In the 21st century, the existence of parallel universes was accepted after quantum mechanics confirmed it. The protagonist was a graduate student specializing in traditional Chinese medicine. After working for forty hours straight, he died suddenly and transmigrated to a parallel Earth's ancient, primitive era.
“Other people transmigrate as princes or feudal lords. I transmigrate as a caveman?”
Fortunately, he brought the theoretical knowledge of the 21st century with him.
“Slash-and-burn farming is too primitive? I'll crossbreed crops!”
“Tribes are still fighting with obsidian? I'll smelt copper and iron!”
“The planes are raising chosen ones to fight transmigrators? Then I'll just become the chosen one!”
