By the numbers: Tupi's debut Instagram post garnered 22,000 likes. Now, any content featuring Tupi is a guaranteed hit — like a video of its tiny "ear wiggles," which racked up nearly 650,000 likes.

Over a period of nearly 10 centuries, the native Tupi speakers of South America prospered and dispersed over vast areas of the continent. In a demographic and migratory event that began almost 3,000 ...