The Sumerians invented a type of hieroglyphic writing that later developed into cuneiform writing. This is the oldest known human writing. Today, more than 100,000 Sumerian texts have been unearthed, most of them engraved on clay tablets. These include personal and business correspondence, money transfers, recipes, encyclopedic lists, laws, hymns, prayers, magical incantations, and scientific articles including mathematical, astronomical, and medical content. Many large buildings such as large sculptures also have text engraved on them. Multiple versions of many articles survive because they were frequently copied. Copying was the only way people could spread articles at that time. After Semitic speakers became rulers of Mesopotamia, Sumerian remained the language of religion and law.