Order of authorship in the original paper: Kanki, McLane... Essex. This paper was followed in the issue of Science by: Daniel, M., Letvi, N. L., King, N.W., "Isolation of T-cell tropic HTLV-3 - like retrovirus from macaques", Science, 228 (1985) 1201-1204. 

Discovery of Simian immunodeficiency virus (SIVs), a species of retrovirus that cause peristent infections in "at least 45 species of African non-human primates."

"Virus strains from two of these primate species, SIVsmm in sooty mangabeys and SIVcpz in chimpanzees, are believed to have crossed the species barrier into humans, resulting in HIV-2 and HIV-1 respectively, the two human immunodeficiency viruses. The most likely route of transmission of HIV-1 to humans involves contact with the blood of chimps that are often hunted for bushmeat in Africa.^[3] It is theorized SIV may have previously crossed the species barrier into human hosts multiple times throughout history, but it was not until recently, after the advent of modern transportation and global commuterism, that it finally took hold, spreading beyond localized decimations of a few individuals or single small tribal populations" (Wikipedia).

(Thanks to Juan Weiss for this reference and its interpretation.)