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Author Topic:   Human Chromosome 2 and the Evolution of Humans
herebedragons
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Message 1 of 56 (667882)
07-12-2012 11:18 AM


I posed this question to dwise in the thread "Creation vs Evolution is a 'Red Herring' argument" (I don't know what the thread ID is or I would have included the link) and never received a response. Since I am curious about the answer to this (and it was a bit off topic over there), I am proposing it as new topic.

Message 68

So just what the frak is this imaginary barrier that you're fretting about?

I suppose the barrier that PaulGL is referring to (although he doesn't appear to know what he is referring to) is the fusion of human chromosome 2. This event could bring about the "hopeful monster" you referred to in Message 62. It is a sudden, major genomic change and contrary to Message 62:

quote:
Rather, the degree to which each generation of a changing population changes is very slight; "sudden" changes in the fossil record (in geologic time, so in thousands of years) result from the accumulation of lots of small gradual changes with each generation (in generational time, so in one to a couple tens of years, depending on the species in question).

This fusion did not occur a little bit at a time, in small gradual changes. It may have made a small difference morphologically, but it was a major event in the genome.

Now whether this fusion event itself caused infertility or not is uncertain. In at least in one situation, the Przewalski's horse (mentioned by Coragyps in Message 61), this type of event does not cause infertility but it does result in offspring with a conglomerate of chromosomes ie. 65 as opposed to 64 or 66.

Since none of the great apes are known to have 46 chromosomes, it is most likely that either:

1. The fusion event caused the split OR ...
2. The fusion event occurred within the human lineage (after the chimp / human split)

If option 1 is true then either

1. The fusion caused infertility in which case we have the dilemma that PaulGL supposes OR ...
2. The fusion did not result in infertility in which case we should have a Homo spp. (living or extinct) with a conglomerate of chromosomes, such as with Przewalski's horse.

If option 2 is true then

We should see Homo spp. (living or extinct) with 48 chromosomes, Homo spp. (living or extinct) with 46 chromosomes AND Homo spp. (living or extinct) with chromosome conglomerates, ie. 47.

Is there any evidence that Homo spp. have chromosome numbers other than 46? Note: I do understand that we don't have access to all DNA from all extinct species and that absence of evidence is not evidence of absence.

You assumed that that event had to have happened within a single generation.

Since chromosome fusion is a relatively rare event, I would assume that this event did occur in a single individual and then spread to that individual's descendants. And since all known hominids except humans (Homo spp.) have 48* chromosomes, it seems most plausible that option 1 is true (that is: the fusion event caused the split). And since it is pretty obvious that the human lineage survived (the event did not cause infertility), shouldn't we expect to find Homo spp. (living or extinct) with a conglomerate of chromosomes?

What is the most accepted understanding of this? Are there any known hybrids of species with different chromosome numbers that do not have a conglomerate of chromosomes?

*edited 24 chromosomes to 48 so that all references were to diploid numbers

HBD


Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for. But until the end of the present exile has come and terminated this our imperfection by which "we know in part," I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca

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Message 2 of 56 (667884)
07-13-2012 7:51 AM


Thread Copied from Proposed New Topics Forum
Thread copied here from the Human Chromosome 2 and the Evolution of Humans thread in the Proposed New Topics forum.
    
Dr Adequate
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Message 3 of 56 (667903)
07-13-2012 12:40 PM
Reply to: Message 1 by herebedragons
07-12-2012 11:18 AM


I don't see why this variation shouldn't have undergone fixation through genetic drift just like any other variation can.
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PaulK
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(1)
Message 4 of 56 (667904)
07-13-2012 1:00 PM
Reply to: Message 1 by herebedragons
07-12-2012 11:18 AM


Of course, the fact that it is a fusion is strong evidence for the evolution of humans (and yet one more example of how evolutionary explanations could be falsified).

