Table 2. Bayesian divergence date estimates in mya (C denotes calibration points). Homo/Pan (C1) Hominoidea/Cercopithecoidea (C3) Colobus/Cercopithecinae Chlorocebus/Papionini Macaca/other Papionini Macaca mulatta/M. sylvanus (Mandrillus+Cercocebus)/(Lophocebus+Theropithecus+Papio+Rungwecebus) Mandrillus/Cercocebus Lophocebus/(Theropithecus+Papio+Rungwecebus) Theropithecus/(Papio+Rungwecebus) (C2) northern Papio (N)/(southern Papio+Rungwecebus) (S) western (W)/eastern (E) Papio P. ursinus (PU5,PU6) (S1)/other southern lineages (S2,S3) P. cynocephalus (PC6,PC7) (S2)/other southern lineages (S3) Rungwecebus/P. cynocephalus (haplotype PC4) doi:10.1371/journal.pone.0004859.t002 baboons living in forests in other parts of Africa (e.g. olive baboons in eastern and north-eastern Democratic Republic of Congo) did not adapt specifically to a more arboreal life [19]. A more likely explanation is that Rungwecebus is the result of introgressive hybridization. Hybridization and introgression events are well known for African papionins including baboons (within Papio [20]; between the genera Theropithecus and Papio, [21,22]; reviewed by [23]). In fact, Figure 2 shows multiple para- and polyphylies of baboon species, which is consistent with previously- described introgressive hybridization and nuclear swamping events among various baboon lineages [e.g. 9–11, 24–28]. For a broader view of Papio phylogeny see [9–11]. Hybridization and introgression have been considered impor- tant in the generation of plant diversity, and an appreciation of their role in the evolutionary diversification of animals has been growing over the past decade [29–32]. Besides papionins, additional examples of hybridization between other primate species, and even genera, are well documented for a number of clades [reviewed in 23,33]. Introgressive hybridization appears to have likely played a role in the evolution of hominoids, including Homo sapiens [23,30,33–36]. There is also evidence that Trachy- pithecus pileatus, Macaca arctoides and Macaca munzala are the products of hybridization or introgression [37–39]. In the case of the kipunji, introgression of the maternally- inherited mitochondrial DNA from baboons into Rungwecebus seems to be likely. A possible scenario that would explain the introgression detected by our study is that a small population of baboons became isolated within the range of kipunjis resulting in female baboons reproducing with male kipunjis until 0.35 (0.09– 0.67) mya. As mentioned above, repeated shifts in the extent of forest and savannah habitats during the Pleistocene may have promoted the isolation of local baboon populations. Given backcrossing of the hybrid offspring with kipunji or proto-kipunji males, over several generations, the frequency of baboon nuclear genes within the hybrid population would have decreased sharply. The result of such a process would have been a population with an almost complete kipunji nuclear genome, but with a baboon mitochondrial genome. This is what we have detected in our analyses. Since morphological characteristics would most likely be Phylogeny of Rungwecebus mean 6.48 23.70 14.26 10.20 9.18 5.86 8.47 3.73 4.95 3.75 2.19 1.69 2.02 1.36 0.35 95% credibility interval 6.10–6.87 21.65–25.55 11.14–18.12 9.27–11.09 7.71–10.48 3.59–8.00 6.98–10.00 2.09–5.68 3.79–6.15 3.20–4.28 1.50–2.98 1.04–2.41 1.37–2.79 0.82–2.01 0.09–0.67 determined by nuclear genes, members of such a population would be expected to resemble kipunjis rather than baboons. Based on our findings, the newly described genus Rungwecebus might be congeneric with baboon species and thus, Rungwecebus would be synonymous with Papio. Alternatively, and we would argue more likely, Rungwecebus represents a sister lineage to Papio and experienced later introgressive hybridization. However, since only a single Rungwecebus individual from the population in the southern highlands was studied, it is not clear whether the entire kipunji population possesses 1) a Papio-like mitochondrial DNA in general or 2) an admixture of Papio-like and undefined Rungwecebus haplotypes. Regardless of whether or not the detected mitochon- drial haplotype of Rungwecebus turns out to be characteristic for only the examined specimen (or for only a subsample of the individuals belonging to this species) the evolution of Rungwecebus has been reticulate. Defining the extent of the reticulate (i.e. introgressive hybridization) events will require further molecular studies that incorporate more individuals and additional mito- chondrial and nuclear loci. Materials and Methods Ethics statement Our work was conducted according to relevant German and international guidelines, including countries where we collected fecal samples. Fecal samples were collected in a non-invasive way without disturbing, threatening or harming the animals. Blood samples were taken from zoo animals by zoo veterinarians for diagnostic reasons to check the health status of the respective individuals. Blood samples were explicitly not taken for our study. Sample collection and preservation In the course of a study on baboon phylogeography [10,11,28] fecal samples from baboons were collected during field surveys from several sites throughout Africa (Figure 1, Tables S1, S2). To ensure that the fecal samples were fresh, of baboon origin, and individually assignable, they were collected directly after baboon individuals were observed to defecate. Samples were preserved following the two-step storage method [40]. Accordingly, fecal PLoS ONE | www.plosone.org 5 March 2009 | Volume 4 | Issue 3 | e4859