Remains of ancient Australopithecus hominins from the Sterkfontein caves in South Africa—including the well-known “Mrs. Ples”—were originally dated to between 2.1 and 2.6 million years ago, but they are actually between 3.4 and 3.6 million years old, a study estimates. The revised dates, published in PNAS on Monday (June 27), would mean they’re older than the famous Lucy fossil unearthed in Ethiopia, which is dated to around 3.2 million years ago, reports The Washington Post.
“This important new dating work pushes the age of some of the most interesting fossils in human evolution research, and one of South Africa’s most iconic fossils, Mrs. Ples, back a million years to a time when, in East Africa, we find other iconic early hominins like Lucy,” study coauthor Dominic Stratford tells the Post.
The archeological community has widely accepted the hypothesis that the early hominin species Australopithecus africanus (e.g. Mrs. Ples) descended from A. afarensis (e.g. Lucy). However, “[t]he contemporaneity of the two species now suggests that a more complex family tree prevailed early in the human evolutionary process,” the study authors write.
Hundreds of Australopithecus fossils have been found at the Sterkfontein caves since the first was discovered in 1936, reports the Post. “But it’s hard to get a good date on them,” study coauthor Darryl Granger tells the outlet.
In East Africa, volcanic ash surrounding fossils can be used for accurate dating, according to Purdue’s news release, but it’s more complicated in South Africa. There, researchers have had to use surrounding animal fossils, which can shift over time, or calcite flowstone deposits, which can settle in areas older than them, leading to underestimates of age. The current analysis employed a newer technique instead, one that directly ages the surrounding sediment. Using mass spectrometry, the researchers measured cosmogenic isotopes in quartz excavated from around the fossils; the relative decay of these elements reveals how old the rocks are.
“South Africa was largely ignored because it was so difficult to date the fossils. They were largely dismissed as not being relevant to the story of human evolution,” study coauthor Ronald Clarke tells The Sydney Morning Herald. “It’s a big deal, this does confirm that these primitive ancestors were all over Africa.