A severe bottleneck in the size of the PV Alu subfamily in the common ancestor of human and gorilla has been used to isolate an Alu source gene. The human PV Alu subfamily consists of about one thousand members which are absent in gorilla and chimpanzee DNA. Exhaustive library screening shows that there are as few as two PV Alus in the gorilla genome. One is gorilla-specific, i.e., absent in the orthologous loci in both human and chimpanzee, suggesting the independent retrotranspositional activity of the PV subfamily in the gorilla lineage. The second of these two gorilla PV Alus is present in both human and chimpanzee DNAs and is the single PV Alu known to precede the radiation of these three species. The orthologous Alu in gibbon DNA resembles the next older Alu subfamily. Thus, this Alu locus is originally templated by a non-PV source gene and acquired characteristic PV sequence variants by mutational drift in situ, consequently becoming the first member and presumptive founder of this PV subfamily. © 1993, Springer-Verlag New York Inc. All rights reserved.