Here are a few possibilities:
1. Oral Tradition: This is the most ancient form of communication, predating writing and even printing. It's how stories, knowledge, and culture were passed down through generations. While not a "medium" in the modern sense, it was certainly a means of mass communication.
2. Cave Paintings: These early forms of art, dating back tens of thousands of years, could be considered a mass medium as they were public displays of information and ideas.
3. Pictograms and Hieroglyphics: These early writing systems, developed around 3000 BCE, allowed for the spread of information and stories to a wider audience than oral tradition alone.
4. Printing: The invention of the printing press in the 15th century by Johannes Gutenberg is often considered a watershed moment. It allowed for mass production of books and other printed materials, making information more accessible to the public.
5. The Newspaper: The first regular newspapers appeared in the 17th century, serving as a central source of news and information for a large audience.
Ultimately, which medium you consider the "first mass medium" depends on your definition of "mass" and "medium." Each of these forms played a crucial role in the development of communication and information dissemination, leading to the modern mass media we know today.