for National Geographic News
A pre-human fossil found in a South African cave may be more than a million years younger than was previously thought, a new study has discovered.
If the new findings are right, the fossil is not a direct human ancestor, as had been believed, but instead belongs to a side branch of the evolutionary tree that eventually led to modern apes and humans.
The fossil, called "Little Foot" because its discovery initially yielded only foot bones, was found in 1997 in South Africa's Sterkfontein cave system, a formation that has produced many early hominid fossils.
(Read related story: "Tooth Study Reveals Diets of Early Humans" [August 3, 2005].)
Previous studies estimated the skeleton's age at 3.3 million years or older, but the complex geology of the cave made it difficult to obtain a precise date.
The new study, by a team of British archaeologists and geophysicists, determined that Little Foot is only 2.2 million years old.
The million-year-difference is significant, the researchers say, because Little Foot's discovery had originally raised the possibility it might be a direct human ancestor, due to its combination of human- and apelike features.
"[But] if Little Foot is as young as our measurements suggest, then Little Foot himself can't have been in the direct line of descent," said study co-author Robert Cliff, a geochronologist from the University of Leeds.
(See a time line of human evolution.)
To date the fossil, scientists in the most recent study looked at the amounts of lead and uranium isotopes in cave formations immediately above and below the skeleton.
Types of lead known as daughter isotopes are produced during the radioactive decay of uranium.
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