The SYNTACTIC head of I is the abstract morpheme [+PAST]. As a tense morpheme, it must be associated with a verb, hence the complement status of VP.
The SEMANTIC head of the sentence is the verb EAT. It is the verb that selects a subject that can indeed perform the action of eating and the Object Apple, which can be eaten. Indeed, a sentence such as : The tree ate the sea perfectly grammatical but it is SEMANTICALLY anomalous, to say the least.
ايش يعني هذا الرمز I