Ambel is a tonal Austronesian (South Halmahera-West New Guinea)language, spoken to the west of New Guinea. There are two dialects of Ambel: Metsam and Metnyo. In this paper, the segments and tones of proto-Ambel monosyllables are reconstructed. Proto-Ambel had two tones, *High and *Rise; toneless monosyllables are also reconstructed. The tonal phonology of Metsam Ambel is identical with the proto Ambel system. The tone system of Metnyo Ambel, however, has undergone two innovations: an unconditioned merger of *Rise and toneless syllables; and a primary split affecting proto-Ambel toneless syllables. Notably, this latter change was conditioned by vowel height: toneless monosyllables with high vowel nuclei (*i or *u) remained toneless, while those with non-high vowel nuclei (*e, *a, or *o) merged with *High tone. The diachronic development of High tone on non-high vowels has not previously been attested. The possible mechanisms that caused this split will therefore be given special attention. Based on experimental phonetic evidence, it will be argued that the split was caused by the neutralisation, in low-pitched contexts, of the intrinsic differences in fundamental frequency found between high and low vowels; combined with the utilisation of Intrinsic Pitch, an auditory mechanism used to compensate for intrinsic f0 differences.