Journal Club

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Journal Club: Age of ‘Little Foot’ provides twist in story of hominid evolution

An early hominid named Little Foot has been dated to 3.67 million years old, making the timeline of human evolution even more complicated.

Little Foot was found in a cave at Sterkfontein in South Africa, an hour’s drive from Johannesburg in 1980, and identified as hominid in 1994. According to co-author Ronald Clarke, a professor at the Evolutionary Studies Institute at the University of Witwatersrand and Little Foot’s discoverer, the name came from a play on the mythical Big Foot, and because four foot bones were discovered first.

The dating would muddle the standard view of human evolution. Based on the results, published in Nature on April 1, Little Foot is a million years older than any hominid found in South Africa, suggesting far more geographic diversity for our oldest ancestors. Generally, it is thought that humans derived from the species Australopithecus afarensis, remains of which have been found in Kenya and Ethiopia (the partial skeleton known as “Lucy” being the most famous). Little Foot’s dating, if accurate, challenges that narrative.

“It means that later hominids did not only derive from Australopithecus afarensis,” Clarke says. Little Foot was morphologically different from the hominids of East Africa living at the same time. “There could well have been other species in different parts of Africa living at the same time but not yet discovered.” Clarke believes Little Foot, a mature female about 1.25-1.5 meters tall, fell into the cave. He says she is part of a new species known as Australopithecus prometheus.

It took 16 years to get the entire skeleton removed from the rock and sand; it was embedded in concrete-like sediment and sealed in flowstone (stalagmite) in the cave. “The bones were broken up and scattered within the deposit and were in very soft, fragile condition,” says Clarke.

Sterkfontein has been the site of archeological digs since the mid 1930’s. But because of the region’s difficult geology, dating fossils found there has been wrought with controversy. In 1999, scientists estimated that Little Foot lived about 3.3 million years ago, but dating of some of the calcium flowstones surrounding the fossil suggested it may be closer to 2.2 million years old.

“The date was questioned because it was always possible the fine sand that came into the cave came from elsewhere in the cave … and was washed in,” says Darryl Granger, a co-author of the Nature paper and a professor of earth, atmospheric and planetary sciences at Purdue University in Indiana.

The recent dating entailed a new technology that Granger developed. Called isochron burial dating, the technique uses the radioactive decay of isotopes of aluminum-26 and beryllium-10 found in the quartz in the rocks, as well as samples of rocks and sand from around a fossil. The method can determine whether a sample has been undisturbed since its burial with the fossil, or whether it was deposited later.

Despite the use of isochron burial dating, some still have doubts. “I would like to see biochronological validation of these new age estimates,” says Tim White, a professor of human evolutionary studies at the University of California at Berkeley. White sees further work ahead even if the results hold. “Their bearing on the significance of the hominid remains will remain unknown until anatomical studies have been completed,” he says.

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