This paper claims that the drastic change in the English language can be better explained in terms of the emergence of functional categories, assuming that language shifts from a lexical-thematic stage to a functional stage over time. The process of first language acquisition of English children also embodies this shift. Old English is supposed to have lacked functional categories such as DP, TP and CP. Many syntactic phenomena related to these functional categories were absent in Old English. They were made possible later after these functional categories emerged. First language acquisition is also described as the process of acquiring functional categories. This parallelism between ontogeny and phylogeny is attested in the relation between aspect and tense.