If the fusion occurred in a single individual then they must have still been fertile with the rest of the population. Otherwise he or she would have left no descendants. This in itself is reason for doubting that the fusion caused the split - or any of the later splits in our lineage.

As to the timing of the event, I will simply note firstly that there are non-homo species between us and our common ancestor with the chimps (likely among the australopithecines) and secondly that we have relatively little data to go on. So far as I know we don't even have a chromosome count for Homo erectus. Which leaves us no basis for saying that all Homo species had 46 chromosomes.

If we simply use chromosome count as a basis the fusion event could have occurred before the split between our ancestors and those of the chimpanzees or after our ancestors split from the Homo erectus population. There's no way to tell without additional data.


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NosyNed
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(7)
Message 5 of 56 (667909)
07-13-2012 2:50 PM
Reply to: Message 4 by PaulK
07-13-2012 1:00 PM


It happened again
http://genetics.thetech.org/original_news/news124

This is a case of a living human who has undergone another fusion down to 44 chromosomes.

Various things can happen if he breeds. If it is with a female with 44 choromosomes it seems it is expected to work just fine. I can't find the site that explains all the other combinations that produce different effects.


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Coragyps
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(3)
Message 6 of 56 (667912)
07-13-2012 3:50 PM
Reply to: Message 5 by NosyNed
07-13-2012 2:50 PM


Hurrah!
Ned! You're back!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Edited by Coragyps, : No reason given.


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jar
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Message 7 of 56 (667913)
07-13-2012 3:57 PM
Reply to: Message 5 by NosyNed
07-13-2012 2:50 PM


Re: It happened again
Hi Ned, welcome home. Pull up a stump and set a spell.

I tried a thread about that just about a year ago that has a link to the article you may be looking for.

The 44 Chromosome man

Never generated much interest I fear.


Anyone so limited that they can only spell a word one way is severely handicapped!

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herebedragons
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Posts: 1413
From: Michigan
Joined: 11-22-2009
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Message 8 of 56 (668205)
07-18-2012 10:21 AM
Reply to: Message 3 by Dr Adequate
07-13-2012 12:40 PM


At first I didn't consider genetic drift to be much of a factor since the probability of fixation seemed unrealistic. But your comment gave me cause to think about it a bit closer. If the population group that the mutation occurred in was rather small, say <100 time?

HBD


Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for. But until the end of the present exile has come and terminated this our imperfection by which "we know in part," I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca

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 Message 3 by Dr Adequate, posted 07-13-2012 12:40 PM Dr Adequate has responded

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herebedragons
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Posts: 1413
From: Michigan
Joined: 11-22-2009
Member Rating: 2.9


Message 9 of 56 (668206)
07-18-2012 10:37 AM
Reply to: Message 5 by NosyNed
07-13-2012 2:50 PM


Re: It happened again
Thanks Ned.

I can't find the site that explains all the other combinations that produce different effects.

This source seems to do a good job of describing all the possible outcomes.

Actually, it wouldn't be hard to imagine that a balanced translocation such as this has happened many times in history and just ended in miscarriages and spontaneous abortions and then dead ended.

In this case, his parents were first cousins that shared the same translocation.

Thanks for the info.

HBD


Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for. But until the end of the present exile has come and terminated this our imperfection by which "we know in part," I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca

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herebedragons
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Posts: 1413
From: Michigan
Joined: 11-22-2009
Member Rating: 2.9


Message 10 of 56 (668208)
07-18-2012 11:37 AM
Reply to: Message 4 by PaulK
07-13-2012 1:00 PM


Of course, the fact that it is a fusion is strong evidence for the evolution of humans

I agree, it does have strong implications.

and yet one more example of how evolutionary explanations could be falsified

I'm sorry, but I don't understand why you added this comment. Would you please expand on what you mean? What would falsify this explanation?

This in itself is reason for doubting that the fusion caused the split - or any of the later splits in our lineage.

