PHILADELPHIA — Weathered and pitted with the passage of 3,000 years, a rock slab found in southern Mexico shows clear evidence of a script that anthropologists say is the oldest writing ever found in the Western Hemisphere.

In Friday's issue of the journal Science, researchers identified the carved symbols as those of the Olmec civilization, an urbanized people who lived along the Mexican Gulf Coast from about 1200 B.C to 400 B.C.

Deciphering the mysterious script, which includes apparent depictions of fish, maize, dart tips and insects, will be a big challenge even if other examples of it can be found. But the discovery nevertheless provides fascinating new clues about the first complex civilization in this part of the world — a society of kings and priests that is poorly understood.

"This find knocked us off our feet," said Brown University anthropology professor Stephen D. Houston, who helped Mexican experts analyze the find. "Writing systems of the ancient world are discovered once in a lifetime."

The 26-pound slab was discovered accidentally in April 1999, in a town called Lomas de Tacamichapa. Road workers unearthed the tablet and other artifacts in a gravel quarry that was both a source of building materials and a known archaeological site. Although archeologists were not able to view the objects in the positions they were found, the researchers said there is no doubt of their authenticity.

That belief was echoed by Simon Martin, a University of Pennsylvania expert in Mayan writing who was not involved with the research.

After the discovery, local authorities asked the National Institute of Anthropology and History in Mexico to investigate.

Married Mexican archaeologists Maria del Carmen Rodriguez Martinez and Ponciano Ortiz Ceballos were the first researchers to study the block and other objects, including ceramic pieces, clay figurine fragments, and ground stone artifacts. Though similar symbols have been found on other Olmec artifacts, the paper's authors called the new find the first unambiguous example of Olmec text.

The style of the accompanying artifacts led Rodriguez and Ortiz to estimate that the block had been carved sometime between 1000 to 800 B.C. They were the lead authors of the paper.

The next oldest Mesoamerican writing to be found probably dates from about 500 B.C., although experts disagree, Houston said.

The new block features 28 distinct symbols, some of them repeated several times in various combinations, for a total of 62 markings.

Among them is an "X" with a circle around it, an apparent religious symbol found previously on Olmec figurines and jewelry, said Mary DeLand Pohl, a professor of anthropology at Florida State University.

The development of writing went hand-in-hand with the evolution of a hierarchical society, said Pohl, who was not involved with the new research but has discovered other Olmec objects with carved symbols.

"It's all being associated with the development of kings and royalty and social stratification," Pohl said.

Anthropologists believe the Olmec heartland was in the modern Mexican Gulf states of Veracruz and Tabasco.

Since the 1940s researchers have found Olmec artwork, jewelry, and giant carved heads. The Olmec were skilled jade carvers, and they built earthen pyramids and the first true cities in what is now Mexico. The later Olmec probably overlapped with early Mayan society, said Penn's Martin.

"Whoever these people were, they were the first to create huge settlements, carve enormous stone monuments, and do really elaborate work," Houston said.

Like other Mesoamerican writing, the Olmec symbols seem to represent a hieroglyphic system, each sign depicting a particular action or object. The symbols are assembled in combinations, sometimes repeated, suggesting some complex meaning and syntax. Authors say they were likely read from left to right.

Yet trying to decipher the actual meaning from just one writing sample is like trying to figure out the entire vocabulary and grammar of the English language from a single greeting card, Houston said.

"This is the supreme frustration," said Houston.

The job would be easier with explanatory imagery accompanying the writing, or with a document like the Rosetta Stone, the slab that enabled anthropologists to decipher Egyptian hieroglyphics from the known Greek script accompanying it.

Without the luxury of a multi-script document, the hunt is on for more Olmec texts. If more writing is found, Houston expects it to contain many new distinct elements besides the 28 inscribed on the block.

The fact that the Olmec were literate indicates they were even more sophisticated than previously thought, perhaps engaged in accounting and keeping records for posterity.

The Olmec script was probably widespread throughout the Olmec heartland, experts said, though it is distinct from later Mesoamerican writings.

And it may always remain what it is today — a tantalizing mystery.

"Finding more samples of text is going to be very, very tough," Martin said. And even if they do, the task is substantial.

"We have 2,000 Maya texts," he said, "and we haven't finished deciphering that."