Mrs Ples: The Sterkfontein Australopithecus africanus Skull

Mrs Ples is the popular name of Sts 5, the most complete skull of Australopithecus africanus ever found. Robert Broom and John Robinson recovered it from the Sterkfontein Caves near Johannesburg, South Africa, on 18 April 1947. Its discovery, together with other finds made the same year, helped persuade sceptical scientists that the australopithecines were early relatives of humans, not apes.

Background: Broom at Sterkfontein

In 1925 Raymond Dart had described the Taung Child, a juvenile skull he named Australopithecus africanus. Most leading scientists dismissed it as a young ape. The Scottish-born palaeontologist Robert Broom, then working as a doctor in South Africa, was one of Dart’s few supporters, and he set out to find adult specimens.

In 1936, now at the Transvaal Museum in Pretoria, Broom began visiting the lime quarries at Sterkfontein, where miners blasted the cave deposits. In August 1936 the quarry manager, G. W. Barlow, gave him part of an adult skull. Broom first named it Australopithecus transvaalensis and in 1938 moved it to a new genus as Plesianthropus transvaalensis, meaning “near man of the Transvaal”. Work stopped during the Second World War and resumed in 1947.

The discovery

On 18 April 1947 Broom and his assistant John T. Robinson used dynamite to break up the hard breccia, a cemented cave deposit, as was then common practice. One blast split a block of rock containing a nearly complete skull into two pieces. Fitting them together revealed a cranium lacking its lower jaw and upper teeth, but otherwise remarkably intact.

The specimen was catalogued as Sts 5. Newspapers quickly dubbed it “Mrs Ples”, from Broom’s genus name and the belief that it came from a middle-aged female.

What the skull shows

Mrs Ples had a small braincase of about 485 cubic centimetres, roughly a third the size of a modern human’s. The face projected forward and the brow was relatively smooth. The base of the skull and the position of the foramen magnum, where the spinal cord enters, were consistent with an upright posture.

Later in 1947 Broom and Robinson also found Sts 14, a partial skeleton with a pelvis and spine adapted to walking upright. Together, these adult fossils confirmed that Dart’s species was a small-brained biped, just as he had argued.

Acceptance and a new name

The 1947 discoveries came at a turning point. The British anatomist Wilfrid Le Gros Clark had already examined the South African fossils and supported their hominin status, and in 1947 Sir Arthur Keith publicly conceded that Dart had been right. Broom, Robinson and Gerrit Schepers published a full description of the Sterkfontein material in 1950.

Broom had given many fossils new genus names, but later researchers concluded that the Sterkfontein adults belonged to the same species as the Taung Child. Plesianthropus was merged into Australopithecus africanus, though the nickname Mrs Ples stuck.

Later debates

In the early 2000s the South African palaeontologist Francis Thackeray and colleagues argued, using CT scans, that the tooth sockets suggested a large canine and that Sts 5 could be a young male. Others continue to see it as female. The debate illustrates how hard it is to sex individual fossils of species whose males and females differed in size.

The age is also uncertain. Mrs Ples comes from Member 4 of the Sterkfontein deposits. Uranium-lead dating of flowstones published in 2010 suggested an age between about 2.6 and 2.0 million years. In 2022 a team led by Darryl Granger used cosmogenic nuclides to argue that Member 4 could be about 3.4 to 3.7 million years old, closer to the disputed age of Little Foot from an older part of the cave. That claim has not been universally accepted.

Sterkfontein today

Sterkfontein has remained one of the most productive hominin sites in the world. Excavations, led for decades by Phillip Tobias and Ron Clarke of the University of the Witwatersrand, have yielded hundreds of australopithecine fossils from Member 4 alone, as well as early stone tools from younger layers.

In 1999 Sterkfontein, Swartkrans, Kromdraai and neighbouring sites were inscribed together as a UNESCO World Heritage Site, later extended and known popularly as the Cradle of Humankind. The Maropeng visitor centre opened nearby in 2005, and the caves themselves can be toured. Mrs Ples, more than any other fossil, has become the public face of the site, appearing in exhibitions, school textbooks and South African media.

Timeline

• 1925: Raymond Dart names Australopithecus africanus from the Taung Child.
• 1936, August: Broom obtains the first adult australopithecine skull from Sterkfontein.
• 1938: Broom names Plesianthropus transvaalensis.
• 1947, 18 April: Broom and Robinson recover Sts 5, Mrs Ples.
• 1947, August: The partial skeleton Sts 14 is found.
• 1999: Sterkfontein becomes part of a UNESCO World Heritage Site.
• 2002: Thackeray and colleagues suggest Sts 5 may be male.

Why it matters

Mrs Ples gave the world its first clear picture of the adult face of Australopithecus africanus. Coming 22 years after Dart’s Taung announcement, it helped end the long rejection of the australopithecines and shifted the search for human origins decisively towards Africa. It remains one of South Africa’s national scientific treasures.