The paleoanthropological cave site of Swartkrans is located in Gauteng, South Africa. It is located about 32 kilometers (20 miles) west of Johannesburg, on the Blaauwbank River in the Cradle of Humankind, a UNESCO World Heritage Site that has a long record of some of the oldest hominin remains. It is 1 kilometer (0.62 miles) from Sterkfontein, a site that has produced similar discoveries from the same time period [1][4][5][6].

Swartkrans is a limestone cave that likely opened to the ground surface during the early Pleistocene. It then began to admit materials of the Swartkrans Formation. The formation is made up of five sequential sedimentary members that are separated by erosional discontinuities [7][8].

Member 1 is the oldest of three Early Pleistocene deposits currently recognized in the Swartkrans Formation. It is divided into three subunits: two distinct masses called the Lower Bank (LB) and the Hanging Remnant (HR), as well as the Lower Bank East Extension (LBEE) [7][8].

The LB was discovered and named in the 1970s following continued site clearing and excavations that exposed breccias more fully [9]. Thousands of macro-mammalian fossils, including many P. robustus and early Homo [10][11], as well as stone tools [12] can be found in the LB [13]. Subsequently, the SPRP identified the LBEE as an eastern extension of the LB. LBEE produced taxonomically and taphonomically consistent lithics and fauna with the rest of the LB [14][8][3].

Previous attempts to improve the faunal age estimate of the earliest deposit, Member 1, had produced results with uranium-lead dating (U-Pb) on flowstones and cosmogenic burial dating of quartz, placing the entire member in the range of >1.7/1.8 Ma and <2.3 Ma. In 2014, two simple burial dates for the Lower Bank, Member 1’s earliest unit, narrowed its age to between ca. 1.8 Ma and 2.2 Ma. A new dating program using the isochron method for burial dating has established an absolute age of 2.22 ± 0.09 Ma for a large portion of the Lower Bank, which can now be identified as containing the earliest Oldowan stone tools and fossils of Paranthropus robustus in South Africa. This date is within one sigma of the previous U-Pb age of 2.25±0.08 taken from the flowstone underlying the Lower Bank [15][13][3].

Most of the original fossils discovered by Broom and Robinson between 1948 and 1953 came from the HR [7]. Several faunal estimates place the HR’s age between 1.5 Ma and 1.8 Ma. However, recent electron spin resonance (ESR) dating has estimated the HR’s age to be at 1.6 Ma [16][17].

Member 2 “brown breccia” is distinguished by the absence of stony inclusions larger than gravel. Member 2 occasionally contains fallen blocks of calcified “pink breccia” from the HR. Member 2 also contains Homo ergaster and Paranthropus fossils, as well as stone and bone artefacts [17][1][13][8].

Member 3 is made up of heavily calcified sediments grading to lightly calcified in the upper-most exposed levels. The deposit contains predominantly clasts of angular to sub-angular rock, including dolomite. Deeper portions of the gully contain a few large to very large clasts of dolomite, some of which may be spall from the original cave roof [8].

Member 3 is particularly significant because of its behaviorally informative archaeological record of Acheulean stone artefacts, bone digging tools, butchered animal bones, and burned bones. This member produced 15 isolated Paranthropus robustus teeth. There are 13 fossil specimens in the hominin postcranial sample, including ten complete or partial manual phalanges, two partial distal humeri, and a fifth metatarsal (MT5) [18][19]. Member 3 was estimated to be 1.0 Ma by fauna-derived estimates [7][20], but more recent cosmogenic nuclide burial dating analysis refined its age to 0.96±0.09 Ma [8][19].

Members 4 and 5 are made up of more recently admitted sediments that accumulated ≤110,980 [14] and ~12,000-9000 years ago, respectively [7][21][8].

History

Swartkrans cave site was first worked on by Robert Broom and John Robinson from 1948 to 1949, then by Robinson from 1951 to 1953, when many fossils were discovered. Swartkrans is the first site in Africa to provide evidence of both Paranthropus robustus and early Homo coexisting [22][13][3][1].

