The female monkey later delivered a fully formed male, making it the world's first live birth of a chimeric non-human primate.

While scientists have used this method to create both monkey-human embryos (which were terminated before further development) and live individuals, it's the first time the introduced genetic material has made up such a significant percentage of the newborn's tissue composition. The injected stem cells formed 92% of the monkey's brain tissue, and overall accounted for 67% of the animal's makeup.

"This is a long-sought goal in the field," said senior author Zhen Liu, of the Chinese Academy of Sciences (CAS). "This work could help us to generate more precise monkey models for studying neurological diseases as well as for other biomedicine studies."