DOWN (OR DONE) WITH THE NEANDERTHALS ... this time?

[lagarvelho]

Dale and Dr. Hawks:

<<Quite true that mtDNA is all or nothing, but really the point is whether a selective or neutral explanation is true. Suppose we assume neutrality for mtDNA. For Neandertal mtDNA to survive in today's population, we require an unbroken chain of female descent from some Neandertal woman to some living woman. If mtDNA is neutral, the odds of this happening depend on the size of the human population and whether the population has been growing: large population size over time makes Neandertal mtDNA survival more likely (more women) and growth also makes it more likely (more children per woman). >>

Okay.  I follow this part, so far.  

<<Let's consider some conditions under which Neandertal mtDNA disappearance might be expected without selection. Approximately 1000 human generations have elapsed since Neandertal morphology disappeared. Suppose that at around the time of Neandertal disappearance, there were 50,000 females globally, and that 500 Neandertal females mated into this population, for an initial frequency of the Neandertal mtDNA type of 1 percent. Further assume that the population never grew in size. Under these assumptions there is about a 35 percent chance that Neandertal mtDNA would not be found in living people. >>

That makes sense.  So far, anyway.

<<Notice that these are the kind of parameters that most replacement advocates would be very comfortable assimilating into their models of complete replacement. If the population was much larger, and growing, as we know to have been true, or if we assert that the Neandertals made up a more substantial part of our ancestry, it becomes quickly certain that Neandertal mtDNA should survive today, if continuity were true. In short, the neutral assumptions make the replacement model appear to be a very likely explanation of the mtDNA data, and this is why so many people find the argument so convincing.>>

Okay.  Here is where I am confused.  Apparently you seem to be saying on the one hand, that if the total Neandertal population was small, then there would be more chance of that particular type of DNA(from Neandertals, at least), would disappear.  YOu don't have to be a mathematician to figure this out.  But then you say "if the population was much larger, and growing, as we know to have been true", seems to imply that the Neandertal population was larger and growing?  Or that the AMH population *was* "larger and growing"?  Or am I misssing something here?   I think I follow you where you say that if such populations were *small*, then it's possible to convincingly demonstrate "replacement", but there's no way to demonstrate continuity?  At least not through mtDNA?  

In the extreme case, it would only take one selection factor, from whatever source, that differentiated between females for mtDNA lineage replacement.  It is very likely that, whatever the case, male expression followed that female-only selection factor requiring only that it be compatible after the fact rather than contributory to that selection factor or factors.

<<Exactly right, and since the mtDNA distribution of living humans shows clear signs of past selection, it follows that the molecule should tell us nothing about whether other Neandertal genes may have been replaced. In my view, there is little to be gained in further, more specific descriptions of how easy it would be for different social systems to cause mtDNA replacement; the hypothesis of selection is consistent with the data and more powerful than social factors. I would note that there is no necessity for this selection to be limited to, or even present in, females; selection on anyone will work. It is the strong probability of past selection that persuades me that the data cannot test continuity.>>

IOW, the real problem is that there *was* "past selection" in some population?
Anne G

[Dale Hoogeveen]

Dale and Dr. Hawks:

(snip)
<<Notice that these are the kind of parameters that most replacement advocates would be very comfortable assimilating into their models of complete replacement. If the population was much larger, and growing, as we know to have been true, or if we assert that the Neandertals made up a more substantial part of our ancestry, it becomes quickly certain that Neandertal mtDNA should survive today, if continuity were true. In short, the neutral assumptions make the replacement model appear to be a very likely explanation of the mtDNA data, and this is why so many people find the argument so convincing.>>

Okay.  Here is where I am confused.  Apparently you seem to be saying on the one hand, that if the total Neandertal population was small, then there would be more chance of that particular type of DNA(from Neandertals, at least), would disappear.  YOu don't have to be a mathematician to figure this out.  But then you say "if the population was much larger, and growing, as we know to have been true", seems to imply that the Neandertal population was larger and growing?  Or that the AMH population *was* "larger and growing"?  Or am I misssing something here?   I think I follow you where you say that if such populations were *small*, then it's possible to convincingly demonstrate "replacement", but there's no way to demonstrate continuity?  At least not through mtDNA?  

Hi Anne,

It is obvious that human populations have exploded in the last 30,000 years.   The first question is how Neanderthal populations may have contributed to the initial seed populations involved in that explosion.  If I read Dr Hawks correctly, his statement is directed at that participation rather than huge underestimation of original Neanderthal populations.

IOW significant participation should result in greater chance of partial retention as that growth occurs.

But there is also a different take on the dynamics of exploding populations where there is postulated a differential favoring more recent genotypes in such a population growth.  IIRC Wolpoff says something to that effect and so does Sarich in his extensive Email at:

http://wilmot.unh.edu/~jel/sarich.html
Search in document for THE MITOCHONDRIAL DNA EVIDENCE
(unless you want to wade through the whole damn thing...  OTOH there are quite a number of references at the end which may be enlightening.)

quoting:

"All we can be reasonably certain of is that the actual base of the human mtDNA tree is much older than the 200,000 or so years given by Cann, Stoneking, and Wilson in their landmark 1987 Nature article.  The only published suggestion as to just how much older is by Wills (1993: 53-4)"

(ref = C. Wills, The Runaway Brain, Basic Books, 1993.)

"It is thus of appreciable interest that DiRienzo and Wilson (1991) reported a similar pattern from their study of Sardinian and Middle Eastern individuals; that is, most of the branches in their tree originated in a "narrow interval of sequence divergence about two-thirds of the way from the root to the tips of the tree." And the "peak at the 0.5-0.75 level of percent sequence divergence suggests that the probability of survival of new mtDNA lineages changed dramatically during the evolution of modern humans."

