Scientists announced yesterday that they have discovered ancient writing, carved in stone, that dramatically pushes back the dawn of writing in the Americas.

The Cascajal block, a slab of rock named for a site in south central Mexico where it was found, dates to about 900 BC, the scientists said. The text is written in previously unknown script and is four centuries older than any known New World writing, they said. It was authored by the Olmec, a people famous for the massive sculpted stone heads they left behind in the dense tropical forest.

The stone is carved with 62 symbols, some of which appear to be depictions of maize, an animal skin, a fish, and a dart tip. They appear in groups that run horizontally, and some are repeated. It is not known what the text means.

The find is significant because it shows the Olmec were literate, adding weight to a controversial theory that they built what scholars call a “mother culture,” laying the groundwork for the Mayans, Aztecs, and the other great civilizations of the Americas that followed. The find also raises the possibility that more Olmec texts could be found and perhaps even deciphered, allowing archeologists to read the records of what is sometimes called the Americas’ first civilization.

“This is the find of a lifetime,” said Stephen D. Houston, an anthropologist at Brown University who coauthored a study of the block in the journal Science.

The discovery is likely to add to the building interest in early Mesoamerican writing. In recent years, archeologists have found other early Olmec symbols and early Mayan writing.

While specialists hailed yesterday’s announcement new writing systems are not often discovered they said there was certain to be controversy over the age of the stone slab. The Cascajal block was found by workers gathering stone from a pile of rubble for a road, not by archeologists, making it difficult to date precisely. The most accurate method of determining something’s age is to use carbon dating on the object, but carbon dating works only on organic materials, not on stone. When archeologists find stone objects, they can sometimes date them by the age of closely associated organic items, like plant material.

The piece was dated using three lines of evidence, Houston said. It was found near ceramics, most of which are in an early style that ended around 900 BC. The style of the symbols seems consistent with individual symbols seen from around 900 BC. And another archeologist has found Olmec figurines, reliably dated to at least 1100 BC, that seem to use some of the same symbols.

A prominent researcher of Mesoamerica who was not involved in the new research, Mary Pohl of Florida State University in Tallahassee, said she found the 900 BC date convincing. But anthropologist David Grove of the University of Florida, Gainesville, said he believes the block is from a later period, between 700 and 500 BC, because some of the individual symbols found on the stone also appear on objects from that later period, and the team had not definitively proven it is earlier.

The block weighs roughly 25 pounds and was found in the late 1990s in Veracruz, Mexico, where the Cascajal site is located, according to Houston. A pair of Mexican archeologists, the lead scientists listed on the paper, first examined the block in April 1999, according to the report in Science. The pair, Maria del Carmen Rodriguez Martinez of the National Institute of Anthropology and History of Mexico and Ponciano Ortiz Ceballos of Veracruz University, then contacted other scientists in Mexico and the United States, who have been studying it since.

Because the stone was not found by archeologists, the team took care to ensure it was not a fake. Geologists found weathering inside the inscriptions, showing them to be ancient, not new carvings on an old stone, according to the Science paper.

Grove said that when he first saw an image of the symbols, he suspected the stone was a fake. The text runs horizontally, instead of in the vertical rows used in later Mesoamerican writing, such as Mayan. But after seeing the weathering evidence presented in the report, Grove said that he was ready to concede the writing was authentic, but that he still found it “puzzling.”

The block was found near the Olmec’s first major center, called San Lorenzo. The dating puts the block at a crucial historical moment. About 900 BC, the Olmec were abandoning San Lorenzo, and a new center was rising up in nearby La Venta.

The inscriptions are carved in a rock called serpentine. The block is about 14 inches long, 8 inches wide, and 5 inches deep. The side with the writing is concave, leading the authors of the study to speculate that the block may have been written on, then sanded blank, many times.

The writing system appears to have died, because there are no signs of direct influence on other known writing systems. The oldest previously known text in the Americas dates to about 500 BC, and was used by a people known as the Zapotec, who also lived in what is now Mexico.

The Olmec arose on the coast of the Gulf of Mexico around 1200 BC, in what is now central Mexico, and they faded from history about 400 BC. The farming of maize played a vital role in building their civilization, and the scientists said that three of the 28 different symbols used on the stone appear to depict maize.

The Maya civilization overlapped chronologically with the Olmec and lived to their east, centered on the Yucatan peninsula. Mayan civilization began about 500 BC and went into decline in the eighth and ninth centuries AD. The Aztec lived in central Mexico like the Olmec, but far later, roughly from the 14th to 16th century AD.

Olmec is an Aztec word meaning “people from the rubber area.” Rubber plants are common where the Olmec lived.

The Olmec may have originated a game played with rubber balls, made using the local plants a distant ancestor, perhaps, of today’s soccer.