It Seems no body knows who first sang this song...
Bonney M, Mariam Makeba and many others have Sang their versions.
Royalties accrued from the song are shared amoung the first set of local musicals from Kenya and Tanzania who first sang the song.
It has been said the tune was brought to Kenya by returning members of the entertainment regiment of the Kings African Rifles that was recruited to entertain African soldiers serving in Burma during the Second World War.
I think its a folk song transmitted orally over generations.