What if you wanted to do a genealogy
Why would you want to do that?
Ancestry, particularly
patrilineal ("agnatic") ancestry, was very important in many ancient societies. For example in ancient Ireland if a man injured another, he was expected to provide financial compensation to ALL the members of the victim's
dearbhfhine, i.e. even 2nd cousins. IIUC the Irish genealogies were generally memorized rather than written down. (A small number of societies -- Picts, Etruscans, and possibly some groups in Eastern Siberia -- focused on
matrilineal genealogies rather than patrilineal.)
Terms like "Tribe" or "Dynasty" (or even "Race") were often used to denote the (putative) agnates of a particular founder. For example, EVERY King of France since 987 AD was an agnate (at least if official pedigrees are believed) of Hugh Capet. Royal lineages 1000 years old or so are mostly correct (ignoring occasional undocumented cuckoldings). 2000 year-old genealogies should be viewed with severe skepticism.
With DNA testing, agnatic relationship is easily discerned via the Y-chromosome. Some of the of the results may be surprising. It is known, for example, that a large majority of Western European males have the Y-chromosome of a single man who lived about 5000 yeas ago.