Note by the first paragraph: In the centuries after the time of the Periplus, Himyar (East Africa's main trading partner) concurs most of present day Yemen. Ethiopia grows out to be a world power. However trade with the East-African coast shifts towards the Persian Gulf, (300-650AD) where a new power, the Sassanians established themselves. Archeological evidence of this is plentiful in the potsherds of the Sassanians found all over East Africa. I presume that the exported rhinos where african, but this is maybe mistaken as there was a local variety of rhinos in the marshes of Iraq, the precise date that it got extinct is unknown.

 

Note by the second paragraph: What this text really tells us is that these envoys bring east African exports (including the horn of Africa) to China. This statement might look strange, except if you take into account that Aden was for several centuries a Roman colony. (It had been an Egyptian colony for several centuries before that). Out of Egypt the Romans had set out to conquer Arabia, and although successful at first the lack of water drove them back. As a result however, most south Arabian princes had to regard the Romans as their overlords. (As is attested in the Periplus). This position of power extended also to the African continent, where the Romans had inherited several towns in Somalia and Eritrea from the Hellenists. It seems normal that any harbor till South Somalia would recognize the Romans as their overlords.

 

The so called envoys to China, well many authors think they were just a bunch of  Greek speaking merchants from Syria, trading with the East African coast, and pretending to be representatives of Rome. Because the tribute they brought really was not at all special, and really did not represent what came from the Roman Empire, but were the raw products that were exported from East Africa. That they came from Syria we know because the description they gave the Emperor is not a description of Italy or Rome, but of Syria. Because they reached China through the commander in Vietnam we know that at least that far they must have traveled by boat.  They were not the first to bring African ivory to China. The grave of king Zhao Mo of Nanyue (122BC) contains African Ivory.   

 

In 226 AD and 281 AD other Roman envoys arrived in China. This time even the official Chinese documents (for 226 AD) call them merchants.

 

You will find the same story of the envoys (of 166AD) repeated by Ma Tuan Lin, Chao Ju Kua, and others.