a molecule that can be used as a molecular clock has a neutral rate of one mutation per 12 million years. how many years ago did two species share a common ancestor if the molecules found in these two species differ by a total of six mutations?

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If the neutral rate of mutation for the molecule is one mutation per 12 million years, and the two species differ by a total of six mutations, then we can calculate the time since their common ancestor by multiplying the number of mutations by the rate:

6 mutations * 12 million years/mutation = 72 million years

Therefore, the two species shared a common ancestor approximately 72 million years ago.