In the late nineteen thirties, palaeontologist Robert Broom discovered a series of Hominin skeletons in a cave system in Sterkfontain, South Africa. These, he assigned to the genus Australopithecus, which had been described in 1925 by Raymond Dart, suggesting that he had found a second species in the same genus. At the time, these discoveries did not receive a great deal of attention from the international scientific community. The cave deposits at Sterkfontain were estimated by Broom to be Late Pleistocene in age. This made Broom's material much younger than Dart's and contemporary with modern Human remains from many parts of the world, which meant that, while interesting, Broom's new 'Ape-man' could not be ancestral to modern Humans.
More locally, however, the discoveries caused considerable interest, leading many amateur fossil hunters to begin exploring the Sterkfontain cave system. One of these was Helmut Kurt Silberberg, a Johannesburg art dealer and antiquarian with an interest in prehistory. In 1942, Silberberg's explorations led to the discovery of a small cave with numerous bones embedded in the walls. Unfortunately, the rock in which the bones were embedded was extremely hard (more so than the bone) making excavation difficult. Silberberg paid a local man to hammer free a chunk of rock, during which process several bones were broken, displeasing Silberberg. When the bones in the rock were examined, they were found to belong to a Hyena and a Baboon.
Discouraged by these findings, Silberberg lost interest in the work, giving his specimens to Broom, and moving onto other projects. While the fossils were not interesting to Silberberg, the material proved to be much more useful to Broom, who recognised the Hyena as a member of the genus Lycyaena, which was otherwise known from the Early Pliocene of Europe and India. Even if this South African specimen was younger than other fossil Hyenas assigned to the genus, it was unlikely to be Late Pleistocene in age. This caused him to re-evaluate the age of the cave system using Mammal fossils to establish an age sequence, which proved that his Australopithecus fossils were much older, and therefore much more significant, than he had originally realised. Broom named the Hyena fossil Lycyaena silberbergi (a name which is no longer used) and the cave where it was found was eventually named Silberberg Grotto.
In 1966, palaeoanthropologists Philip Tobias and Alun Hughs began a new series of excavations at the Sterkfontein Caves, working methodically through the cave system from west to east, and uncovering several new specimens, albeit mostly fragmentary. In 1978 they revisited Silberberg Grotto, removing large sample of breccia, which was cleaned by technicians at the site to recover a larger amount of fossil material, most of which was moved to the University of the Witwatersrand, while some remained in storage at Sterkfontein.
In 1992 Tobias' team returned to Silberberg Grotto, this time employing two mining engineers, John Cruise and Dusty van Rooyen, to blast away a section of the fossil-rich wall of the cave. Analysis of the material recovered by this process in 1994 by palaeoanthropologist Ronald Clarke revealed something surprising. The rocks had produced numerous bones associated with Carnivores and Monkeys, but for some reason almost no Bovids (usually the most abundant Mammals in South African deposits). This led him to return to the material stored at Witwatersrand, which he also analysed for the proportions of different Mammal groups.
While examining this material, Clarke discovered what appeared to be the talus, navicular, medial cuneiform and proximal half of the first metatarsal of an unknown Hominin. This was surprising for two reasons. Firstly, the material had been examined previously by specialists palaeoanthropologists looking for exactly this sort of material, and secondly because the Silberberg Grotto had by this time been identified as somewhat older than the previous Hominin-producing deposits at Sterkfontein, implying any fossil Hominin found here could be the oldest yet identified specimen from the cave system. In addition to these four bones, Clarke found what appeared to be a badly damaged lateral cuneiform from the right foot, and a fragment of a possible Hominid calcaneum.
This material suggested a small Hominin, capable of standing upright like a Human, but still retaining an disposable big toe like an Ape, which suggested that at least some of its time was still spent within the trees. This specimen was given the official designation StW 573, but quickly became known as 'Little Foot'.
In 1997 Clarke discovered a box labelled 'D18 Cercopithecoids' (Old World Monkeys), which when inspected was found to also contain material labelled 'D20 Cercopithecoids'. Since D20 implied the layer of material exposed at Silberberg Grotto, and Clarke was interested in the fauna of this cave, he took down this box to inspect its contents. Within this material he found a Hominid intermediate cuneiform, which he found fit with the lateral cuneiform associated with Little Foot. Along with this was the left lateral cuneiform, the proximal end of the left second metatarsal and the left distal fibula. A subsequent search of related material uncovered the disto-medial portion of a Hominid tibia. This did not fit exactly with the Little Foot talus, but was close enough that, once the damage to the material was taken into account, Clarke considered it likely to have come from the same individual.
