The Horn of Africa has seen considerable human movement, primarily between the Horn and:
You don't need to be a physical anthropologist to see that people of the Horn combine East African and Arabian genes, as well as some from farther north.
The highlands of Eritrea and Ethiopia
1 CE: The region was never part of the Roman Empire, but by dominating shipping through the gate of the Red Sea, it controlled trade between the Roman Mediterranean and the Indian Ocean. Thus, if your ancestors were in the Horn in 500 CE, their ancestors might also have been in southern Arabia or anywhere around the shores of the Red Sea in 1 CE.
First - sixth centuries: The kingdom of Aksum in what is now Eritrea and northwest Ethiopia grew rich on Red Sea trade. Aksum's royal family converted to Christianity during the fourth century. During the sixth century, Aksum briefly conquered and ruled western Yemen. Thus, if your ancestors were in the Horn in 1000 CE, their ancestors might have been in southern Arabia in 500 CE. (Or vice versa.)
Seventh - eight centuries: The rise of Islam both confirmed and fundamentally altered the demographic relationship between Yemen, the Horn, and circum-Indian Ocean trade. Indeed, many of the first generation of Muslims sought refuge in Aksum prior to the Muslims' victory over the Meccans. Soon, however, the founding of Baghdad (eighth century) diverted trade between the Indian Ocean and the West from the Red Sea to the Persian Gulf. Moreover, the incorporation of Egypt and Sudan into the Muslim sphere effectively cut the Ethiopian highlands off from the Christian world. For several centuries, Ethiopia was a backwater.
Eleventh- fifteenth centuries: Trade and human traffic began to return as Christian Ethiopians began to expand southward from their base in northwestern Ethiopia into the pagan highlands of southern Ethiopia. Many captives taken in these campaigns, primarily women, were sold as slaves in Yemen. As conquest proceeded, Christian governors and missionaries moved south, adding their genes. For the first time, northern and southern Ethiopia and Eritrea formed a major meling pot. Thus, if your ancestors were in Yemen in 1490 CE, their ancestors might have been in southern Ethiopia in 1000 CE (But genes flowed both ways. See below). If they were in southern Ethiopia in 1490 CE, their ancestors could have been in northern Ethiopia and Eritrea in 1000 CE. Ironically, some of the Ethiopian conquests overran outpost Muslim communities founded by Arab pioneer-traders trying to get in on the trade in southern Ethiopian pagan slaves and forest products. Things looked bad for Muslims in the Horn of Africa until.....
Fourteenth century: The semi-nomadic Somali nation embraced Islam. With this infusion of manpower and the help of capable leaders, Muslims of the Horn held their own against Christian Ethiopia through a series of destructive wars. By 1490 CE, the Horn of Africa was divided in half along a north-south line with Christian Ethiopia to the west and Muslim Somalia to the east. In terms of gene flow, the net beneficiary was Yemen, which received captives taken by both sides as slaves. The Horn, itself, became underpopulated, an invitation for visitors from.....
Somalia, Djibouti, and the Harrar Plateau
Sixteenth century: The Oromos, nomadic pastoralists originating in today's northwestern and central Kenya began migrating into the demographic vacuum of southern Ethiopia and Somalia. This was not a military invasion, but rather a piece-meal opportunistic expansion. The highlands of Ethiopia suited their way of life, and by 1600 they were the majority in the southern half of Ethiopia. Depending on where they settled, they became either Christians or Muslims. During this time, central government in Ethiopia and Somalia collapsed. Thus, if your ancestors were Oromos in the Horn of Africa in 1800 CE, their ancestors would have also lived in Northern Kenya in 1000 CE .
At the same time, modern Eritrea became a possession of the Ottoman Empire. This fact facilitated trade in (you guessed it) slaves - this time to Egypt and the Eastern Mediterranean, and may have added the genes of some Ottoman soldiers and administrators to the local mix. Remember, the Ottomans were known for conscripting boys from Christian portions of the empire and training them for duty anywhere. Thus, if your ancestors were in Eritrea in 1800 CE, their ancestors would have also lived anywhere in the Ottoman Empire in 1490 CE .
Nineteenth century: What's remarkable about this century is what didn't happen - Thanks to able leadership, Ethiopia was not conquered by foreign powers, despite attempts by Egypt (1850s) Britain (1868) and Italy (1896). No such luck for Eritrea (colonized by Italy) and Somalia (Britain and Italy). Thus, if your ancestors were in Somalia or Eritrea in 1900, their ancestors might also have lived in Italy or Britain in 1800 CE. Trust your family folklore and the evidence of your senses for guidance.