Halliday & Matthiessen (1999: 236-7):
… from the standpoint of the evolution of language, when we say they are the congruent forms we are claiming not merely that they evolved first, but that this is why they evolved. One of the contexts in which grammar came into being — one of its metafunctions — was that of construing human experience; and, as we have seen, the model that emerged was one that construed the continuum of goings-on into taxonomies: taxonomies of parts (meronymic) and taxonomies of kinds (hyponymic). The central construct was that of the ‘figure’; figures could be further constructed into ‘sequences’ and also deconstructed into ‘elements’. How did the grammar construe this hierarchy of phenomena? — as clauses, clauses complexes, and elements in the structure of the clause …