Laura Spinney traces the remarkable journey of Proto-Indo-European, the reconstructed ancestor of languages spoken by nearly half of humanity. This tongue, never written down, is known only through the painstaking work of linguists comparing Sanskrit, Greek, Latin, Germanic, and Celtic languages to deduce their common source. Spinney follows the trail from the Pontic steppe, where PIE speakers likely lived around 4000 BCE, across continents as their descendants carried their language through migration, trade, and conquest. The book profiles the scholars who developed comparative linguistics and the sometimes bitter disputes about where PIE speakers originated and how their language spread. Spinney explains how bronze-age technologies like the wheel and horse-drawn wagons enabled the vast migrations that brought Indo-European languages to India, Persia, and Western Europe. But she also traces the more recent journeys—medieval monks preserving Latin, British colonizers spreading English, Sanskrit's influence on Indian nationalism. The book makes linguistics accessible while exploring deeper questions about what language reveals about human history and cultural transmission. Readers come away understanding how a single tongue spoken by steppe nomads five millennia ago became the ancestor of English, Hindi, Russian, and Spanish.