Someone further back than a grandparent
In everyday English an ancestor is a person you descend from, usually more remote than a parent or a grandparent. The same noun also names an earlier plant or animal from which a later kind evolved, and, more loosely, an older tool, custom, or language form that something modern grew out of. Law keeps a tighter sense still: the person from whom an estate has come down, the counterpart of the heir.
The family-tree meaning feels old. The English word is not a native Germanic heirloom. The origin of the word ancestor is a medieval French borrowing of a Latin agent noun whose first job was to name someone who went physically in front.
Two French forms, several English spellings
By about 1300, Middle English writers were already using a cluster of related shapes: ancestre, auncestre, ancessour, antecessour. They point back to Old French, which kept two living descendants of one Latin noun. The nominative, or subject-case, form was ancestre. The oblique form was ancessor or anceisor. Later French trimmed the nominative into modern ancêtre.
English tried all of those streams. Sources do not quite agree on when the present spelling ancestor became the usual one. One standard account puts the current form in the early fifteenth century; another places the settled modern spelling in the sixteenth. The disagreement is about orthography, not about the borrowing itself. The word came in from French in the thirteenth century. It was not inherited from Old English, which had used other kinship words, including ieldra, an elder in the line, for a similar idea.
The t that English speakers now hear in ancestor is not an English insertion. It is already there in Old French ancestre. On the way from Latin antecessor, unstressed vowels dropped, and French, as it often did between s and r, developed a t in that cluster. English kept the consonant it received.
A prefix, a verb, an agent ending
The Latin source is Late Latin antecessor, built on the verb antecedere, to go before. The morphology is plain once it is unpacked. The prefix ante means before. The verb cedere means to go. The ending -tor marks the person who does the action. Put together, the noun is a fore-goer: someone who goes in front.
That same verb family is still productive in English. Cede, precede, proceed, recede, succeed, and access all rest on cedere. Ancestor and antecedent are, at root, two views of one compound. One names the person who went first. The other names whatever comes before.
Behind the Latin pieces sit the usual scholarly reconstructions. The prefix is traced to a Proto-Indo-European root for the front or forehead, with derivatives meaning in front of or before. The verb is traced to a root meaning to go or to yield. Those are reconstructed forms, not words written in an ancient word-list, and they should be treated as such.
Scout, office-holder, then kin
Family descent was not the first sense in Latin. The earliest uses of antecessor are military: a soldier sent ahead of the column, a scout. From there the noun shifted to a predecessor in an office, the person who held a post before you did. Only in medieval Latin did it take on the meaning French and English now treat as central: people who lived before the present generation, and more narrowly the ascendants in a family who stand beyond the grandfather. In that role it covered ground that classical Latin had given to maiores, the elders of the line.
Old French already had the kinship sense in the twelfth century. The nominative ancestre is recorded in Wace in the 1160s and 1170s. An earlier oblique form, anceisor, appears in the mid-eleventh century; some historians of French link that spelling to the influence of Old French anceis, previously. By the late twelfth century the plural could mean the run of ascendants stretching back before the father.
English took over that kinship center of gravity. The legal sense, a person from whom land or goods descend, sits close to the older idea of a predecessor. It does not have to name a beloved forebear in a story. It names the one who stood in the chain before the heir. The biological sense, an earlier species from which a later one evolved, and the figurative sense, an early bicycle or an older form of a word, are later widenings of the same picture: whatever came before in a line of development.
A rare feminine, ancestress, is recorded from the 1570s. It never became the ordinary word. English, unlike some languages, generally lets ancestor cover men and women alike.
Sisters and leftovers
Cognates keep the Latin skeleton in view. French ancêtre is the direct sister. Catalan keeps ancestre. Spanish and Italian more often reserve forms close to Latin antecessor for a predecessor in office, and they frequently carry kinship with other nouns, such as antepasado or antenato. English itself once used the more Latinate doublet antecessor for an ancestor or a predecessor; that form slipped out of everyday speech and now looks like a specialist leftover.
The useful fact to keep is the first Latin job. A scout went in front of the army. A medieval predecessor went in front in time. English kept the time sense and, for most speakers, narrowed it to blood and species. When biologists speak of a common ancestor of lizards and turtles, they are closer to the Latin than the family album suggests. They are not pointing at a grandmother in a portrait. They are naming the one who went before.