According to Halliday, language is differentiated from all other semiotic systems by being constituted by a stratified content plane (the 'signified' of Saussure's sign), such that semantics is realised by lexicogrammar, and it was this stratification of the content plane that created modern humans. Halliday & Matthiessen (2014: 25):
The semiotic means through which this stratification could come about is suggested by the model of metaphor in Systemic Functional Linguistic Theory.
This stratification of the content plane had immense significance in the evolution of the human species – it is not an exaggeration to say that it turned Homo ... into Homo sapiens. It opened up the power of language and in so doing created the modern human brain.
As Halliday & Matthiessen (1999: 288) explain, the deployment of metaphor involves an internal stratification of semantics, in the sense that a lower level token (metaphorical meaning) realises a higher level value (congruent meaning).
By the same token, in the emergence of language from protolanguage, the content plane becomes stratified when a lower level token (lexicogrammatical wording) comes to realise a higher level value (semantic meaning).
It is this prior emergence of a lower level token (lexicogrammar) on the content plane that makes possible the emergence of lower level tokens (metaphors) on the semantic stratum, and it is these lower level tokens on the semantic stratum that enormously expand the meaning potential of language, as demonstrated by the use of lexical metaphor in the reconstruals of meaning in the field of mythology, and the use of grammatical metaphor in the reconstruals of meaning in the fields of science.
By the same token, in the emergence of language from protolanguage, the content plane becomes stratified when a lower level token (lexicogrammatical wording) comes to realise a higher level value (semantic meaning).
It is this prior emergence of a lower level token (lexicogrammar) on the content plane that makes possible the emergence of lower level tokens (metaphors) on the semantic stratum, and it is these lower level tokens on the semantic stratum that enormously expand the meaning potential of language, as demonstrated by the use of lexical metaphor in the reconstruals of meaning in the field of mythology, and the use of grammatical metaphor in the reconstruals of meaning in the fields of science.