Final answer:
Jomo Kenyatta is the East African writer who published an anthropological treatise of the Gikuyu language in 1938, before becoming a key political figure and Kenya's first President.
Step-by-step explanation:
The East African writer in question is Jomo Kenyatta, who published an anthropological treatise of the Gikuyu language in 1938. This work was part of his broader contributions to East African literature and politics, and it highlighted his commitment to his native Gikuyu culture. Later, Kenyatta became a significant political figure, leading the Kenyan African National Union during the struggle for independence and eventually becoming Kenya's first President after independence. His early work on Gikuyu culture and language, along with his political leadership, cemented his status as a historically significant figure in Kenyan history.