Charlie Bell wrote:
On 07/07/2006, at 12:55 AM, David Hobby wrote:
I'm not arguing against a figure of 10,000 years, that's a long time.
Alberto and others were pushing for a much lower figure, around 1000
years, for EVERYONE to share ancestry from some person who lived then.
Were they? I thought the common ancestor, as being pushed by the paper,
was "a few thousand years ago". 4 - 6 thousand. It's a statistical
study, but what it suggests is very plausible.
True, that's what the part Ticia quoted said. But then Alberto
pushed a bit harder:
I am - but I think 2000-3000 is too conservative. I would bet that
_everybody_ descends from Gengis Khan, who lived less than 1000 ya.
The math is too simple: just imagine that "being a descendant of
G-K" is a disease, and that the rate of non-infected people
gets squared at each generation.
Treat semi-isolated groups with care, but once there is contact -
a single outsider f---ing a tribe woman - the group will be doomed
to be "infected" in a few generations.
Alberto Monteiro
Charlie wrote:
...
It doesn't matter - as i showed in another post, even the smallest
amount of interbreeding means that there is ancestry, even if there's
little measurable genetic relatedness in the main chromosomes.
Yes, of course.
Third, totally isolated populations tend
to die out.
You mean because of genetic drift, or what?
No, inbreeding.
But it's genetic drift that CAUSES the inbreeding, isn't
it? (It's not the fall that kills, but the impact. : ) )
I grabbed some information from a randomly chosen page:
http://www.ucl.ac.uk/~ucbhdjm/courses/b242/InbrDrift/InbrDrift.html
It states that the effective rate of inbreeding due to
drift increases roughly as 1/2N per generation, where N
is the population size. So it would take a population of
500 around 1000*ln(2), or 700 generations to double the
effective rate of inbreeding. I presume that would be
a tolerable amount?
I imagine that
would take awhile. Couldn't a population of a few hundred
be isolated for over 1000 years and survive? (Again, I'm
not taking issue with the general argument. I just feel that
1000 or 2000 years is too small a number.)
Maybe so. But a couple of theoretically totally isolated populations,
even isolated for a thousand years - where did they come from? and even
if the paper is wrong, and it's not *all* humanity alive today, it's all
bar one group in a mountain somewhere, the point is hardly changed.
Yes, I agree, the point is hardly changed. But then why the
big argument when I pointed out in the first place that it
probably wasn't ALL humanity that was descended from one
individual in the recent past? (I blame the authors, who
claimed "all" without having too much basis for it. Wouldn't
"almost all" have grabbed headlines as well?)
...
Their are other candidate groups. Google "uncontacted peoples".
You seem to have trouble grasping the timescales here. It's a very
common problem when talking evolution.
Charlie
Charlie-- That last bit is insulting. (Pouts.) It's pretty hard
to know exactly how much contact there was between different groups
unless the groups kept records. One could argue that there was no
new influx of genes to the Americas between 10000 BPE and 1492.
This takes a bunch of assumptions, that the land bridge was closed
and that the Norse (etc) contributed no genes. If so, one who wanted
to prove that everybody was descended from a Eurasian of 5000 years
ago would have to show that all of the native peoples of the Americas
had picked up some European blood in 20 to 25 generations. Even
tribes deep in the Amazon jungle...
It could be true, or not. I doubt we'll ever know.
---David
"All" means all, Maru
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