While Tezuka was a very important figure in the history of manga, he was definately not the creator of the first manga. Before manga was even a thing, ancient scrolls covered in paintings and drawings have been found all over Japan.
The word "manga" dates back to the late 18th century and translates directly to "cartoon", or more accurately, "comic". During this time period, many mangakas, or manga artists flourished, including influencers like Katsushika Hokusai and Rakuten Kitazawa.
However, in the early 19th century, literature, art, and other mixed media from foreign countries began to come to Japan. This cultural renaissance heavily infuenced many mangakas, as their art styles became less artsy and realistic and more cartoony and cute. This is where Tezuka comes in.