Sentence:  The most important lesson of the full year course in ancient history was the inculcation of the sweep of time: that a nation's life was not lived in isolation from its neighbors, that the acts of men have great and grave influences on their own times and for the future, and that the <WSD>thread of life</WSD> beginning in ancient times extends to the present. 
Meaning of the word marked with <WSD> tags:  The link of inheritance from past to present.