The Fossils of Sterkfontein Caves
Mrs Ples, Little Foot, and nearly a century of excavation — the story of how Sterkfontein became the richest early-hominin fossil site on Earth.
Sterkfontein's reputation rests on two finds: “Mrs Ples”, lifted from the breccia in 1947, and “Little Foot”, spotted in a box of misfiled bone in 1994, located in the rock in 1997 and not unveiled until 2017. Between them the caves have produced more than 700 hominin specimens across nearly a century of excavation — and a cosmogenic re-dating study published in 2022 moved most of that assemblage roughly a million years further into the past. This guide covers both discoveries, what the new dates changed, how the site was excavated, and how much of the research a visitor can actually see today.
Who discovered Mrs Ples, and why does it matter?
On 18 April 1947 Robert Broom and John T. Robinson lifted a near-complete cranium out of the Sterkfontein breccia. Catalogued as Sts 5 and nicknamed “Mrs Ples”, it was identified as Australopithecus africanus — the species Raymond Dart had named in 1924 from the Taung Child, the discovery that drew Broom to Sterkfontein in the first place. Broom had already recovered the first adult A. africanus fossil here in 1936; Sts 5 was the specimen that made the case unarguable, because its completeness let anatomists read an early hominin skull rather than infer one from fragments. The specimen number is worth holding onto: some material, including one of the operator's own pages, attaches the nickname to Sts 14, which is a partial skeleton. Mrs Ples is Sts 5.
The name has not aged as cleanly as the fossil. Broom read Sts 5 as an adult female on the basis of its small canine sockets, and the nickname stuck. The sex is genuinely unsettled: later researchers have argued the skull belongs to an adolescent male, while a 2012 reassessment defended Broom's original reading of an adult female. The disagreement turns on features that stay ambiguous in a cranium this heavily mineralised — canine root dimensions, facial proportions, the state of dental development — and no consensus has emerged. The specimen came out of Member 4, the deposit the scientific literature describes as the most prolific single source of Australopithecus fossils anywhere, and which runs around ten metres deep at the excavation faces. Member 4 is also where the argument about age begins.
What is Little Foot, and why is it so significant?
Little Foot was found twice. On 6 September 1994 Ronald Clarke was sorting boxes of bone catalogued as animal material from excavations run in the late 1970s when he picked out four small hominid foot bones; Phillip Tobias supplied the nickname. The bones had come from Member 2 breccia in the Silberberg Grotto and had sat unrecognised in storage for close to two decades. On 3 July 1997 Clarke located the rest of the individual still in place in the Silberberg Grotto — not a scatter of bones but a single articulated skeleton locked into breccia as hard as concrete. Freeing and preparing it took a further twenty years or so. The reconstructed skeleton, catalogued StW 573, was unveiled on 6 December 2017 at the Hominin Vault of the Evolutionary Studies Institute at Wits.
The completeness is the point. StW 573 is more than 90 per cent complete, against roughly 40 per cent for the far better-known “Lucy”, making it the most complete Australopithecus skeleton ever recovered and the most complete skeleton of any human ancestor older than 1.5 million years. Having a skull, spine, limbs and both feet from one individual lets researchers ask questions that a scatter of jaw fragments cannot. Its cranial capacity is at least 408 cm³, about a third of a modern human's; the skeleton combines clear bipedal adaptations with features suited to climbing; and the bony labyrinth of the inner ear supports upright walking. Small stature and skull features suggest an aged female. Its excavators assigned it to Australopithecus prometheus, but that is contested — a 2025 reassessment argues the attribution is unsupported and places the specimen nearer A. africanus.
How old are the fossils — and why did the dates change?
In 2022 a team led by Darryl Granger published cosmogenic nuclide burial dates for Sterkfontein in PNAS, with Dominic Stratford — still the site's research lead — alongside Laurent Bruxelles, Ryan Gibbon, Ronald Clarke and Kathleen Kuman as co-authors. The method dates the sediment the fossils are buried in rather than the flowstone layers around them, and the results moved almost everything. Member 2 in the Silberberg Grotto, which held Little Foot, returned 3.67 ± 0.16 million years. The lower-middle of Member 4 came back at 3.41 ± 0.11 million years and the upper-middle at 3.49 ± 0.19 million; the Jacovec Cavern at 3.61 ± 0.09 million. The base of the Oldowan stone-tool unit in Member 5 dated to 2.18 ± 0.21 million years. Earlier flowstone-based estimates for Member 4 had run 2.03 to 2.65 million years.
The consequence is that Sterkfontein's australopiths are no longer the younger southern cousins of the East African finds. Granger's team concluded that nearly the entire Australopithecus assemblage here sits in the mid-Pliocene, contemporaneous with Australopithecus afarensis — Lucy's species — rather than following it. Mrs Ples is the clearest casualty of the revision. She was long dated to roughly 2.1 to 2.5 million years; Wits now cites approximately 3.3 million on its research pages, about a million years older than previously estimated, while another of the operator's own pages still gives 2.4 million. If you meet conflicting figures on site, that is why. The re-dating has not gone unchallenged either — an objection drawing on monkey-fossil biostratigraphy was published and rebutted — so treat any single number as a position in a live argument.
How was Sterkfontein excavated?
Fossils were first turned up at Sterkfontein in the 1890s, and serious scientific research only began in the 1930s. Robert Broom recovered the first adult Australopithecus africanus fossil here in 1936, then Sts 5 in 1947. In 1956 C.K. Brain located Middle Stone Age artefacts at the site, and later excavation of the same deposits revealed early Acheulean tools — evidence that the cave records tool-making populations as well as australopiths. In 1966 Phillip Tobias initiated the excavation programme that still runs today, and Wits has managed the site from that year onward. Alun Hughes directed the fieldwork through to 1991, recovering significant hominin fossils from the mid-1970s on — including, as it turned out, the boxed material that Clarke would re-examine in 1994.
