot bones of early hominid. Science 271:1301-1302.
Field AS. 1999. An analytic and comparative study of the Earlier Stone Age archaeology of the Sterkfontein Valley. MasterÕs thesis, University of the Witswatersrand.
Kuman K, Clarke RJ. 2000. Stratigraphy, artefact industries and hominid associations for Sterkfontein Member 5. J Hum Evol 38:827-847.
McKee JK. 1996. Faunal evidence and Sterkfontein Member 2 foot bones of early hominid. Science 271:1301.
Partridge TC, Granger DE, Caffee MW, Clarke RJ. 2003. Lower Pliocene hominid remains from Sterkfontein. Science 300:607-612.
Sterkfontein Member 2 paleoenvironment
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.
References:
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.
John Hawks Department of Anthropology
University of Wisconsin—Madison
Copyright © 2007 John Hawks