Ancient Apelike Fossil Not Human Ancestor, Study Finds

Richard A. Lovett
for National Geographic News
December 8, 2006
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.)

Radioactive Decay

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.

"The technique takes samples from the layers, extracts uranium and lead from them, and measures the proportion of the daughter lead isotope compared to the remaining uranium," Cliff said.

"The older the sample, the more daughter product there will be and the less uranium."

Uranium decays very slowly, however, through a series of intermediate stages. This makes it difficult to use the technique on relatively young rocks such as those in Sterkfontein.

Further complicating matters, the rocks in the cave contain unusual background levels of another element that also decays into lead.

The scientists took these background traces into account to arrive at the new date for the fossil. They report their findings in the current issue of the journal Science.

Paul Renne of the University of California, Berkeley's Geochronology Center, says the new study appears to have "convincingly resolved" a longstanding issue, opening the door for similar work on other South African fossils.

"It is very exciting," he said by email.

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