From 1965 to 1986, Brain excavated the Swartkrans deposits in a systematic manner, focusing on the stratigraphy and taphonomy of its fauna, producing many more fossils and artefacts in the process. He was the first researcher to describe the cyclical nature of the filling, erosion, and refilling of karst caves site in the Bloubank River valley, which contains cemented underground cavern infills [23][24][7][13][3].

Swartkrans was mined for lime from the early to mid-1900s, and this activity obscured in situ cave fills with dumps of cementer cave earth discarded across the site. Brain spend the first seven years of his career at the site, beginning in 1965, processing these dumps for fossils and information on the ex situ breccia. He spent the remainder of his 21 years excavating, interpreting, and reinterpreting the strata. In 2005, the Swartkrans Palaeoanthropology Research Project (SPRP) began a new program of excavations [25][11][26][13][3].

Discoveries

The Swartkrans limestone cave has accumulated fossils through surface openings that occurred sporadically over the course of geologic time. The majority of the early bone deposits appear to be the result of carnivore kills [4]. Hominin fossils include remains from early Hominin species such as Homo ergaster, Paranthropus robustus, and Homo habilis, as well as other artefacts, greatly contributing to our understanding of hominin evolution in South Africa [3].

The hominin remains in the cave are thought to be the victims of big cat predation. Initially, the leopard Panthera pardus was proposed as the most likely predator responsible for primate fossil accumulation. This is in stark contrast to the previously held belief that hominids were dominant predators rather than prey. Other carnivores, such as the Dinofelis sp. an extinct saber-toothed cat, and the hunting hyenids Chasmoporthetes (formerly Euryboas) and Hyaenictis, were proposed as possible candidates [23][24]. Recent evidence from stable carbon isotope ratios extracted from the fossil tooth enamel of a number of large predators, however, suggests that leopards, the saber-tooth Megantereon and the hyaena, Crocuta are the most likely candidates [27][28].

Swartkrans have produced some of the earliest evidence of the controlled use of fire by early humans, dating up to 0.96 ± 0.09 Ma [8][19]. The histology and chemistry of burnt modified bones recovered in Member 3 reveal that they had been heated to a range of temperatures consistent with that occurring in campfires. The presence of these burnt bones combined with their distribution in the cave is one of the earliest direct evidence for use of fire by hominids in the fossil record. Despite similar environmental conditions, no evidence of fire use has been found in Members 1 and 2, implying that the discovery of fire occurred in the interval between Members 2 and 3 [18][1].

The site has also revealed evidence of modified bone tools found in Members 1 to 3. Because both Paranthropus robustus and early Homo inhabited the cave around the same time, these tools could have been made by either [29][1]. The bone tools were made from weathered long bones [29] as well as fresh bones [2]. These tools were thought to have been used to dig up tubers at first. However, subsequent research indicates that they may have been used to harvest termites. Termites, which are high in protein, served as an additional food source for early hominins. The light weight, efficiency, and nature of polish and wear on the Swartkran artefacts made it probable that selected bone tools were carried by hominins due to their portability and used for a variety of tasks [1][2].

The reported incidence of neoplasia or abnormal growth of cells or tissues in extinct hominins is rare, with only a few confirmed cases from the Middle or Later Pleistocene. Furthermore, it is widely assumed that those rare cases were benign. This makes the discovery of SK 7923, a 1.8-1.6 Ma hominin metatarsal highly interesting. This unclassified hominin showed evidence of osteosarcoma, making it the earliest identified case of cancer in an early human ancestor. This demonstrates that tumors unrelated to modern lifestyle and in younger individuals developed even in ancient hominins [17][1].

Sources

Earliest hominin cancer. 1.7-million-year-old osteosarcoma from Swartkrans Cave, South Africa

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