But if we found two individuals that we considered to be both say Homo erectus by morphological data, and we found that one had 46 chromosomes and one had 48 chromosomes, wouldn't we then reclassify them as separate species (considering that weren't found in the same cave from the same time period or something)? I guess that is what I would consider "cause". Since we could not expect to point to one individual in history and say "this was the first of this species", we say things like geographical isolation "caused" the split; genetic drift "caused" the split; chromosome translocation "caused" the split. Is this improper thinking?

If we simply use chromosome count as a basis the fusion event could have occurred before the split between our ancestors and those of the chimpanzees or after our ancestors split from the Homo erectus population. There's no way to tell without additional data.

Before or after ... well that narrows it down

So, what data do we have? Which ancestral humans do we have genetic data for (at least karyotype for this topic)?

Thanks

HBD


Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for. But until the end of the present exile has come and terminated this our imperfection by which "we know in part," I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca

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PaulK
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(1)
Message 11 of 56 (668216)
07-18-2012 12:51 PM
Reply to: Message 10 by herebedragons
07-18-2012 11:37 AM


quote:

I'm sorry, but I don't understand why you added this comment. Would you please expand on what you mean? What would falsify this explanation?

The evolution of humans from (other) apes would be seriously called into question if we had no plausible explanation of the difference in chromosomal arrangement. The fact that not only do we have an explanation, but that the explanation is strongly supported by the evidence shows evolution passing a significant test with flying colours.

quote:

But if we found two individuals that we considered to be both say Homo erectus by morphological data, and we found that one had 46 chromosomes and one had 48 chromosomes, wouldn't we then reclassify them as separate species

Do we classify the '44 chromosome man' as a different species? And on what criterion would we do so? Not on the morphological criterion or the criterion of reproductive isolation, which are the two most likely to be applied.

quote:

I guess that is what I would consider "cause". Since we could not expect to point to one individual in history and say "this was the first of this species", we say things like geographical isolation "caused" the split; genetic drift "caused" the split; chromosome translocation "caused" the split. Is this improper thinking?

The split would have to occur at the point where the populations diverge and interbreeding ends (there are nuances, but that's close enough). Did the chromosomal arrangement cause that? I don't think that there is a strong case that it did - and even if it did, it wouldn't happen immediately.

quote:

Before or after ... well that narrows it down


which was my point :-)

quote:

So, what data do we have? Which ancestral humans do we have genetic data for (at least karyotype for this topic)?

Neanderthal and Denisovan are the only two, I think.


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Dr Adequate
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Message 12 of 56 (668238)
07-18-2012 7:27 PM
Reply to: Message 8 by herebedragons
07-18-2012 10:21 AM


At first I didn't consider genetic drift to be much of a factor since the probability of fixation seemed unrealistic. But your comment gave me cause to think about it a bit closer. If the population group that the mutation occurred in was rather small, say <100 time?

Actually, it's rather neat how it works out. If you have a larger population, then the probability of a neutral mutation being fixed is that much smaller. But the probability of it happening is that much bigger. And these effects, if you do the math, cancel out exactly, giving us the nice result that if all mutations were neutral (which is a good first approximation to the truth) the rate of fixation of mutations in the gene pool per generation is exactly equal to the rate of mutation in the individual.

So yes, a large population makes it more unlikely that any particular balanced translocation will get fixed, but it also means that there are that many more balanced translocations which will arise and so be candidates for fixation, and these two effects will exactly cancel one another out. Therefore, if we knew how often balanced translocations happen to individuals (and I'm not sure that we do) then we could also say the rate at which we'd expect them to become fixed in the population, and this would be completely independent of the size of the population.

So if you ask yourself: "What were the chances of this balanced translocation being fixed by genetic drift?" then population size is important; but if you ask yourself: "What were the chances of a balanced translocation being fixed by genetic drift?" then it isn't.

Edited by Dr Adequate, : No reason given.


