Stumbled upon this question today while going through synonyms of various terms. I was advised that they are not synonymous to each other.
"Ancestors" has only one meaning. Your ancestors are your relatives several generations back who lived, and, it is commonly understood, had already died, by the time you were born.
"My ancestors arrived in Australia from Ireland in the 1790s."
"Hominids are our human ancestors."
English-speakers sometimes use "forefather" as a synonym for "ancestor."
"Our forefathers passed these customs down to us."
But it also has a more fanciful meaning, often referring to the originator of an idea or tradition, or a person who established an important precedent in some sort of field, like "forerunner."
"Alan Turing is the forefather of modern computing."
Ancestors and forefathers are terms used interchangeably in some instances, as when referring to a general shared historical past, and there is an implied assumption that the deceased persons being referenced are both blood-related ancestors, and "persons of an earlier historical period and with a common heritage with us" - i.e., forefathers. Example: "our Pilgrim ancestors," or alternatively, "our Pilgrim forefathers." When one but not both of those factors are present, one word is more applicable than the other.
I like to think of the difference as this: ancestor refers to individual and personal family tree predecessors (from whom we inherit DNA internal to ourselves), and forefathers refers to multiple persons and communal, regional, or national predecessors (from whom we inherit language, customs, etc. external to our individual selves). I would say "my ancestors," but not "my forefathers," and - unless I was at a family reunion, I would say, "our forefathers and mothers," not "our ancestors."
Stated another way, ancestors implies a blood and familial relation, and forefathers does not imply a blood and familial relation; forefathers implies a common cultural, regional heritage and roots, and ancestors does not imply a common cultural, regional heritage and roots. Those of us Americans with immigrant roots have ancestors who lived and spoke in ways radically different from us, and were not our forefathers. And conversely, those of us Americans with immigrant roots are expected, as I was, to accept the Pilgrims as among our American forefathers as the history books portray them. As an aside, I believe the range of persons who ought to be included in the American forefathers category should be broadly expanded to include at a minimum certain Native American peoples and African captives who were sold into slavery. Just saying'
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