Mesoamerica, along with Mesopotamia and China, is one of three known places in the world where writing is thought to have developed independently. Mesoamerican scripts deciphered to date are a combination of logographic and syllabic systems. They are often called hieroglyphs due to the iconic shapes of many of the glyphs, a pattern superficially similar to Egyptian hieroglyphs. Fifteen distinct writing systems have been identified in pre-Columbian Mesoamerica, many from a single inscription. The limits of archaeological dating methods make it difficult to establish which was the earliest and hence the progenitor from which the others developed. The best documented and deciphered Mesoamerican writing system, and the most widely known, is the classic Maya script. Earlier scripts with poorer and varying levels of decipherment include the Olmec hieroglyphs, the Zapotec script, and the Isthmian script, all of which date back to the 1st millennium BC. An extensive Mesoamerican literature has been conserved, partly in indigenous scripts and partly in postconquest transcriptions in the Latin script. Pre-Classic and Classic Period In Mesoamerica, writing emerged during the Pre-classic Period, with Zapotec and Maya writing flourishing during the Classic Period. Olmec writing Early Olmec ceramics show representations of something that may be codices, suggesting that amatl bark codices, and by extension well-developed writing, existed in Olmec times. It was also long thought that many of the glyphs present on Olmec monumental sculpture, such as those on the so-called "Ambassador Monument" (La Venta Monument 13), represented an early Olmec script. This suspicion was reinforced in 2002 by the announcement of the discovery of similar glyphs at San Andres. In September 2006, a report published in Science magazine announced the discovery of the Cascajal block, a writing-tablet-sized block of serpentine with 62 characters unlike any yet seen in Mesoamerica. This block was discovered by locals in the Olmec heartland and was dated by the archaeologists to approximately 900 BCE based on other debris. If the authenticity and date can be verified, this will prove to be the earliest writing yet found in Mesoamerica. Zapotec writing Another candidate for earliest writing system in Mesoamerica is the writing system of the Zapotec culture. Rising in the late Pre-Classic era after the decline of the Olmec civilization, the Zapotecs of present-day Oaxaca built an empire around Monte Albán. On a few monuments at this archaeological site, archaeologists have found extended text in a glyphic script. Some signs can