The Frame/Content theory of the evolution of speech production (MacNeilage,

1998) predicts that only modern humans superimpose a continual rhythmic

alternation between an open and a closed mouth-defined as a Frame-on the

sound production process. MacNeilage emphasizes that the question of skill

development in speech production requires some background. An important

issue to note in this perspective is that most work on sound preferences in

babbling and in early words has been done on consonants. But the question of

the emergence and control of these sounds has not been developed as explicitly

as for vowels. Since Lindblom (1986), principles explaining the emergence of

vowels in sound systems of human languages are better understood. Lindblom

and Maddieson (1988) suggested a consonant classification in three levels of

difficulty in terms of the number of separate active subcomponents required.