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Figure 21.12 Dating the Origin of HIV-1 in Human Populations (A) A phylogenetic tree for samples of the main group of HIV-1 virus. The dates indicate the years in which the samples were taken. (For clarity, only a small fraction of the samples that were examined in the original study are shown.) (B) A plot of sample year versus genetic divergence from the common ancestor provided an average rate of divergence, or a molecular clock. (C) The molecular clock was used to date a sample taken in 1959 (as a test of the clock) and to estimate the date of origin of the HIV-1 main group (about 1930). Branch length from a common ancestor represents the average number of substitutions per nucleotide.