Disco appeared in the 70s when affros were hip. Disco is a kind of dance that's usually done without a partner on a dance floor that lights up to the music. There are many quick beats in disco which makes for a particular dancing style that involves both feet and arms. The dancer usually wears white clothes and platform shoes. When it's a man who's dancing, his shirt's collar is open to show off some manly chest hair. But this dancer would be nothing without the all important afro.
Disco is also a lifestyle that grew and expanded during the 1970s. After the 60s, many things in the world had changed: evolution of woman's rights, evolution of music towards a more synthesized form, economic prosperity... People started letting themselves go a little more and the notion of "cool" appeared. All this contributed to the appearance of disco.