Genetic differences between human and chimpanzee include ~50,000 amino acid changes, ~30,000,000 point mutations in non-coding sequences, and millions of insertions, deletions, inversions, genomic rearrangements, and transposable element movements . The vast majority of these genetic changes are neutral  . For neutral mutations, the expected number of generations required to reach fixation in the population is the reciprocal of the mutation rate per site per generation . This holds irrespective of selection at other linked loci, changes in population size, or almost any other conceivable complication . The human mutation rate, μ, is ~10-8 per site per generation . Although μ can be orders of magnitude greater at certain sites, the fact that, as cited above, millions of neutral mutations occurred in the human line since its divergence from the chimpanzee line indicates that at least thousands of mutations occurred at the average rate of 10-8 per site per generation. Since the expected number of generations required for any one of these mutations to reach fixation in the population is the reciprocal of μ, this number equals ~108 generations. Since a generation is ~20 years for humans , the time required for any one of these thousands of neutral mutations to reach fixation is ~109 years. According to the currently-known fossil record, the human line diverged from the chimpanzee line between 6 and 8 million years ago . How is this major discrepancy between population genetics and the fossil record to be reconciled?
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