Aging ancestors. A new method dates these hominid bones to a surprisingly ancient 4 million years old.

Ancient Cave Dwellers Age Even More

Certain limestone caves in South Africa have been called the cradle of humankind because they contain nearly one-third of known early human fossils. Using a new dating method, researchers now suggest that hominid remains from the Sterkfontein caves are a stunningly ancient 4 million years old.

Age estimates for Sterkfontein's 500 hominid fossils have ranged widely, from 3.5 million to 1.5 million years old. The new estimates, reported in the 25 April issue of Science, are based on a technique developed by Darryl Granger, a geologist at Purdue University in West Lafayette, Indiana. When cosmic rays bombard Earth's surface, they collide with the nuclei of atoms such as oxygen, producing unstable isotopes of beryllium and aluminum. The older the rock, the more isotopes it accumulates; when it is buried, the bombardment stops and the isotopes decay (Science, 11 January 2002, p. 256).

Geologist Tim Partridge of the University of the Witwatersrand in Johannesburg, South Africa, sent Granger five samples of a fossil known as Little Foot from the Sterkfontein Caves. Granger dated samples of the cementlike matrix of rock and sand surrounding Little Foot to 4.17 million years ago, plus or minus about 350,000 years. If that's correct, then Little Foot's contemporary was Australopithecus anamensis, who lived 4 million years ago in east Africa. It also means that “hominids were basically all over Africa 4 million years ago,” says paleontologist Fred Spoor of University College London.

However, not everyone's convinced. “These dates do not stand up to close scrutiny,” says Witwatersrand paleoanthropologist Lee Berger. Those familiar with Sterkfontein add that Granger, who didn't visit the caves, is on shaky ground using his method there. The site is notoriously complex, with collapsed ceilings and overhead shafts that may have allowed debris from the surface and cave walls to fall in and mix with dated sediments, says geochemist Henry Schwarcz of McMaster University in Hamilton, Canada.

Partridge counters that the chance of mixing is “very remote indeed,” because the cave's dolomite walls don't shed small debris, and because the skeleton was apparently buried rapidly.