This tibia had a break across its shaft consistent with the sort of damage caused by mine blasting, Clarke next turned to the material stored at Sterkfontein. Here, in a bag of material labelled as Bovid tibiae, he found another Hominid tibia, which this time fit perfectly with the left talus of StW 573, leading him to realise that the previously discovered tibia was in fact from the right leg. Further inspection of this material also produced a second piece of cuneiform, which fit together with the one from the original Little Foot material, giving a complete bone. This had a bowl-like surface on its articular surface with the cuboid bone, indicating that these bones still articulated (could be moved relative to one-another), as in Chimpanzees, and unlike the condition seen in Modern Humans, where the two are immobilised by a keel-and-socket arrangement, which gives more stability when walking and running upright.
This gave Clarke a collection of twelve bones from the feet and lower legs of a single Hominin, including the left tibia and fibula, which joined to an articulated set of eight foot and ankle bones, and the distal fragment of a right tibia and right lateral cuneiform.
Convinced that more material might still be found in Silberberg Grotto, Clarke prepared a cast of the distal fragment of the right tibia of StW 573, which he gave to two perpetrators based at Sterkfontein, Nkwane Molefe and Stephen Motsumi. Molefe and Motsumi then searched the area exposed by blasting within Silberberg Grotto, looking for an exposed cross section of bone which would fit this cast.
Clarke was not optimistic that this approach would work. The various excavations had turned the grotto into a cavern of some size, with exposed breccia on the walls, floor and ceiling, all of which might contain the matching bone fragment - if it was there at all. As if this was not difficult enough, there was neither natural light nor a rigged lighting system within the cave, forcing the two investigators to search using hand-held torches.
After two days Molefe and Motsumi reported finding a bone embedded in a talus slope at the western end of the grotto, which was a perfect match for the cast given to them by Clarke. This was unexpected, as this was not the area where Cruise and van Rooyen had carried out their blasting, but an older area of excavations where blasting had been carried out by commercial miners extracting limestone for use in the construction industry, an operation which had stopped 65 years before. Furthermore, a second section of bone could be seen to the left of this bone, which turned out to be a perfect match for the broken left tibia.
Following this discovery, Clarke, Molefe, and Motsumi began excavating the site using hand tools, taking almost a year to uncover the left tibia shaft and the distal ends with lower shafts of the left and right femurs. After which, however, the material apparently stopped, until Motsumi hammered away a section of travertine (dense limestone deposited by hot springs) higher up the slope exposing more breccia, and the distal and of a Hominid humerus and mandible. The skeleton had apparently been entombed in rock, broken in half by an ancient rockfall, and covered by travertine from an ancient hot spring, before eventually being exposed by limestone miners some time in the early twentieth century.
In the following months, and eventually years, the skull, left forearm, left hand, pelvis, ribs, vertebrae, humerus, and most of the lower limb bones were extracted. The final skeleton was 90% complete, the most complete non-Human Hominin skeleton ever recovered. For comparison, the famous Lucy (or Dink'inesh) skeleton AL 288-1, considered to be the most complete specimen of Australopithecus afarensis, is only 40% complete.
The Sterkfontein cave system, now part of the Maropeng Cradle of Humankind World Heritage Site, are a karstic cave system on the south side of the Bloubank River, about 50 km to the northwest of Johannesburg. Karstic cave systems form as water percolates through limestones, dissolving some of the mineral to create voids. This is a complex process, as cave systems form, are enlarge by the hollowing-out of water erosion, then collapse bringing sediments from above down to lower levels and often redirecting the water flow which caused the hollowing out. Furthermore, the limestone dissolved in one place is typically re-deposited in another, as travertines or flowstones. All of this makes establishing timelines for the karstic deposits extremely complicated.
Sterkfontein Caves began to form about six million years ago, towards the end of the Miocene. Throughout much of the intervening time the local environment has been a mosaic of grasslands, woodlands, and lakes, albeit with warmer, wetter phases, and cooler, dryer phases. At least five different Hominin species have been found within the caves, some of whom appear to have sought shelter within cave systems, others to have been dragged their for consumption by cave-dwelling carnivores, while others appear to have fallen into sinkholes.
Skeleton StW 573, or Little Foot, was found closely associated with a large Baboon (an animal quite capable of predating a small Hominin), but does not show any signs of having been consumed. Rather, it seems to have fallen face-down into a shallow puddle of water within a sinkhole. Possibly the Hominin and Baboon fell together following a struggle, but this is guesswork we will never be able to prove. The chamber where it was found contained Pliocene Hyenas, and was directly below the chamber which produced Bloom's most famous Australopithecus specimen, Mrs. Ples (STS 5), and therefore must be older than that specimen.
