contradicts nor confirms the hypothesis that the divergence of ancestral Semitic from Afroasiatic occurred in Africa. Christopher Ehret has hypothesized that genetic analyses (specifically those of Y chromosome phylogeography and TaqI 49a,f haplotypes) shows populations of proto-Semitic speakers may have moved from the Horn of Africa or southeastern Sahara northwards to the Nile Valley, northwest Africa, the Levant, and Aegean. North Africa hypothesis Edward Lipiński believes that support for an African origin is provided by what he describes as a possible relationship between an early Afroasiatic language and the Niger–Congo languages, whose Urheimat probably lies in Nigeria–Cameroon. According to this theory, the earliest wave of Semitic speakers entered the Fertile Crescent via Israel and Syria and eventually founded the Akkadian Empire. Their relatives, the Amorites, followed them and settled Syria before 2500 BC. Late Bronze Age collapse in Israel led the southern Semites southwards, where they reached the highlands of Yemen after the 20th century BC. Those crossed back to the Horn of Africa between 1500 and 500 BC. Phonology Vowels Proto-Semitic had a simple vowel system, with three qualities *a, *i, *u, and phonemic vowel length, conventionally indicated by a macron: *ā, *ī, *ū. This system is preserved in Akkadian, Ugaritic and Classical Arabic. Consonants The reconstruction of Proto-Semitic was originally based primarily on Arabic, whose phonology and morphology (particularly in Classical Arabic) is extremely conservative, and which preserves as contrastive 28 out of the evident 29 consonantal phonemes. Thus, the phonemic inventory of reconstructed Proto-Semitic is very similar to that of Arabic, with only one phoneme fewer in Arabic than in reconstructed Proto-Semitic, with and merging into Arabic and becoming Arabic . As such, Proto-Semitic is generally reconstructed as having the following phonemes (as usually transcribed in Semitology): The reconstructed phonemes *s *z *ṣ *ś *ṣ́ *ṯ̣, which are shown to be phonetically affricates in the table above, may also be interpreted as fricatives (), as discussed below. This was the traditional reconstruction and is reflected in the choice of signs. The Proto-Semitic consonant system is based on triads of related voiceless, voiced and "emphatic" consonants. Five such triads are reconstructed in Proto-Semitic: Dental stops *d *t *ṭ Velar stops *g *k *ḳ (normally written *g *k *q) Dental sibilants *z *s *ṣ Interdental (written *ḏ *ṯ *ṯ̣) Lateral (normally written *l *ś *ṣ́) The probable phonetic realization of most consonants is straightforward and is indicated in the