common ancestor
Used in biology for a shared evolutionary ancestor, and in linguistics for a proto-language. Technical but widely understood.
The common ancestor of humans and chimpanzees lived about 7 million years ago.
These languages are thought to have diverged from a common ancestor.
祖先 is a general term for ancestors or forebears, while 共通祖先 specifically refers to a shared ancestor in evolutionary or historical contexts.
Compound of 共通 (common) and 祖先 (ancestor). The term is a direct calque of the English 'common ancestor' and is used primarily in scientific contexts.