2 Questions about Evolution and Genetics

[DRT23]

I was reading 'The Genetic Ancestor' part of Britannica Illustrated Science Library's Evolution and Genetics Encyclopedia when I was confused by two information. So, I want to ask them here:

1. 'Africa is where the greatest number of mutations is found. This leads to the supposition that humans have lived there the longest.' Which means all humans are originated from the continent of Africa. But, my question is this: They calculate mutations from samples of people who live today. So, a man from Africa and me (for example) should have same amount of mutations (but different mutations) although we live in different continents. Because, my ancestors back to first humans and his ancestors back to first humans are from different lineages, but have passed through the same amount of time. And time is the matter what is linked to number of mutations.

(So, why we think humans are originated in Africa? It's probably because of the sets of mutations every sample person have since the scientists trace these motifs and fills the puzzle.)

2. 'In genetic terms, DNA enables us to conceive of a primordial Adam and Eve, our genetic ancestors. However, the common ancestor of all humans alive today is quite a different matter. Several scientific hypotheses estimate that an ancestor to whom we are all related lived between 1,000 and 10,000 years ago.' How could this be true? This passage made me imagine that all people but only one died suddenly some time between 1,000 and 10,000 years ago and the person who survived is our ancestor (by the way, this person must be hermaphrodite ). It's obvious that they don't want to mention this. So, what am I missing? Could you explain what they aim to mean with this passage?

[Jonl1408]

Here is a link, with a very detailed explanation of DNA, please take the time to read the whole page.
http://www.hauns.com/~DCQu4E5g/DNA.html

[canalon]

Or don't if you value your sight and your sanity.
Your sight because of the choice of color. My screen might be bright, but the webmaster here should rethink his color choice.

Your sanity because of course the author knows some maths, but obviously nothing about genetics or evolution... and assume that every organism has a genome of same size, and that it was created in one pass. My ribs are hurting.

to the OP:
1- To try to make that clear: In the original population mutations accumulate randomly and you will see them over time. Among that popoulation small group are going to emigrate with already a given set of mutations. Since reversion is not very common, what you will see is that as they move away they are going to start accumulating their own muattaions, but at the same time they will mix less of the old one when reproducing, so the pool of muations will be reduced compared to the original population. So over time the original population by mixing more old mutations with the set of new accumulation will end up with more diversity that the populations that branched off.


2- No the idea is more that for each human, there are 2 parents, 4 grand parents, 8 great grand parents etc... Now The population on earth is not exactly reducing. So that mean that when comparing 2 persons genetic tree, the more likely you are to find common ancestors. So that way you could argue that ~10000 year ago there was a small troop of related hominids from which we all descend from.

[DRT23]

Thank you, canalon! Now, I see what the author wants to mean in these sentences.