Humans and chimpanzees diverged about 6 million years ago.If 20 nucleotide substitutions have occurred between human and chimpanzee lineages in a sequence of mitochondrial DNA, and 30 have occurred between humans and gorillas, then, according to the molecular clock hypothesis, humans and gorillas separated _______ million years ago.

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The molecular clock hypothesis assumes a constant rate of nucleotide substitution over time. If we know the number of substitutions between two species, we can estimate the time since their last common ancestor by dividing the number of differences by the rate of substitution per unit time. Using this method, if 20 nucleotide substitutions have occurred between humans and chimpanzees, and 30 have occurred between humans and gorillas, we can calculate the rate of substitution as (20+30)/2=25 substitutions per 6 million years. Therefore, the number of substitutions expected between humans and gorillas will be 25*(time since their separation). Solving for time, we get time=30/25=1.2 million years since their separation. Therefore, humans and gorillas separated 6+1.2=7.2 million years ago.

Humans and Chimpanzees Diverged About 6 Million Years Ago