Ronald Clarke took over direction of the excavation in 1991, working on the deposit's complexity and the ancient cave collapses that shuffled its layers. Cosmogenic dating campaigns then ran from 2015 to 2022. In total Sterkfontein has produced more than 700 hominin specimens, and Wits holds the world's largest hominin fossil repository. The most recent changes are administrative and physical rather than scientific. From 1 April 2024 the Wits Faculty of Science took over all research and visitor operations at both Sterkfontein and Swartkrans, ending Maropeng's role in running the site. Severe flooding at the cave entrance in December 2022 had already forced the first public closure in the site's history; it reopened on 15 April 2025. Dr Job Kibii has headed daily operations since April 2025, Professor Dominic Stratford remains research lead, and Ntobeko Mbuyisa is operations manager.
Why is the Cradle of Humankind a UNESCO World Heritage Site?
UNESCO inscribed the area in 1999 under the serial listing “Fossil Hominid Sites of South Africa”, and extended it in 2005 to take in further fossil localities across other provinces. The domestic protection is separate and in practical terms the sharper instrument: SAHRA, the South African Heritage Resources Agency, records 15 declared National Heritage Sites within the Cradle of Humankind — originally 12, declared on 5 November 2004 — alongside the Makapan Valley site in Limpopo and the Taung Child site in North West. All are protected under the National Heritage Resources Act of 1999, which SAHRA administers. SAHRA describes the cluster as holding some of the richest concentrations of fossil-hominid-bearing sites in the world, with evidence spanning the past 3.5 million years, held predominantly in dolomitic caves.
Sterkfontein is the most extensively excavated site in that cluster, with its 700-plus hominin specimens — South African Tourism gives a more conservative figure of more than 500. You will often read that the caves account for more than a third of all early hominin fossils found worldwide prior to 2010; the qualifier matters, and the claim is widely repeated rather than authoritatively published, so treat it as an order of magnitude rather than a statistic. The wider Cradle has produced Karabo, the Australopithecus sediba type specimen, the Homo naledi material, and evidence of controlled fire use at Swartkrans. Underneath all of it is the geology: dolomite laid down around 2.5 billion years ago, carrying stromatolite structures built by the earliest photosynthesising life on Earth, into which the cave system itself dissolved only 20 to 30 million years ago.
What does ongoing research at Sterkfontein involve?
The research is not confined to the past tense. In the on-site fossil preparation laboratory a technical team works alongside postgraduate students and researchers to clean, sort, document and catalogue fossil material — and not only Sterkfontein's, since the lab processes finds from numerous sites across the Cradle of Humankind. The questions being asked there span environmental change and its effect on early hominins, physical and cognitive evolution in the Cradle, and the relationship between southern and East African hominin species, across a record of more than three million years. The methods have moved on accordingly: stratigraphic work reconstructs how the deposits accumulated, and a machine-learning analysis of tooth marks on Sterkfontein hominin fossils identified scavenging by brown hyenas — which says something about how these individuals came to be in the cave at all.
Visitors see more of this than they used to. The preparation laboratory is now public-facing, modernised with support from the GENUS research centre ahead of the 2025 reopening, and being able to watch and talk to the people doing the work is a first for the site. The newly developed temporary exhibition space shows iconic finds from both Sterkfontein and Swartkrans, including one of only two full casts of the Little Foot skeleton and the first complete leg of a Paranthropus, recovered from Swartkrans Cave. The two most famous originals are elsewhere: Little Foot is curated in the Hominin Vault at the Evolutionary Studies Institute at Wits in Johannesburg, and Mrs Ples is held by the Ditsong National Museum of Natural History in Pretoria. The site draws 80,000 to 100,000 visitors a year.
Frequently asked
When was Mrs Ples found?
Robert Broom and John T. Robinson discovered the cranium at Sterkfontein on 18 April 1947. It is catalogued as Sts 5 — not Sts 14, which some material confuses it with — and identified as Australopithecus africanus.
How old is Mrs Ples?
Unsettled. The traditional figure was roughly 2.1 to 2.5 million years, but cosmogenic re-dating of the Member 4 deposits published in 2022 made the assemblage about a million years older, and Wits now cites around 3.3 million.
Was Mrs Ples male or female?
Genuinely disputed. Broom read the small canine sockets as female; later researchers argued for an adolescent male; a 2012 reassessment defended the adult female reading. No consensus has emerged, and guides tend to present both cases.
When was Little Foot found?
Ronald Clarke picked out four foot bones from stored material on 6 September 1994 and located the skeleton in the Silberberg Grotto on 3 July 1997. Preparation took some twenty years; it was unveiled in December 2017.
What species was Little Foot?
Its excavators assigned StW 573 to Australopithecus prometheus, but the attribution is contested — a 2025 reassessment argues it is unsupported and places the specimen closer to Australopithecus africanus. The debate is unresolved.
Why is Little Foot so important?
It is more than 90 per cent complete against roughly 40 per cent for Lucy — the most complete Australopithecus skeleton ever found, and the most complete of any human ancestor older than 1.5 million years.
Is excavation still happening at Sterkfontein?
Yes. The on-site fossil preparation laboratory is an active research facility processing material from across the Cradle of Humankind, and since 1 April 2024 all research and visitor operations have been run by Wits.