这种古代鱼类被认为是现代哺乳动物的祖先。

This ancient fish is considered an ___ancestor of modern mammals.

我的祖先在200年前移民到了这个国家。

My ___ancestors immigrated to this country 200 years ago.

蒸汽机被认为是现代内燃机的雏形。

The steam engine is considered the ___ancestor of modern internal combustion engines.