An ancestor or forebear is a parent or (recursively) the parent of an ancestor (i.e., a grandparent, great-grandparent, great-great-grandparent, and so forth). Ancestor is "any person from whom one is descended. In law the person from whom an estate has been inherited."

Two individuals have a genetic relationship if one is the ancestor of the other, or if they share a common ancestor. In evolutionary theory, species which share an evolutionary ancestor are said to be of common descent. However, this concept of ancestry does not apply to some bacteria and other organisms capable of horizontal gene transfer.

Assuming that all of an individual's ancestors are otherwise unrelated to each other, that individual has 2n ancestors in the nth generation before him and a total of about 2g+1 ancestors in the g generations before him. In practice, however, it is clear that the vast majority of ancestors of humans (and indeed any other species) are multiply related (see Pedigree collapse). Consider n = 40: the human species is more than 40 generations old, yet the number 240, approximately 1012 or one trillion, dwarfs the number of humans that have ever lived.

Ignoring the possibility of other inter-relationships (even distant ones) among ancestors, an individual has a total of 2046 ancestors up to the 10th generation, 1024 of which are 10th generation ancestors. With the same assumption, any given person has over a billion 30th generation ancestors (who lived roughly 1000 years ago) and this theoretical number increases past the estimated total population of the world in around AD 1000. (All of these ancestors will have contributed to one's autosomal DNA is concerned: this excludes Y-chromosomal DNA and mitochondrial DNA.)

Some cultures confer reverence to ancestors, both living and dead; in contrast, some more youth-oriented cultural contexts display less veneration of elders. In other cultural contexts, some people seek providence from their deceased ancestors; this practice is sometimes known as ancestor worship or, more accurately, ancestor veneration.

Famous quotes containing the word ancestor:

    I cannot yet begin to understand
    Why we are proud that an ancestor knew
    The crazy Poe, who was not of our kind
    Bats in the belfry that round and round flew
    In vapors not quite wholesome for the mind.
    Allen Tate (1899–1979)

    In the core of God’s abysm,—
    Was a weed of self and schism;
    And ever the Daemonic Love
    Is the ancestor of wars,
    And the parent of remorse.
    Ralph Waldo Emerson (1803–1882)

    We rarely quote nowadays to appeal to authority ... though we quote sometimes to display our sapience and erudition. Some authors we quote against. Some we quote not at all, offering them our scrupulous avoidance, and so make them part of our “white mythology.” Other authors we constantly invoke, chanting their names in cerebral rituals of propitiation or ancestor worship.
    Ihab Hassan (b. 1925)