How can one use molecular DNA as a chronometer?
The mitochondrial chromosome, as I said, is inherited maternally and it is-so far as we can tell at least-not recombined the way the nuclear genes tend to be mixed up and recombined every generation. So it is inherited as a unit. And essentially, then, as these mitochondrial chromosomes are passed down from one generation to the next they begin to diverge, not because they’re mixing up their genes but because of accumulating mutations, so that as time goes on, you might have mitochondrial chromosome of a particular type which is then left to many, many progeny in the future. And as time goes on, these different chromosomes begin to diverge from each other; these different copies of that chromosome begin to diverge from each other. The longer the period of time the greater the divergence. So that’s turned out to be very useful in understanding how human evolution might have happened, because any human group essentially can trace its mitochondrial chromosomes back to one ancestor. So sma
