MRS PLES (STS 5) FROM THE STERKFONTEIN CAVES, SOUTH AFRICA
By: Francis Thackeray, Honorary Research Associate, Evolutionary Studies Institute, University of the Witwatersrand and Lazarus Kgasi, Plio-Pleistocene Palaeontology, DITSONG: National Museum of Natural History
“Mrs Ples” (Sts 5) is the nickname of a Plio-Pleistocene skull (Fig. 1) attributed to Australopithecus africanus, from the Sterkfontein Caves in the Cradle of Humankind World Heritage Site, situated about 70 km south-west of Pretoria (Fig. 2). It is curated at the DITSONG: National Museum of Natural History (DNMNH) in Pretoria. The fossil was discovered by Dr Robert Broom and John Robinson (Fig. 3) in April 1947. The fossils were exposed as a result of a dynamite explosion. The impact of the explosion broke the skull in two parts. It was initially called Plesianthropus, meaning “almost human” (Broom and Robinson, 1950), i.e., a distant relative of all humankind.
Fig. 1. Mrs Ples, Australopithecus africanus, from Sterkfontein (Photo: Lazarus Kgasi).
Fig. 2. Aerial view of the Sterkfontein Caves.
Fig. 3. Robert Broom (right) and John Robinson (left) with Mrs Ples (broken in two parts) at Sterkfontein, April 18, 1947.
Geological ages
The exact age of this specimen is uncertain, but it is known to have come from a stratigraphic unit known as Member 4. Faunal estimates for the age of this Member are listed in Table 1, ranging between 2 and 3 million years ago (Ma), based in part on bovids (antelope) studied by Elisabeth Vrba (1982); suids (pigs) studied by Basil Cooke (1971); and cercopithecids (monkeys) studied by Steve Frost et al (2022, 2023). Cosmogenic dates as early as 3.4 Ma for Member 4 have been estimated by Darryl Granger et al (2022, 2023), using chert samples near the base of an excavation by Alun Hughes. Using a biochronological approach, Thackeray and Dykes (2022) have suggested (from their calculations of the ages of first lower molars of A. africanus) that Member 4 can be dated to a period between about 2 and 3.5 million years, thereby reconciling the “Sterkfontein Debate” between Granger et al (2022, 2023) and Frost et al (2022, 2023). The mean age for Member 4 determined by Thackeray and Dykes (2023) is 2.7 Ma. It has been claimed by Andy Herries and his colleagues that the top of Member 4 dates to about 2 Ma. In fact they claim that the age for Mrs Ples ranges between 2.01 and 2.05 Ma, based on palaeomagnetic and uranium-lead analyses (Herries and Shaw, 2011; Pickering and Kramers, 2010, Herries et al, 2013).
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Table 1. Dates for Sterkfontein Member 4
Range (Ma) Mean of range (Ma)
3.4 – 3.7 3.55 Granger et al (2022, 2023), cosmogenic nuclides.
2.0 – 2.6 2.30 Frost et al (2022, 2023), cercopithecids.
1.8 – 3.5 2.65 Thackeray and Dykes (2022), hominins.
2.5 – 3.0 2.75 Cooke (1974), suids.
2.3 – 2.8 2.55 Vrba (1982), bovids
2.2 – 2.7 2.45 Johanson and Edey (1981), mammals.
2.4 – 2.6 2.50 McKee (1993), mammals.
2.05 – 2.58 2.31 Herries and Shaw (2011), palaeomagnetism.
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Is Mrs Ples male or female?
Yoel Rak (1983) thought that Mrs Ples was male on the basis of anterior pillars associated with canine roots. Mirriam Tawane and Francis Thackeray (2018) could demonstrate from a study of canine sockets that Sts 5 is probably a small male (Fig. 4). This contradicts Villamoare et al (2013) who (like Broom) claimed that it was female. It also contradicts Grine et al (2012) who wrote an article entitled: “Sex at Sterkfontein: ‘Mrs. Ples’ is still an adult female”.
Fig. 4. Tawane and Thackeray (2018) presented evidence to demonstrate from a study of canine sockets that Mrs Ples represents a small male (green circle).
Developmental age
Unfortunately, Mrs Ples was not preserved with teeth but broken roots of some of the maxillary dentition can be recognised from CT scans (Fig. 5).
Fig. 5. Reconstruction of Mrs Ples based on CT scans (Treil, Thackeray and Braga). The white areas represent broken fragments of dentition, including dentine and roots. The enamel crowns of the teeth had been broken before fossilisation.
