According to Vennemann, Afroasiatic seafarers settled the European Atlantic coast and are to be associated with the European Megalithic Culture. They left a superstratum in the Germanic languages and a substratum in the development of Insular Celtic. He claims that "Atlantic" (Semitic or Semitidic) speakers founded coastal colonies beginning in the fifth millennium BC. Thus "Atlantic" influenced the lexicon and structure of Germanic and the structure of Insular Celtic. According to Vennemann, migrating Indo-European speakers encountered non-IE speakers in northern Europe who had already named rivers, mountains and settlements in a language he called "Vasconic". He considered that there were toponyms on the Atlantic coast that were neither Vasconic nor Indo-European. These he considers derive from languages related to the Mediterranean Hamito-Semitic group.
Vennemann bases his theory on the claim that Germanic words without cognates in other Indo-European languages very often belong to semantic fields that are typical for loanwords from a superstratum language, such as warfare, law and communal life. Likewise, he proposes Semitic etymologies for words of unknown or disputed origin; for instance he relates the word bee to Egyptian bj-t or the name Éire, older *īwerijū to *ʔj-wrʔ(m), 'island (of) copper', as in Akkadian weriʔum 'copper'.
Other evidences he adduces for a Semitic superstratum are a Semitic influence on the Germanic form of the Indo-European ablaut system and similarities between Germanic paganism and Mesopotamian mythology, for instance the parallelism between Freyja and Ishtar, goddesses of war and love.
The idea that there is a connection between Insular Celtic and Afroasiatic goes back to John Davies (1632). It was expanded by John Morris-Jones in 1913 and developed further by Vennemann. This position is supported by Pokorny (1927-49) and Vennemann identifies Phoenicians as the likely people. A key factor is the dominant word order in Insular Celtic compared to other IE languages, together with lexical correspondances. Another important factor is the identification of the people later known as Picts. Venemann holds the position that they spoke an Atlantic language. This belief was also held by Zimmer (1898) but is not generally accepted.
Hayim Y. Sheynin, adjunct professor of Jewish Literature at Gratz College, critically reviewed the work Europa Vasconica - Europa Semitica (2003) in which Vennemann lays out his arguments for the existence of a Semitic (or "Semitidic") superstatum in the Germanic languages. He concludes that Vennemann’s arguments are unacceptable on several grounds. He notes that Vennemann bases important parts of his main claim on long-outdated and critically rejected literature, that many of the words presented by Vennemann as evidence of an Atlantic (Semitidic) superstratum display nothing more than "mere ad hoc sound similarities", and that Vennemann’s claims made in reference to Semitic range from "objectionable" to "ridiculous". In summary, Sheynin concludes "that (Vennemann) failed in this book not only as comparative linguist, or etymologist, but even in his narrow specialization as a Germanist. ... In short, we consider the book a complete failure."
The book has also been reviewed by Baldi and Page (Lingua 116, 2006). They too are critical of his German part of the theory. There are no Phoenician inscriptions in Britain though traders may have visited the island so the Insular Celtic part of the theory depends on linguistic evidence. The period of the fifth millennium is very early for Celtic speakers in Britain compared with other theories, for example Mallory suggests a date around 1000 BC though more recently a third or fourth millennium date has been suggested by Gray and Atkinson (and more controversially by Forster and Toth). Vennemann's view of the establishment of megaliths is not supported by mainstream archaeologists who view their construction as having a local origin. Eska (1994) argues that the change from V-noninitial word order in Continental Celtic to V-initial in Insular Celtic is internally motivated. Baldi and Page say that the strength of Vennemann' proposals lies in his lexical arguments and that these merit serious consideration. The origin of the Picts is unknown, see discussions by Jackson and by Wainright as well as those by Kitson and by Forsyth.