April 9, 2012
In 1934, paleontologist Robert Broom set out to find the first fossil of an adult Australopithecus. Discovering a grown-up version of Raymond Dart’s Taung Child, the first Australopithecus specimen ever found, would help sway skeptics who doubted the fossil was a human ancestor, Broom thought. Throughout the 1930s, Broom found bits of australopithecine fossils in several caves in South Africa. But his most spectacular find came in 1947.
After blasting through limestone with dynamite at a cave called Sterkfontein, Broom recovered a nearly complete skull, missing only the teeth. He determined that it belonged to a middle-aged female of the species Plesianthropous transvaalensis (later scientists placed the skull in the species Australopithecus africanus). Known formally as Sts 5, the approximately 2.5-million-year-old skull is better known today as Mrs. Ples.
Paleoanthropologists agree that Mrs. Ples is the most complete, undistorted A. africanus skull ever found, but they quibble over whether the fossil is really a he or a she. Researchers who published a new study in the Journal of Human Evolution conclude that Broom was right all along, and anthropologists should continue to address the fossil as “Mrs.”
Doubts over Mrs. Ples’ sex first popped up in the 1980s. The most recent challenge came from Francis Thackeray of the University of the Witwatersrand in South Africa and colleagues. Over the past decade, they have argued that Mrs. Ples was a juvenile male. One reason for the confusion: The hole where the canine tooth sat was eroded away after the boy died, making it appear smaller and more feminine.
To address these claims, Frederick Grine of Stony Brook University in New York and his colleagues looked at virtual reconstructions made with CT scans of Mrs. Ples and several other A. africanus fossils. The fossils, representing both adults and youngsters, were all discovered at Sterkfontein. First, they concluded that Mrs Ples was an adult at the time of death. The roots of the individual’s wisdom teeth were fully formed. People today reach this stage of development between the ages of 17 and 21.
Furthermore, in comparison to the other australopithecine fossils, Mrs. Ples doesn’t appear to have suffered any bone loss in its upper jaw. So the size of the canine socket accurately reflects the size of the tooth when Mrs. Ples died. The canine’s small size suggests Mrs. Ples was indeed female.
Mrs. Ples isn’t the only A. africanus specimen whose sex has been hard to figure out. Several other fossils have also been identified as male or female by different research teams. Determining the true sex of these fossils is important because A. africanus is one of the most physically variable hominid species, Grine’s team notes. In the early days, researchers like Broom thought A. africanus fossils actually represented several different species. Today, anthropologists chalk up at least some of the diversity to sexual dimorphism, in which the physical traits of males and females in a species differ in size, shape and color. Knowing the sex of the most complete A. africanus skull, Mrs. Ples, will help anthropologists better understand the nature of the species’ variation.
(If you want to learn more about Robert Broom’s contributions to human evolution, consider reading Martin Meredith’s Born in Africa.)
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