Like the price of oil, the age of the famous hominid fossil known as Little Foot has fluctuated dramatically in the past decade. Named for its rare foot and toe bones, the South African australopithecine was originally estimated to be 3 million to 3.5 million years old, making it one of the oldest members of the human family. Other estimates dated Little Foot back as far as 4.1 million years, or as recently as 2.5 million years. Now a team of geochronologists thinks it has finally nailed down the fossil's true age--2.2 million years old--perhaps indicating that ancient hominids arrived in South Africa much later than currently thought.
Little Foot's age ambiguity stems from its final resting place--a cave in Sterkfontein, South Africa. Unlike other paleontological sites in east Africa, the rock here does not contain volcanic sediment, which typically provides researchers with a reliable way to date fossils. Instead, paleoanthropologists have depended on less precise dating methods, which have given widely different ages for the fossil (Science, 25 April 2003, p. 562).
A team led by geochronologist Robert Cliff of the University of Leeds in the United Kingdom tried to get a more precise age estimate by using a new application of a longstanding method called uranium-lead dating, which has traditionally been used to date minerals in ancient rock but not in cave stone. The method relies on measuring the decay of an uranium isotope--238 U--into a daughter isotope of lead, 206 Pb. The more time that passes, the more lead is produced. Unlike earlier dating methods, uranium-lead dating cannot be skewed by the intrusion of older or younger sediments.
Ten samples taken from the layers of cave stone surrounding Little Foot produced a consistent date of 2.1 million to 2.2 million years for the fossil, Cliff's team reports tomorrow in Science. This suggests the earliest hominids arrived in South Africa 2 million to 4 million years after they arose in eastern or central Africa. Had the fossil been closer to 4 million years old, it might have given clues to how upright walking evolved, but the new age estimate indicates Little Foot will shed little light on the earliest stages of this adaptation.
"They have nailed it," says Paul Renne, a geochronologist at the Berkeley Geochronology Center in California. "This is the most reliable age yet determined for Little Foot." But University of Witwatersrand geologist Tim Partridge, who heads the team that initially dated the fossil, says the new results are "implausible" because Little Foot would now be as young as another famous australopithecine, known as Mrs. Ples, found in another younger layer of limestone at Sterkfontein--and the extinct animals in those two layers are of different ages. "A great deal more work and many more results will be needed to resolve these issues," he says.