From my research, I have discovered that the ancestry of the Beja and Agaw are derived from two different people than the Somali and Oromo.
The Beja are descended from Pinedjem I, the Libu Viceroy of Kush for the 21st Dynasty of Egypt who took control after the civil war in 1088 BCE. When the Libu 21st Dynasty of Egypt were deposed by the Meshwesh (another Libyan tribe) 22nd Dynasty, Pinedjem's descendents remained in control of the red sea coast and became the Beja. The reason why Beja is a "Cushite" languages are similar that both Kush and Libu are ancient Libyan tribes.
The Agaw are descended from Moses because, during Moses' tenure as Viceroy of Kush, he founded a viceroyal dynasty when he deposed the king of Kush and married his daughter Helena. Moses and Helena had a son named Amenhotep (Awawa or Agaua in ancient Libyan). His son Paser I and grandson Amenemopet ruled as viceroys of Kush for the Egyptian Pharaohs, but Amenemopet rebelled against Seti I and was defeated.
The Lowland Cushites (Sidamo, Oromo, Somali etc) are, believe it or not, the actual monarchy of Ethiopia. Kandake Makeda Nicaule, a royal Kushite princess is the Queen of Sheba, or rather Soba, a ruined city near Khartoum, was the one who visited Solomon and gave birth to Menelik I. Menelik's descendant, Alara, conquered the Agaw and Beja and made them subjects of Kush, thus reunifying Sudan. Alara's grandson Kashta and great-grandson Piye managed through clever politics to conquer a divided Egypt around 750 BCE.
Kashta's descendents then ruled Egypt until the Assyrians invaded in 671 BCE, driving Pharaoh Tantamani south out of Egypt and a pro-Assyrian Egyptian dynasty put in their place, though the Kushites continued to rule Thebes until 653 BCE.
After this the Kushites began to suffer under two different incursions. Initially the Kushites moved south to their former capital at Napata but the Egyptians attacked and sacked Napata, thus spurring a move to Meroe in 591 BCE by Emperor Aspelta.
The Beja broke away from Kush in 100 CE and conquered the first to the fifth cataracts and the Agaw broke away from Kush to establish kingdoms in the Ethiopian highlands, parts of which were part of Kush, thus reaching the nadir of the Kushite Empire's power for over a century until Emperor Gadarat in 221 CE. The Beja allied with the Roman Empire and began to expand at the expense of Kush, as well as expansion of the Nuba people (a Nilote tribe from Darfur and Kordofan) who took all of Kush's land up to the Nile.
Gadarat conquered the Ge'ez (the descendants of the Qahtani immigrants who settled in Ethiopia in 2500 BCE) and the Agaw, and then sailed across the sea and conquered parts of Yemen, resuscitating the Kushite Empire. Gadarat moved his capital to the rich Ge'ez lands, to Axum.
In 297 CE the Roman Emperor Diocletian switched allies to the Nuba against the Beja, and the Nuba crossed the Nile and captured the land from the fifth to third cataracts, making the Nuba the aggressive neighbors of Kush. A few years later the Nuba invaded Kush and captured Soba. Mass exodus of Kushites from Sudan into Ethiopia occurs.
Around 330 CE the greatest Kushite emperor, Ezana, stopped the advance of the Nuba at Meroe and soundly defeated them (though Meroe was by this time almost uninhabited). Ezana pragmatically reduced the Nilote tribes including the Nuba to vassalage. Ezana then sailed across to Yemen to further the conquests of his ancestor Gadarat.
in 615 CE Muhammed took refuge in Axum with the help of Emperor Ashama, recorded in the Qu'ran as al-Negashi. Oromo is descended from al-Negashi, who is recorded in some genealogies as Ram Nag.
I believe that in the following century, the royal family of Axum converted to Islam.
In 867 CE Yodit, a Jewish queen of the Agaw, attacked Axum and sacked it. I believe that at this time religious and ethnic chaos erupted in the Axumite Empire. The Christian Ge'ez, Kushites and Agaw attacked the Muslim Ge'ez and Kushites and Jewish Agaw and Falashas and all those who adhered to the old religion.
The traditionalist Kushites who adhered to Kandake Makeda's religion became the Oromo. The Christian Kushites who rebelled against their king became the Amhara and increasingly Ge'ezized, while the "original" Ge'ez became the Tigray. The only legitimate bloodline with "Royal" blood that could be crowned were the royal Agaw lineage, the Zagwe, descended from Moses, as the Cushite royalty had moved to Shoa.
The Islamic Cushites however, the royal family, became the Somali. The last Emperor of Axum founds the Makhzumi sultanate of Shoa in the southern highlands.
The Church however considered this a dire threat. The legitimate, Islamic dynasty could find support amongst many peoples and overthrow the new Christian kingdom, which under the Zagwe was weak. And so was concocted the Great Lie.
A secret descendant of Menelik was "discovered" in the king of Tigray, who was given the crown of Axum by the church. The fake Solomonid dynasty then destroyed the Sultanate of Shoa with many Arabian mercenaries. The Arabian-led Sultanate of Ifat was then put in Shoa's place, which was supposed to be an obedient vassal of the Solomonids.
The Somali migrated east into their current lands.
Darood actually arrived in East Africa while it was still the Islamic Axumite Empire, thus his nickname "Jabarti". Darood became a Somali clan while the Somali still lived in the Ethiopian Highlands.
