HIV-1 was introduced into human populations in the early 20th
century following multiple transmissions of a chimpanzee virus, known as SIVcpz 
. The other, less virulent human lentivirus, HIV-2, resulted from transmissions of SIVsm, a virus found in sooty mangabeys 
. In fact, more than 40 non-human primate species in sub-Saharan Africa are infected with species-specific strains of SIV 
. Known as “natural hosts,” these species either co-evolved with their respective lentivirus or were infected more recently via cross-species transmission from other primates 
. In either case, the association between natural hosts and SIV is thought to be considerably older than that of humans and HIV 
. Natural SIV infections do not generally cause an AIDS-like immunodeficiency in their autologous host species, leading to the hypothesis that the virus-host relationship has evolved towards an apathogenic state 
. However, the age and pathogenic potential of wild SIV infections in diverse primate taxa remain largely uncharacterized.
Initial attempts at calculating the age of SIV using phylogenetics produced widely disparate, but all relatively recent, estimates 
. However, two significant findings have pushed back considerably the age estimates of primate lentiviruses. First, the discovery of a full-length endogenous SIV in the genomes of lemurs indicates that lentiviruses were present in prosimians at least 4 million years ago (MYA) 
. However, SIV is not currently found in prosimians, while it is common in simian primates like OWM. Thus, the age of lentiviral infections in current natural hosts of SIV cannot be addressed by endogenous lentiviruses.
Second, a date to calibrate SIV phylogenetics in OWM was made possible with the identification of SIV strains endemic to the African island of Bioko. Here, each virus found on the island shares ancestry with a mainland virus, and their respective hosts belong to the same genus, demonstrating that lentiviruses have been infecting OWM for tens of thousands of years 
. Nonetheless, the use of viral sequences to establish the age of virus families is problematic because rapid evolution obscures phylogenetic signals, and because many viral lineages have gone extinct in the past 
. On the other hand, because of the process of virus-driven evolution of host innate immunity, it is possible to estimate the true evolutionary age of viruses by tracking and dating the evolution of antiviral genes 
. Ideal candidate genes for this type of analysis are restriction factors, cellular proteins that coordinate the cell-intrinsic innate immune response to virus infections. Moreover, if the virus encodes an antagonist of the restriction factor, and the interactive interface between host and viral factors is known, then evolution at the site(s) of interaction can be used to infer past instances of infection 
The host restriction factor APOBEC3G (A3G) is a cytidine deaminase that restricts lentivirus replication by hypermutating viral DNA and by inhibiting reverse transcription 
. To overcome this block, all known primate lentiviruses encode the accessory protein Vif 
, which links A3G to a cellular E3 ubiquitin ligase complex and accelerates its turnover at the proteasome 
. The early birth and ongoing retention of vif
within all circulating primate lentiviruses 
suggest that antagonism of A3G is crucial to lentivirus spread and survival. Therefore, A3G
is a likely substrate for signatures of lentivirus-driven selection, from which a detailed account of past viral challenges can be reconstructed.
Previously, we studied the co-evolution of A3G
in the setting of natural SIVagm infections in African green monkeys (AGM). We found that the A3G
is subject to recent diversifying selection in wild monkey populations, with single nucleotide polymorphisms (SNPs) encoding charge altering amino acid changes at surfaces targeted by Vif 
. Our data support that these naturally occurring mutations in A3G
were selected to allow evasion of SIVagm Vif proteins, implicating Vif as the selective pressure responsible. Adaptive evolution at the A3G-Vif interface in recently diverged primate populations implies that some modern SIV infections can incur a cost to host fitness, whether it be overt immunodeficiency or more subtle phenotypes that decrease host survivability or fertility 
In the present study, we trace the co-evolution of A3G and Vif through deep evolutionary time using an array of diverse primate species and SIV isolates. Our work allow us to provide a minimum age estimate for simian primate lentivirus infections, as well as an illustration of the dynamic flux of a host-pathogen interaction over time. We find that multiple species of the Old World Monkey (OWM) subfamily Cercopithecinae possess mutations in the Vif interaction site of A3G and that each allows escape from antagonism by Vif proteins. The recurrence and deep ancestry of such mutations suggest that a lentivirus encoding Vif existed at least 5–6 MYA. In response, contemporary Vif proteins have counter-evolved to these various Vif-resistant forms of A3G by tolerating amino acid variation at the canonical Vif interaction site. Moreover, we reveal an even older ancestral insertion event in the N-terminus of A3G of the Colobinae subfamily that conceals the Vif-binding site and precludes interaction with Vif proteins, suggesting that lentiviruses may have infected primates as much as 12 MYA. Coincident with this unique host adaptation, a Vif protein from a lentivirus currently infecting one of the Colobinae species has evolved to recognize a novel surface of A3G. Furthermore, we highlight the adaptability of lentiviral Vif proteins and the possible impact that this evolution may have on cross-species transmission and virus emergence. For example, Vif from a lentivirus infecting sooty mangabeys (SIVsm) and its descendants (SIVmac and HIV-2), exhibit exceptional breadth, possibly explaining in part how SIVsm was able to successfully colonize both humans and macaque species. Together, these data suggest that infections of OWM by primate lentiviruses are older than previous thought, driving selective changes in antiviral genes of their natural hosts and inciting an evolutionary arms race that continues to this day.