In 1978, geomorphologist Timothy Cooper was part of the team led by Tobias and Hughes which re-examined the Silberberg Grotto. He established that the chamber had formed as a cone-shaped void, the walls of which had then been covered by flowstone (dissolved and then re-deposited limestone), before an opening had reached the surface, forming a sinkhole into which surface sediments, and various Animals now preserved as fossils, had fallen, filling in the cone. These sediments were then covered by a second flowstone deposit. The sediments within this cone were thought to be about 2.8 million years old (Late Pliocene) on the basis of the fossil fauna they contained, implying a similar age for Little Foot.
However, Molefe and Motsumi had established that Little Foot had in fact come from below the lower flowstone deposit, with only a small portion displaced into the accessible sediments by an ancient rockfall and the actions of early twentieth century miners. Various attempts have been made to date this flowstone deposit, including cosmogenic uranium-lead series nuclide dating, palaeomagnatism. These studies produced dates between 4 million years and 1.07 million years ago, although most fell into the range between 2.8 and 2.2 million years ago. However, subsequent studies also established that the flowstone itself contained pockets of younger, redeposited material, making all dating of this layer suspect.
Cosmogenic aluminium²⁶ and beryllium¹⁰ dating works because both aluminium²⁶ and beryllium¹⁰ are produced in quartz exposed to cosmic rays (i.e. on the surface) at a steady rate, producing a ration of 6.75 aluminium²⁶ to 1.0 beryllium¹⁰. Once the quartz is buried no more of either radioisotope is formed. Since the half-life of aluminium²⁶ is roughly half that of beryllium¹⁰, over time the ratio between the two radioisotopes will change, enabling the precise measurement of both how long a piece of quartz was exposed on the surface, and how long ago this happened. In April 2015 a study led by Darryl Granger of Purdue University produced a date for the sediments in which StW 573 was directly lying, based upon cosmogenic aluminium²⁶ and beryllium¹⁰, yielding an age of 3.67 million years before the present (equivalent to the dates for early Australopithecus afarensis specimens from East Africa.
This also makes StW 573 the oldest Hominin fossil discovered in South Africa, as well as the most complete pre-Human Hominin fossil ever found. It also predates the oldest stone tools in South Africa by over a million years, and the oldest known stone tools anywhere by over 300 000 years, suggesting that it came from a population which did not yet have this technology.
Studies of Little Foot have suggested that she was female, and stood about 1.20-1.30 m tall, with longer legs than arms (similar to modern Humans, but unlike Chimpanzees). She had an 'S' shaped collarbone, which is a more Human-like trait, and but the shape of her pectoral girdle and shoulder blades, suggest that she was comfortable moving in trees suspended by her arms. The structure of its foot suggests that it was able to walk in an erect manner, similar to modern Humans, but still had an opposable big toe, implying it was still a comfortable climber (Pliocene Hominins are in general thought to have been woodland species, with the preference for open grasslands emerging in the Pleistocene when a cooler, dryer climate led to the retreat of forests and expansion of grasslands).
This fits well with the environment in which her remains were found. While the Silberberg Grotto produced the remains of some terrestrial Animals, such as Antelope and Hyenas, the majority of the fossils are of Animals associated with an arboreal lifestyle, such as small Monkeys and Felids. Such Animals might have climbed down into the sinkhole in order to obtain water and become trapped, or possibly have fallen directly from trees into the hole - in Little Foot's case, possibly while trying to escape from a large Baboon.
Surprisingly, despite the completeness and significance of Little Foot, she has never been assigned to a specific species. She was referred to by Ronald Clarke as possibly a specimen of Australopithecus prometheus. However, this species name is now considered to be invalid; it was used by Raymond Dart to refer to a partial occipital bone (part of the back of the skull) from Makapansgat, but this has now been assessed as a specimen of Australopithecus africanus, a species common in South Africa from about 3.3 to about 2.1 million years ago. However, Little Foot is not generally accepted as belonging to Australopithecus africanus, which it predates as well as differing from in its brain capacity (smaller in Little Foot) and shoulder arrangements (more Ape-like in Australopithecus africanus). Nor does she conform to Australopithecus afarensis, a species whose time range she overlaps, but which is not found in Southern Africa.
Thus Little Foot appears to be an otherwise entirely unknown species of Australopithecus from South Africa, with an arboreal lifestyle and no use of stone tools. This is not completely surprising, as species such as Australopithecus africanus and Australopithecus afarensis are clearly already some way removed from their closest Ape relatives. and species which live in trees seldom have good (if any) fossil records.
See also...
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