Out of Africa

[zool3]

So I'm doing an exam question, multiple choice, and it asks how would the mtDNA evidence support the Out of Africe hypothesis.
And the answer is "variation in mtDNA to be greater in African populations than in population outside of Africa".

Can someone please explain this Thanks!!

[Reckoner]

Because the out of Africa hypothesis is that H. sapiens originated in Africa, the African populations would therefore have been around for longer than populations that emigrated. The emigrated populations would then have had less time for the mtDNA to mutate, hence more variation in the African populations.