Ancestry

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 from the past. 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 2^g+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 who have ever lived.

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.

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