How come scientist say that horn of Africans are a distinct biological group in Africa?

I thought they were native and related to other black Africans. Not including (North Africa). How come scientist say this? I saw this on wikipedia.
Cal king if east African share more in common with people in North Africa and the Middle east, then how come the majority of them have dark skin and Afro textured hair compared to people in the middle east. How do scientist find out stuff like this?

---

Orignal From: How come scientist say that horn of Africans are a distinct biological group in Africa?