(ref = A. Di Rienzo, A. C. Wilson,  Branching pattern in the evolutionary tree for human mitochondrial DNA, Proc. Nat. Acad. Sci. USA 88:1597-1601 (1991) )

"These would then, I believe, tend to render the search for the geographical origin of the "mitochondrial Eve" a pointless exercise, and make her a statistical artefact of no biological significance."

All quotes are from Sarich's note To: jel@christa.unh.edu  on line at: http://wilmot.unh.edu/~jel/sarich.html  Date: Thu, 31 Mar 1994 16:03:35 -0800

(Dutch:)
In the extreme case, it would only take one selection factor, from whatever source, that differentiated between females for mtDNA lineage replacement.  It is very likely that, whatever the case, male expression followed that female-only selection factor requiring only that it be compatible after the fact rather than contributory to that selection factor or factors.

(Dr Hawks:)
<<Exactly right, and since the mtDNA distribution of living humans shows clear signs of past selection, it follows that the molecule should tell us nothing about whether other Neandertal genes may have been replaced. In my view, there is little to be gained in further, more specific descriptions of how easy it would be for different social systems to cause mtDNA replacement; the hypothesis of selection is consistent with the data and more powerful than social factors. I would note that there is no necessity for this selection to be limited to, or even present in, females; selection on anyone will work. It is the strong probability of past selection that persuades me that the data cannot test continuity.>>

IOW, the real problem is that there *was* "past selection" in some population?
Anne G

It's not a problem.  It is the nature of the material.

Note from the Sarich quote of A. Di Rienzo, A. C. Wilson, above that not only was there apparant mtDNA selection but it's parameters appear to have changed very recently in human history, making long term calculations based on current mtDNA selective factors very suspect, even for mtDNA itself and as Dr Hawks states it then is totally unreliable about previous selection for other genetics.

Dutch

[John Hawks]

Dale and Dr. Hawks:

Okay.  Here is where I am confused.  Apparently you seem to be saying on the one hand, that if the total Neandertal population was small, then there would be more chance of that particular type of DNA(from Neandertals, at least), would disappear.  YOu don't have to be a mathematician to figure this out.  But then you say "if the population was much larger, and growing, as we know to have been true", seems to imply that the Neandertal population was larger and growing?  Or that the AMH population *was* "larger and growing"?  Or am I misssing something here?  

Yeah, I should have been more specific. My comments on population size refer to the entire global human population, not merely Neandertals. The 1 percent was an assumption about how much of the ancestry of the later global population had been Neandertal; obviously we don't know what that figure might actually have been. My point was, 50,000 is probably an underestimate of the global population 30,000 years ago, one percent is probably an underestimate of Neandertal ancestry under a (neutral) continuity model, and zero is probably an underestimate of population growth. Thus it is quite probable that Neandertal mtDNA should survive in living humans, if there was population continuity and no selection.

I think I follow you where you say that if such populations were *small*, then it's possible to convincingly demonstrate "replacement", but there's no way to demonstrate continuity?  At least not through mtDNA?  

I think it would be possible to demonstrate continuity; for example if a few dozen Europeans showed up with Neandertal-like mtDNA sequences. But since this appears increasingly unlikely to happen, I tend to limit my considerations to scenarios where the currently observed mtDNA differences (Neandertals real different, humans real similar) are likely to occur. With that constraint, there is no good way to demonstrate continuity (i.e. refute replacement), unless a significant proportion of post-Neandertal fossils came out with Neandertal sequences.

But I don't expect that to happen, because if the Neandertal mtDNA type survived for any length of time into later populations, the growth of those populations would make it more and more likely that Neandertal-like mtDNA would be found in living people.

My own feeling about this is by no means immune to logical attack. There is increasing evidence that most of the genes of living Europeans owe their patterns of variation to the spread of agriculture within the past 10,000 years. Moreover, if mtDNA was strongly selected, as I think it was, then there is no reason to expect that the first post-Neandertal Europeans should have necessarily had mtDNA like that of later Europeans. For these reasons, there is hope that some early modern fossils might provide evidence of Neandertal ancestry.

IOW, the real problem is that there *was* "past selection" in some population?
Anne G

That's what I think. The questions remain, "what population?--i.e. the entire global population or some subset of it?" and "was mtDNA evolution somehow coincident with the morphological evolution of modern humans?" I think these are interesting questions, and knowing the answers would get us a lot closer to the truth.

[Dale Hoogeveen]

That's what I think. The questions remain, "what population?--i.e. the entire global population or some subset of it?" and "was mtDNA evolution somehow coincident with the morphological evolution of modern humans?" I think these are interesting questions, and knowing the answers would get us a lot closer to the truth.

There was some very interesting work done on compatibility between various types of primate mtDNA and human chromosomal DNA reported in the following paper:

Proc. Natl. Acad. Sci. USA
Vol. 94, pp. 9131±9135, August 1997
Cell Biology
Expanding the functional human mitochondrial DNA database by the establishment of primate xenomitochondrial cybrids
LESLEY KENYON AND CARLOS T. MORAES
free download:
http://www.pnas.org/cgi/reprint/94/17/9131.pdf

Kenyon and Morales were able to get Gorilla and Pan mtDNA and human chromosomal DNA to combine to produce respirating and reproducing clone lines.  They could not produce that with Pongo nor with any other primate, showing that any such general compatibility has definite boundries.

That is a long way from full development from fertilization to full maturation, but it does suggest that at least inside the human species there may well be broad enough general compatibility between mtDNA lineages and chromosomal ones to question  the necessity of coincident evolution of the two types of DNA.

Dutch