- Simian retroviruses pose a serious threat to public health, as two human pathogenic retroviruses, HIV and HTLV, have been already proved to originate from such non-human viruses. Therefore, studying their natural prevalence among wild non-human primates is important for planning strategies to prevent the emergence of additional human retroviral pathogens. This article is focused on tracing the origin and evolution of the human T-cell leukemia viruses HTLV-I and HTLV-II in comparison to that of the simian lymphotropic viruses STLV-I, STLV-II and STLV-L, which are phylo-genically classified into a common group called primate T-lymphotropic viruses (PTLV). Thus, HTLV-I and STLV-I are referred to as PTLV-I and HTLV-II and STLV-II as PTLV-II, whereas STLV-L, which is highly divergent from both HTLV types, comprises a third subgroup called PTLV-L. The phylogeny of PTLV indicates that both, HTLV-I and HTLV-II emerged from a simian origin, but their subsequent evolution continued in different patterns. HTLV-I includes 6 subtypes which evolved from STLV-I through several times of different geographic interspecies transmission between simian and human hosts. These repeated invasions to new primate species are likely to give rise to viral strains with increasing pathogenic potential. On the other hand, HTLV-II includes 4 subtypes which appear to originate from a common human ancestor virus that emerged from only one simian to human transmission, whereas the subsequent evolution of HTLV-II and STLV-II strains continued separately only within the Homo sapiens and Pan paniscus species respectively, without repeated interspecies jumps. Such evolution pattern likely involves less genetic changes and selection of viral strains with low pathogenic virulence that could co-exist with their hosts for long time. These different evolution patterns can explain the much wider implication of HTLV-I with human clinical disorders than HTLV-II. Of note, however, more recently HTLV-II started spreading much more rapidly through intravenous drug users to many geographical regions, with a 150-350 fold higher mutation rate than that of its previous strictly endemic strains. This change in the mode of the virus spread creates a serious risk for emergence of HTLV-II strains with higher virulence.