In recent years, certain eminent authorities have advanced the theory that the “Celtic” language developed in the Atlantic region of Western Europe and subsequently spread eastwards. This theory is opposed to the generally held view that the Celtic language developed in Central Europe and expanded from there.
The latter view is entirely correct, but the other view is not entirely wrong. In fact, the Atlantic region of Western Europe was the area in which the Celtoid (rather than Celtic) language developed.
My view is that the Insular “Celtic” languages are an offshoot of Celtic rather than being Celtic; I use the term Celtoid rather than Celtic for these Insular “Celtic” languages. Given the eccentric particularities of the Celtoid languages (VSO word order, initial consonant mutations, conjugated prepositions, etc.) which are not found in any other Indo-European language, it is simply bizarre to believe that these languages could belong in the same group as the Celtic language which was centered in Central Europe between the relatively typical Indo-European Italic and Germanic languages.
Some researchers have tried their best to prove some kind of linguistic unity between Celtic and the Celtoid languages, even going so far as to find Insular “Celtic” features in Gaulish texts that no one really understands properly. I can understand that this mission is dear to the hearts (for some reason), but it’s wrong.
Speaking of a lack of unity, comparing Gaelic and Brittonic reveals many more differences than similarities, particularly in terms of vocabulary. Surely these two can’t equally be derived from Proto-Celtic in the same way that East Germanic, North Germanic and West Germanic are derived from Proto-Germanic.
By the way, it is important to bear in mind that only the “Continental Celts” were called Celts in Antiquity. No ancient writer ever called the peoples living in the British Isles “Celts”. In fact, it was only in the Early Modern Era that the Gaelic and Brittonic peoples started calling themselves “Celts”.
Here’s how I see the development of the Celtic and Celtoid languages:
The origins of the Celts are in the Yamnaya Culture which dominated the Pontic Steppe between c3300 and c2600 BC. During the latter part of this period, groups of Yamnaya people migrated into the lower Danube River valley and followed the Danube into Central Europe. The establishment of these Yamnaya ancestors of the Celts in Central Europe probably dates to the time of the early Bronze Age Bell Beaker Culture which existed between c2800 and c2300 BC.
The origins of the Bell Beaker Culture are debated, but it covered most of Western and Central Europe. After their arrival in Central Europe, the Yamnaya ancestors of the Celts apparently adopted the Bell Beaker Culture. This adoption of the Bell Beaker Culture marks the beginning of the Celts.
Sometime before 2000 BC, a portion of the earliest Celts of Central Europe migrated westwards. They first migrated into the present-day Netherlands – producing the Hilversum Culture. Further movements westwards brought these earliest Celts into Great Britain – producing the Wessex Culture – and further into Ireland, while others migrated into the Northwest corner of modern France – a region that was called Aremorica (are-mori “by the sea”) two thousand years ago, and which includes Brittany and adjacent areas.
The men among these migrants carried the R1b-P312 Y-DNA haplotype. After their migration, the R1b-L21 haplotype developed among them. This is by far the most common Y-DNA haplotype among modern speakers of the Celtoid languages. Meanwhile, the R1b-U152 Y-DNA haplotype developed among the Celts staying in Central Europe who developed the Unetice Culture (as well as the nearby Italic peoples who were then settled in the area of modern Hungary).
The language that the R1b-L21 folks spoke was originally part of the Proto-Celtic language. But as their language developed among the Atlantic peoples of Western Europe in relative isolation from the Proto-Celtic core territory in Central Europe, it soon diverged significantly from Proto-Celtic. It may well be that the eccentric particularities of the Celtoid language came from the Atlantic peoples of Western Europe among which the R1b-L21 migrants established themselves.
By the middle of the second millennium BC, there were probably two distinct descendants of the Proto-Celtic language: the Celtic language of Central Europe which directly continued Proto-Celtic (successive Tumulus and Urnfield cultures), and the divergent Celtoid language of Western Europe (Atlantic Bronze Age – c1300 to c700 BC). A later offshoot from the Central European Celtic territory (probably during the Urnfield period – c1300 to c750 BC) was the Celtiberian language which established itself in the northwest of Iberia.
The Celtoid language was originally spoken in Aremorica, Great Britain and Ireland, and possibly also in northern France and the Low Countries. It probably also spread southwards up to Gascony, and may even have spread eastwards to some extent, but the Celtoid language on the Continent was probably largely assimilated to the Celtic language as it spread westwards from Central Europe during the Urnfield and Hallstatt periods. Yet, there may have been transitional dialects between Celtic and Celtoid in some areas, especially in Aremorica.
In Great Britain, the influence of Celtic on the Celtoid language during the Hallstatt and LaTène periods (including the kw > p change) produced the Brittonic language. [The change of the kw sound to the p sound in Celtic probably occurred sometime in the first half of the first millennium BC.] This Celtic influence was largely due to the expansion of the Celtic language into southeast Great Britain in the LaTène period. On the other hand, there was little if any influence of the Celtic language on the Celtoid language of Ireland which developed into the Gaelic (or Goidelic) language. This accounts for the important differences between Brittonic and Gaelic.
So, all the modern languages that are called “Celtic” are really Celtoid languages (Gaelic and Brittonic). The Celtic languages, on the other hand, have been extinct for at least one and a half millennia; the last of these probably disappeared by the fifth century AD.
I might add that everything that we call “Celtic culture” in modern times should really be called Celtoid culture, and that much if not most of this culture is a continuation of the cultures of the pre-Celtoid peoples of Atlantic Europe (rather than Indo-European culture).