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Theories of ethnosyntax and evolutionary approaches to language have described certain patterns of many languages partially spoken by today missing features known among the so-called full grammar languages. There are languages without subordinate clauses, embedding, recursion, passive, plural, many tenses, etc. Likewise research found that even the full grammar languages of today have a past where they likewise shared these simpler traits, having gone through several processes of grammaticalization during certain historical periods. More, research found resemblances to language acquisition in ontogeny. During certain stages of language acquisition modern children share traits with those of archaic languages, a fact already known by some fathers of language research. This article here shows that neither universal grammar theory nor evolutionary respectively Darwinian approaches to language can explain these parallels. It is argued that developmental psychology and Piagetian cross-cultural psychology can explain these parallels. Modern children do not recapitulate the history of language, as Haeckel once believed, but conversely, preoperational stage structures in the mind both of children and ancient humans cause the parallels in language and grammar. Therefore, the knowledge of developmental psychology concerning the psychology of the preoperational stage is necessary in order to explain each single pattern of language and grammar on their earlier stages on the one side and the emergence of full grammar language on the other side. The article gives an overview over a wide range of parallels concerning language patterns, coming to the conclusion that developmental psychology delivers the foundations both to linguistics and the description of the history of language.