Timing the ancestor of the HIV-1 pandemic strains

HIV-1 sequences were analyzed to estimate the timing of the ancestral seque nce of the main group of HIV-1, the strains responsible for the AIDS pandem ic. Using parallel supercomputers and assuming a constant rate of evolution , we applied maximum-likelihood phylogenetic methods to unprecedented amoun ts of data for this calculation. We validated our approach by correctly est imating the timing of two historically documented points. Using a comprehen sive full-length envelope sequence alignment, we estimated the date of the Last common ancestor of the main group of HIV-1 to be 1931 (1915-41). Analy sis of a gag gene alignment, subregions of envelope including additional se quences, and a method that relaxed the assumption of a strict molecular clo ck also supported these results.