There are few things more thoroughly human than our impulse to say something about ourselves in the way we dress or decorate our bodies and our clothing.
A wedding ring, a sailor's tattoo, a Muslim chador or a Yankees baseball cap -- each is intended to deliver an unspoken message about the wearer that is intelligible to those who see it.
Scientists aren't sure when this modern human trait first appeared. But anthropologists digging in the sandy floor of Blombos Cave, a rocky shelter high above the Indian Ocean in South Africa, believe they have found the earliest evidence yet of human symbolic ornamentation.
A cache of small mussel shells -- closely matched in size, with identical holes that might have been used to string them together -- has been dated to the Middle Stone Age, at least 75,000 years ago.
Until this discovery, the oldest known examples of ornamental beads were about 40,000 years old, including ostrich-shell beads found in Kenya.
The director of the dig at Blombos Cave, Christopher Henshilwood of the Centre for Developmental Studies at the University of Bergen, in Norway, said the discovery "provides important new evidence for early symbolically organized behavior in Africa."
Once our ancestors learned to use symbolic behavior, he said, "it meant communication strategies rapidly shifted, leading to the transmission of individual and widely shared cultural values -- traits that typify our own behavior."
Not everyone is persuaded. Stanford University anthropologist Richard Klein was skeptical.
"This is a serious claim, but there is a lack of support," Klein told the Los Angeles Times. "I don't think the evidence is compelling."
Randall White, an expert at New York University, called Henshilwood's claims "incredibly premature."
Anthropologists are deeply divided about how soon after the appearance of anatomically modern people (perhaps 150,000 years ago in Africa) they began to behave in recognizably "modern" ways, said John Yellen. He is the program officer for archaeology at the National Science Foundation, which helped support the internationally sponsored dig.
There's not even agreement on what constitutes "modern" human behavior, he said.
One school argues that such modern behaviors evolved gradually among the anatomically modern people of Africa. Another says that they appeared suddenly, perhaps as a result of a genetic mutation, as recently as 50,000 years ago, not long before they turned up at sites in Europe.
In 1999 and 2000, Blombos Cave gave up several blocks of ochre -- a kind of iron oxide stone -- inscribed with X-shaped markings that Henshilwood argued were evidence of abstract thinking and "symbolic intent" 75,000 years ago.
Critics questioned Henshilwood's interpretation, but Yellen said the new discovery would be harder to dismiss.
"These shell beads are really important because they're personal ornamentation," Yellen said. "It says something about the person that's wearing it. It's making a symbolic statement -- `Look at me; I'm a beautiful person, or a rich person, or of this society that has status to it.'"
The Blombos Cave find, he said, is strong evidence that modern human behavior was present in Africa at least 75,000 years ago. And while it might not lay the debate to rest, "this is like another nail in the coffin, and it's a good nail."
Henshilwood has been digging for more than a decade at Blombos Cave, which he discovered on a rocky, seaside slope near Still Bay, where he now has a holiday home.
It's a remote site, said Karen van Niekerk, a co-author of Henshilwood's paper on the shell bead discovery in the April 15 issue of the journal Science.
Reached by telephone after a day of digging, she said the cave is 45 minutes from the nearest town. The house where the team is staying gets its electricity from a generator and its water from a spring.
"It's not a cave in the sense of a French cave," the deep, tunnel-like caverns where scientists have found Stone Age paintings perhaps 35,000 years old, van Niekerk said.
The opening is less than three feet high and five or six yards wide; its whole area is barely 55 square yards. Rock falls and windblown sand over thousands of years had raised the floor of the cave nearly to the roof.
The debris buried evidence of successive human habitations. Archaeologists have carefully peeled them back, descending slowly through deposits from the Late Stone Age -- less than 2,000 years old at Blombos -- into the Middle Stone Age, which lasted from about 280,000 to 45,000 years ago.
And they're not finished yet, van Niekerk said. "There have been a lot of rock falls from the roof ages ago. There could be a lot more deposits if we can remove those rocks."
The shells were found in clusters of up to 17. They belonged to a species of fingernail-sized mollusk, Nassarius kraussianus, that lives in tidal estuaries. The nearest are in rivers 12 miles east and west of the cave.