Short answers, pulled from the story.
An ethnicity is a group of humans who identify with each other based on perceived shared attributes that distinguish them from other groups. Those attributes can include language, culture, ancestry, traditions, religion, history, or shared social treatment. Ethnicities are maintained through long-term endogamy and may carry narrow, broad, or mixed genetic ancestry.
Primordialism holds that ethnic groups are real phenomena whose distinct characteristics have endured since the distant past, rooted in kinship and biological heritage. Constructivism, which gained ground after the 1960s, views ethnic groups as social constructs whose identity is assigned by societal rules. Max Weber argued that ethnic groups were artificial because they rested on a subjective belief in shared community.
The word ethnic derives from the Greek ethnos, through its adjective ethnikos, loaned into Latin as ethnicus. In Early Modern English and until the mid-19th century it meant heathen or pagan, the foreign nations outside the Christian world. The phrase ethnic group was first recorded in 1935 and entered the Oxford English Dictionary in 1972.
Herodotus, writing around 480 BC, named four marks of Greek identity in his account at 8.144.2. They were shared descent or homaimon, shared language or homoglosson, shared sanctuaries and sacrifices, and shared customs or ethea homotropa. Isocrates offered a rival view in his speech Panegyricus, arguing that the name Hellenes suggested an intelligence and shared culture rather than common blood.
Ethnicity theory was put forward by the sociologist Robert E. Park in the 1920s, arguing that race is a social category and only one of several factors determining ethnicity, alongside religion, language, customs, nationality, and political identification. It was based on an assimilation model with four steps: contact, conflict, accommodation, and assimilation. Michael Omi and Howard Winant later challenged it in Racial Formation in the United States.
Africa is the most ethnically and linguistically diverse continent, with over 3,000 ethnic groups and more than 2,000 languages spoken across 54 countries. Those languages belong to families such as Niger-Congo, Afroasiatic, Nilo-Saharan, and Khoisan. India alone has more than 2,000 distinct ethnic groups, and Indonesia has more than 600.