Salmoneus wrote:Tropylium wrote:Yes, that indicates a much greater amount of variety early on for sure. But that's going to also make drawing any conclusions from these lineages' specifics hazardous. If the full spectrum of population variety in ~ 3000 BP was actually equivalent to something like 3000 equivalently divergent lineages, of which we have found 84 and let's say some 100 others entirely survive, then it's unlikely that the earliest-split groups were among the 84 samples found and analyzed so far, and the common ancestor of all of these is most likely going to be much farther back.
That doesn't follow at all! That assumes some constant rate of branching - that adding more branches means adding more years. But what actually seems to have happened is a big explosive expansion. Lots of branches formed very rapidly from a narrow base.
OK, fair. If an explosive initial spread is to be assumed, then even if archaic lineages have been lost, they might not amount to more than a couple thousand years' worth of extra time-depth.
Salmoneus wrote:If there were a branch with a more distant common ancestor, then giving the period of inbreeding, that ancestor would have to be at least 23,000BP, and then you'd have to hypothesise TWO beringian refuge population without any genetic contact at all,
Nope: mtDNA is completely unaffected by crossbreeding. A single refuge population or even any single tribe is likely going to contain numerous mtDNA lineages.
In studies like these we are therefore not operating with
single common ancestors, but with ancestor
pools. It's long been known that America's indigenous people fall in five main mtDNA haplogroups:
A,
B,
C,
D and
X. The original split between {C, D} and {A, B, X} dates already way back to Africa, and the other splits took place pretty early on in Eurasia as well. The Beringian refuge population would have already included women from all five (and men, but they're irrelevant here). Whatever splits happened during their period in isolation would create a five-fold "forest" of haplotypes, not a single tree rooted in a single Beringian Eve.
My point, then, is that since there existed dozens of sub-lineages already at the onset of the "explosion", several of them may have gotten lost between Alaska and Mesoamerica, due to successive founder effects. Indeed, we
know that this is the case with X, which is not at all present in Southern and Mesoamerica.
Salmoneus wrote:Again, nobody's suggesting a series of settled, expansive populations reaching capacity and sending an offshoot to bud off further down the coast in an orderly, linnear fashion.
Indeed, me neither. I'm suggesting that a population at let's say 17000 BP begets a slightly more southern population at 16950 BP, which begets another population slightly to the south at 16900 BP, which begets another population slightly to the south at 16850 BP, etc. Nobody came to Mesoamerica directly by airplane from Alaska, they all must have trekked southward generation by generation. Founder effects don't stop being founder effects just because they're happening rapidly.
Even if we assume that people intently claimed homelands far away from other groups, it is still unreasonable to assume more than 1000 km dispersal per generation, which already suffices to set up a dozen or so founding effects before there's an initial chain of people from the Pacific NW to Chile.
Youre right that there likely were numerous "holes" at first that only got filled later. This is necessarily a slower process, though (centuries rather than decades). Any given unsettled area or "ethnographical niche" is most likely to be filled by descendants of people who got roughly into the same area already at the time of the initial settling explosion. You can assume again some 1000 km of variation in any direction, but it's still extremely unlikely that somehow a new lineage from NAm wormed its way into SAm in this way, thru an already rudimentarily settled Mesoamerica.
Salmoneus wrote:In fact, they're saying that that DIDN'T happen, not just because it happened so fast but because the 84 linneages are all divided almost all the way back to the original explosion. That's my impression, at least.
mtDNA lineage splits themselves have nothing to do with population splits and everything to do with time depth. That's the reason we study mtDNA in the first place: it accrues mutations at a relatively stable rate. The event corresponding to an mtDNA lineage split is "a mother has 2+ daughters and one of them by happenstance gains a mutation". It's completely irrelevant whether the daughters end up living in the same household, or 2000 kilometers apart.
Now the precise relationships between the lineages could be telling, of course. What the
the Science article shows is that the most recent common ancestors of each main individual mtDNA group goes 15k+ years back, i.e. all the way to the Beringian refugium. But the later splits leading to the full set of 84 are indeed spread fairly evenly across later history; they do not
all emerge right away.
Salmoneus wrote:Tropylium wrote:The "isolation" could also simply result the period of spreading from Asia to Mesoamerica (with various NAm offshoots excluded from the data), not from isolation altogether.
That would require this small, inbreeding population to stick together as a single breeding community all the way from Beringia down to Mesoamerica... and THEN to shatter explosively into a hundred different groups with little contact with one another.
No, they could have well been constantly shedding off splinter groups that just
didn't end up heading in the direction of Mesoamerica. Whichever fringes of the NAm half of the explosion that first reached Mesoamerica would have already brought with them all the mtDNA lineages required.
Salmoneus wrote:Whereas assuming a common amerind radiating out of beringia explains the expansion (uninhabited land), the relatedness (all related) and the distinctness relative to all other populations (founder effects in the small population in beringia).
You seem to be confused about some concepts here.
Founder effects are what happens during expansions; isolation instead produces
genetic drift.