The historically attested Germanic languages provide evidence to justify the construct "Germanic" when we find in them common innovations not shared by other Indo-European languages. It is possible that the primal seed of the Germanic languages was gently sown when an unknown number of Indo-Europeans started articulating voiceless stops as fricatives (e.g., */t/ > */þ/); later on, hitherto voiced stops lost voicing (*/d/ > */t/); and some time thereafter, aspirated voiced stops came to be realised as voiced fricatives, then later in most Germanic dialects as voiced stops (*/dh/ > */ð/ > */d/). This three-part "chain shift" is known famously as Grimm's Law and marks among the first Germanic innovations -- preserved in all languages of the family, being essential and limited to them. Further sound changes paralleled innovations in intonation, morphology, syntax, and vocabulary, increasingly differentiating Proto-Germanic as a dialect of Proto-Indo-European (PIE). During this time, it seems also to have shared innovations with other, geographically proximate PIE dialects, while they yet remained mutually intelligible.
However, at some point Proto-Germanic had come to assume the character of a new language. In their reconstructions of this unattested language, linguists attempt to arrest it at its most advanced age -- before any split into Germanic tongues -- as the last stage of Proto-Germanic. On the other hand, assigning a linguistic "beginning" to Proto-Germanic is something of a moot point, given its origin as a PIE dialect: where linguists draw the boundary between dialect and language is often a matter of opinion.
Theories regarding the geographic location of Proto-Germanic will remain speculative as long as conflicting evidence from history, linguistics, and archaeology leaves conclusions in doubt. Most proposals locate the language community in northern continental Europe, where the majority of Germanic languages have been spoken throughout recorded history; more specific programmes assign the language variously to south Scandinavia, the north European plain, and elsewhere.
A singular bronze helmet labeled "Negau B," dating to the last few centuries BC, bears an inscription written in the Etruscan alphabet: harikastiteiva. If, as many scholars believe, the first element stands for the name Harigastiz, this inscription would constitute the oldest attestation of a language that is identifiably Germanic. As such, it would further make a strong case for being a surviving example of Proto-Germanic. The Harigastiz element clearly shows that the inscription postdates Grimm's Law (PIE *k > *h) and other common Germanic changes (PIE *o > *a), but the second element teiva would seem to have it predate another development that occurred during the Proto-Germanic period (PIE *ei > *ī). All Germanic languages attested later reflect this change, making the harikastiteiva inscription uniquely primitive in the history of Germanic languages.
Traces of a Germanic tongue are next detected in certain toponyms and personal names recorded in Julius Caesar's De Bello Gallico (1st century BC); but it is uncertain whether his description of the milk-and-meat eating Germani refers to an actual Germanic ethnicity, or to some Celtic tribe -- potentially making "German" a misnomer of ancient date. In any case the Roman author Tacitus likewise gives the name "Germans" to the subject of his ethnographic treatise Germania (ca. AD 98), which continues to shape our conceptions of the early Germanic peoples.
The Raetic (or Etruscan) alphabet that had been borrowed by early Germanic tribes was meanwhile morphing into a distinctly Germanic script. New letters were added to better reflect the Germanic sound system, while the alphabet assumed an angular look as the natural outgrowth of writing on beech bark, an early Germanic practice. Such were the origins of runic writing; whatever was beech-written has vanished along with the beech, but writings on stone, bone and ore have survived from the 2nd century AD, comprising the earliest records of Germanic sentences. As for the "language" of early runic, we may more freely characterize it as a linguistic continuum of the diverse samples that survive. This early runic is considered extremely archaic in light of traits such as verb-final word order; however, effects of linguistic drift from Proto-Germanic are evident even in the first of these records. Given that these inscriptions are essentially limited to Scandinavia, where use of runes thrived in later ages and persisted longest, scholars long assumed that the language reflected in them was solely ancestral to Scandinavian languages. In recent decades a revisionary consensus has emerged that early runic could equally have been ancestral to languages such as English, Dutch, and German; the language is now therefore termed Northwest Germanic, as the plausible common ancestor of the North Germanic and West Germanic languages respectively (see below).
Records of a Germanic language not belonging to the northwest continuum have survived from the 4th century AD. Excepting a certain runic inscription, the Gothic language is first attested as the literary legacy of the Arian bishop Ulfilas (Wulfila). His fourth-century translation of the Bible from Greek into Gothic -- which, notably, predates the Latin Vulgate of Jerome -- inestimably furthered the Christianizing mission to the pagan Goths. Wulfila rendered his work, not in runes, but in a new alphabet based primarily upon that of Greek. Of his complete translation we possess only extensive sections of the New Testament, and little aside from this remains of "Gothic" literature. After centuries of expansion and conquest, the fortunes of the Goths succumbed to disasters that saw their seats in Italy and Spain destroyed in turn by Justinian and the Umayyad Caliphate, making the Goths a historical non-entity in Europe by the mid-eighth century. In the East, a community of Goths apparently survived into the early modern era, dwelling in the region of Crimea. A letter has been preserved from the 16th century recording a brief list of words that seem to be Gothic: tentatively its final attestation.
Philologists have traditionally connected Gothic with two other Germanic languages, Vandalic and Burgundian, forming an East Germanic family. However, the latter are so marginally attested that there is not enough evidence to justify committing them to a particularly close relationship with each other or with Gothic. Regarding any association of the three, we may only say with certainty that they are "not Northwest Germanic."
The later runic inscriptions in Scandinavia reflect changes specific to Old Norse in its earliest attestations, whence derive the modern North Germanic languages of the region. By the High Middle Ages a dialect division had appeared, splitting Old Norse into East Norse, spoken in Sweden and Denmark, and West Norse, spoken in Norway and Iceland after it was colonized in the 9th century. A vast literature of poetry, histories, and sagas has been preserved in Old Norse, most of it composed by Icelanders. Modern North Germanic languages include Swedish, Danish, Norwegian, Faeroese, and the extinct Norn.
Relatively few runic writings were composed in West Germanic languages. North Sea Germanic is a dialect grouping which includes Old English, Old Frisian, and Old Saxon. Old English is attested from the second half of the first millenium AD with a literature comparable in size to that of Old Norse. Old Frisian is most closely related to Old English but is not attested until the 13th century, eventually transitioning into modern Fries. Old Saxon is attested from the 9th century in such works as the Heliand, a Germanic poetic remix of the New Testament. This language interacted with Old Low Franconian (attested from the 10th century) and other dialects in the Low Countries to yield modern Dutch. Finally, Old High German is attested from the 8th century, giving rise to Middle High German by the High Middle Ages and modern German by the Early Modern Era.