From my research into the history of the Somali people, I have come to several conclusions.
The Somali people are descended primarily from two groups of people, two related and now forgotten people the Saamaalo and Orma, and the Arabs that conquered them around 700 C.E. (or about 50 A.H.).
The Saamaalo today are split. Half of the Saamaalo have joined the Boorana confederation (Boorana means east-facing) and today their descendants make up the Tuulama Boorana Oromo (Jarso, Dacci, Kono, Jille, etc). The other half were conquered by the Caliphate and today make up the Irir and Gardheere Samaale (Dir, Hawiye, Rendille etc)
The Orma are also split today. Some Orma remained simply Orma, others joined the Boorana confederation and make up most of the "Sabbo" moeity. Others were conquered by the Caliphate and today make up the Digil and Mirifle people.
Both the Orma and the Saamaalo in the pre-Islamic traditions claim descent from their ancestor, Horo, or Horu, thus making them "Horomo". When the various members of the Boorana and Barentuma confederations were trying to forge an identity for themselves seperate from the "Galla" name assigned by their various enemies and oppressors, they chose "Oromo", due to these common legends.
It is my belief that Horo or Horu is none other than Heru, otherwise known as Horus, who has been identified by several scholars with Cush, who in Biblical and Qu'ranic tradition is the son of Ham.
The Arabian conquerers arrived in Eritrea and Mogadishu about 700 C.E. and established several lineages, such as the Jeberti. In successive waves the conquerors expanded to include much of the horn of Africa in the Caliphate by 800 C.E. When the Abbasid Caliphate collapsed in around 900 C.E., many regions re-asserted their traditional independence from the Caliphate, such as the Argobba in Shewa and the Hadiya and related people around Harar. Arabian lineages dating back to the first invasion, such as the Jeberti established themselves throughout the horn.
After this date, more reliable history begins and you don't need me to tell you what happened after this.