In a pioneering study it was suggested that Mrs Ples was young because roots of the third molars appeared to be open (Thackeray et al, 2002). Later studies (Bonmati et al, 2008; Grine et al, 2012 and Villamoare et al, 2013) have shown that the upper third molar roots were closed. Thackeray has found that the M3 of Mrs Ples would probably have had more than “slight” wear (see below) at the time of death, confirming that Mrs Ples had reached adulthood.
Sagittal as well as lambdoidal sutures were open (Potze and Thackeray, 2010) but Villmoare et al (2013) concluded that Mrs Ples was a very old (“senescent”) individual. This is reflected in the title of their paper: “Continuous dental eruption identifies Sts 5 as the developmentally oldest fossil hominin and informs the taxonomy of Australopithecus africanus“. There is a problem with this interpretation. How can Mrs Ples be a very old individual, when Potze and Thackeray (2010) could demonstrate that the cranial sutures were still open?
Villmoare et al (2013) focussed on anterior dentition. They said: “The extremely short and vertically oriented anterior roots observed in Sts 5 suggest that the specimen represents an aged female specimen with extremely worn dentition…the condition seen in Mrs Ples is consistent with that seen in senescent extant hominids with heavy tooth wear”. The views expressed by Villamoare et al need to be assessed not only in terms of the anterior dentition of Sts 5 but also in the light of evidence obtained from temporal lines.
Temporal lines
Temporal lines of Mrs Ples were described by Potze and Thackeray (2010). The distance between right and left lines across bregma (DBTL) is 22 mm. This can be compared to mean DBTL values obtained from male chimpanzees (Pan troglodytes) based on data published by Thackeray et al (2005, Table 2):
Stage 4 M3 in slight wear, mean DBTL = 33.2 mm
Stage 5 M3 more than moderate wear, mean DBTL = 13.5 mm
If the data for male chimpanzees are considered to be analogous to the condition in male A. africanus, we can recognise that a DBTL value of 22 mm for Mrs Ples is intermediate between “slight” and “more than moderate” wear. This would contradict the views expressed by Villamoare et al (2013), firstly that Mrs Ples “would have had extremely heavily worn teeth at the time of death”, and secondly that “Sts 5 represents a senescent adult”. Instead, in human terms, Mrs Ples could be considered to represent a “middle-aged” adult.
The virtual brain of Mrs Ples
A virtual endocast of Sts 5 was prepared by José Braga and Francis Thackeray in 2007 (Fig. 6). A slight bulge in “Broca’s area”, on the left side of the brain, suggests that Mrs Ples had the incipient capacity for speech. Amélie Beaudet has reconstructed the brains of other australopithecine specimens.
Fig. 6. Virtual brain of Mrs Ples, reconstructed from CT scans (Braga and Thackeray).
The skeleton of Mrs Ples?
It has been suggested by Thackeray, Gommery and Braga (2002) that Sts 14 represents a partial skeleton of Mrs Ples. The skull of Sts 5 and the pelvis of Mrs Ples are both considered to be male, not only by Tawane and Thackeray (2018) but also by Gommery and Thackeray (2006) on the basis of the anterior inferior iliac spine of the pelvis. Furthermore, they were discovered close together in 1947, supporting the view that the skull and skeleton represent the same individual (Thackeray, J.F. & Gommery, D. 2002.) They are shown together in Fig 7, representing a potential analogue of the Ethiopian skeleton nicknamed “Lucy”.
Fig. 7. Sts 14 (skeleton) and the skull of Mrs Ples. Tawane and Thackeray (2018) demonstrated that the skull represents a male.
Conclusions
This study indicates the following:
- Mrs Ples (Australopithecus africanus) represents a male.
- Mrs Ples was adult at the time of death.
- Mrs Ples was not adolescent, nor senescent.
- In human terms, Mrs Ples was a “middle aged” adult.
- Sts 14 is likely to represent the skeleton of Mrs Ples.
- Mrs Ples was discovered in Member 4 at Sterkfontein, South Africa.
- Using measurements of first lower molars of Australopithecus africanus from Member 4, Thackeray and Dykes (2023) have indicated a mean age of 2.7 Ma for this Member, associated with a wide range (1.8 – 3.5 Ma).
- Herries et al (2013) claim that Mrs Ples is about 2 million years old.
- Mrs Ples had the incipient capacity for speech.
- Mrs Ples represents a distant relative of all humankind.
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