From: Kragen Sitaker (kragen@pobox.com)
Date: Wed Nov 01 2000 - 15:38:31 PPET
I have some questions about this.
> http://news.bbc.co.uk/low/english/sci/tech/newsid_999000/999030.stm
>
> The most recent ancestor of all males living today was a man who lived in
> Africa around 59,000 years ago, according to an international team of
> researchers.
>
> The scientists from eight countries have drawn up a genetic family tree of
> mankind by studying variations in the Y chromosome of more than a thousand
> men from different communities around the world.
> . . .
> To find the common paternal ancestor, the team drew up a genetic family tree
> of mankind. They mapped small variations in the Y chromosomes of 1,062 men
> in 22 geographical areas, including Pakistan, India, Cambodia, Laos,
> Australia, New Guinea, America, Mali, Sudan, Ethiopia and Japan.
This part sounds like they've found a common ancestor Y chromosome,
from which all our current Y chromosomes mutated. But the common
ancestor of all our Y chromosomes is not necessarily our most recent
common ancestor. For example, if my mother's niece Karrie had a son,
his Y chromosome would not be closely related to mine, even though his
great-grandfather would be my grandfather. [0] My Y chromosome came
from my father, whose lineage might converge with Karrie's notional
partner only ten or twenty generations back.
So the description of the research makes it sound as if it couldn't
possibly confirm or deny the lead paragraph. Y chromosomes give you
very valuable information (because of the lack of crossover) that can't
possibly hope to support assertions like "the most recent ancestor
lived X years ago" (because of the lack of crossover).
I would be rather surprised if our most recent common ancestor were any
further back than 10,000 years, or 400-1000 generations. I would
expect there to be quite a number of them.
> "You can ultimately trace every female lineage back to a single
> Mitochondrial Eve who lived in Africa about 150,000 years ago," said Dr
> Spencer Wells of the Wellcome Trust Centre for Human Genetics in Oxford, UK,
> who was part of the team.
>
> "The Y chromosome we trace again back to Africa but the date is about 80,000
> years ago.
> . . .
> But the finding raises new questions, not least because our most recent
> paternal ancestor would have been about 84,000 years younger than our
> maternal one.
>
> The team believes there is an explanation. They propose that the human
> genetic blueprint evolved as a mosaic, with different pieces of modern DNA
> emerging and spreading throughout the human population at different times.
> . . .
> He [Wells] told BBC News Online that the two studies could be reconciled.
> "There's a different evolutionary history for each region of the
> genome but they all are consistent in placing the ancestor of all
> modern humans alive today in Africa."
I don't understand what the "new questions" are supposed to be; it
sounds like the reporter just got confused.
I would assert that there are many different people who lived a
thousand or more generations ago who are all ancestors of all modern
humans, living all over the world, whose descendants didn't meet each
other until tens of generations later. It's just that none of them
have had straight male or female bloodlines that have embraced the
whole human population, like the Y-Chromosome Adam and Mitochondrial
Eve, so they're harder to identify.
> Some modern-day men living in what is now Sudan, Ethiopia and southern
> Africa are believed to be the closest living descendants of the first humans
> to set out on that great journey tens of thousands of years ago.
Closest in what sense? Geographically? Genetically? Fewest
generations? Did evolution work slower in the stay-at-homes than in
the foolhardy souls that went to colonize Indonesia?
--
<kragen@pobox.com> Kragen Sitaker <http://www.pobox.com/~kragen/>
Perilous to all of us are the devices of an art deeper than we ourselves
possess.
-- Gandalf the Grey [J.R.R. Tolkien, "Lord of the Rings"]
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