Re: Mitochondrial Eve a 'fact'?


Ron O wrote:
Nic wrote:
floyda@xxxxxxxxx wrote:
Nic wrote:
floyda@xxxxxxxxx wrote:
Friar Broccoli wrote:
Ron O wrote:

The multiregional hypotheses is only talking about the most
recent events and not the millions of years since we separated
from other great apes. It only takes into consideration from
Homo erectus to Homo sapiens sapiens.

Thanks for the clarification

[snip]


The mitochondrial data does not support this hypothesis. In fact, it
strongly supports the alternative that modern humans arose in Africa
(the original estimate was that "eve" existed 80,000 to 250,000 years
ago

[snip]

The multiregional adherants might claim that we haven't sampled every
human on earth so we don't really know the answer.

Hope their position isn't this silly.


It's not. It's still almost certainly wrong, but the reasons why the
multiregional model are wrong are not so obvious as that. Wolpoff is
mistaken, but he's not foolish.

There *were problems with Cann et al's original sample (e.g. use of
African American mtDNA as a substitute for African mtDNA, which was a
dumb mistake), and of course there are always small sample problems.
(IIRC, the 1987 study used something like 170 people.) But more recent
analyses give no comfort to the multiregionalists.




They might also claim something like the original
mitochondrial type of the Homo erectus population that left
Africa and colonized the old world on the order of a million
years ago, was not as good in some respects to a later
version that arose in Africa around a hundred thousand years
ago and spread through all the existing human populations of
the world. There could have been mixing with indigenous
populations, but we don't see evidence of it in the maternal
lineage because all the indigenous mitochondrial types are
extinct.

I like this. (And again back to low junk rates in mDNA.)


They can also reasonably argue that with such low populations of humans
at the time we're discussing, the loss of mt lineages to drift is not
at all unlikely.

That puzzles me. I mean what is good for the goose is good for the
gander. Doesn't the loss of mt lineages to drift when populations are
low apply mutatis mutandis to Y chromosome lineages?


Sure, assuming monogamous mating pairs, it almost has to. A man who
has only daughters is the end of that particular Y-chromosome lineage.
Can you expand upon what's puzzling you, please? I'm not quite
following (could be the hangover).

I don't think I am puzzled now - but have a hangover.
Initially I thought (because I'd heard of mt Eve) that the human line
had been through a population bottleneck at some time in the past, and
moreover one worth remarking on for genetic reasons. So I had been
wondering it had anything to do with the last speciation event, and had
been wondering generally whether you can tell how far a species is into
'mid-life' from things like the amount of genetic load it has
accumulated.

Humans have around 1/5 the genetic variation as other species like
chimps. The current thinking is that humans suffered a population
bottle neck around 80,000 to 100,000 years ago and the population may
have crashed to around 1,000 breeding individuals. The Y chromosome
ancestor dates to around this period, but the mitochondrial ancestor
may predate the bottle neck. Who knows if this was a speciation event.
There may have been other hominid populations around (archaic Homo
sapiens and Neandertals) but only that small group left descendents in
the current population.

1/5 is interesting. Chimps all look the same to me. The 1/5 could
mean we're 1/5 as old? Or it possibly reflect a greater propensity to
pick on individuals with characteristics away from the norm. But
pigeons do that - how much variation do they have?

On the subject of whether anything of evolutionary significance
happened in the human lineage 80 to 100K years ago, I don't know.
Obviously the spectacular events of the last 10K years would imply that
an important change happened relatively recently. My thinking on this
is that it is not necessarily so. The 'take off' effect, most
spectacularly displayed in the industrial revolution of the 19th
century may be accounted for by nothing more than population densities
alone. Sheer weight of population affords opportunites for
specialisation. A hunter gatherer, even of a sister species to us, may
well have been capable 250K years ago of learning to be a time domain
reflectometrist - there just wasn't the demand!

You can't tell much about genetic load because it depends on selection
intensity and the amount of inbreeding. The genetic load of obligate
outcrossing species can be much higher than populations like humans
that tend to inbreed.

SNIP:

Ron Okimoto

.

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