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herebedragons
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Posts: 1413
From: Michigan
Joined: 11-22-2009
Member Rating: 2.9


Message 13 of 56 (668275)
07-19-2012 10:19 AM
Reply to: Message 12 by Dr Adequate
07-18-2012 7:27 PM


I'm sorry, my last response must have got clipped off between "100" and "time"??? It didn't really make sense.

So if you ask yourself: "What were the chances of this balanced translocation being fixed by genetic drift?" then population size is important; but if you ask yourself: "What were the chances of a balanced translocation being fixed by genetic drift?" then it isn't.

Yes, but in this case we are talking about a specific mutation. So the probability of fixation of this mutation once it occurred is 1/(2N). So effective population size and inbreeding coefficient would play a role as to how likely this mutation would be to become fixed. True we don't know for sure what the mutation rate is for balanced translocations (B-T). Robertsonian translocations are estimated at 1 in 1000 births, but that is not necessarily whole chromosome fusion is it?

I also would not consider this to be a neutral mutation. As the article explains, when a person with a B-T mates with an individual with normal chromosomes there is a 67% chance the offspring will be nonviable and a 0% chance of the offspring having a B-T. So, the individual must mate with another individual with a B-T. The only way this could happen without invoking another spontaneous B-T is through inbreeding. Which in the case of the 44 chromosome man is what happened. Even when mating with another B-T individual there is only a 1-in-36 chance of having an offspring with a two B-Ts. There is only a 8-in-36 of producing viable offspring at all (but only one of those would not have a B-T). I would say that is a drastic reduction in reproductive fitness, not neutral. The article states that this is a 1 in a 7 billion type of thing. So quite unlikely indeed, not a neutral mutation.

Two explanations the article suggests for the success of the fusion of human chromosome 2 is that it either resulted in a new gene or genes that provided some advantage to the individual or just dumb luck. I am unaware of any evidence of any new genes created by the fusion. I understand that the region of fusion is mostly telomere repeats. Dumb luck is not really possible to test and although I realize that chance plays a role, I don't find it particularly convincing of an explanation in this case. So I feel there is a lack of a plausible explanation as to how this fusion came to be (or at least I don't know of a plausible explanation yet).

Don't get me wrong, I am not trying to say that this disproves anything. When you see that chimps have 48 chromosomes and humans have 46 and then you predict that there must have been a fusion event that reduced the number, and then you find evidence of such an event ... that's pretty convincing. But IMHO, in order to be conclusive there needs to be a plausible explanation. That's all I'm trying to discuss here.

HBD


Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for. But until the end of the present exile has come and terminated this our imperfection by which "we know in part," I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca

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herebedragons
Member
Posts: 1413
From: Michigan
Joined: 11-22-2009
Member Rating: 2.9


Message 14 of 56 (668287)
07-19-2012 11:29 AM
Reply to: Message 11 by PaulK
07-18-2012 12:51 PM


The evolution of humans from (other) apes would be seriously called into question if we had no plausible explanation of the difference in chromosomal arrangement. The fact that not only do we have an explanation, but that the explanation is strongly supported by the evidence shows evolution passing a significant test with flying colours.

That's kinda what I thought you meant, which is why I asked for clarification. Because that's not really true is it? Isn't that what IDers and creationists do with irreducible complexity? Since there is no plausible explanation as to how this could have evolved then it must be intelligently designed. And the answer is usually "just because we don't have an explanation right now doesn't mean there isn't one." So lack of a plausible explanation may cause us to question the hypothesis, but is not a means of falsification. Contradictory evidence is what would falsify the theory. If I were in charge of AIG or ICR I would put all my resources into excavations that would possibly (or theoretically) yield human remains in antediluvian sediments (what ever those are - pre-Devonian should suffice). That would put the issue to rest, would it not?

Do we classify the '44 chromosome man' as a different species? And on what criterion would we do so? Not on the morphological criterion or the criterion of reproductive isolation, which are the two most likely to be applied

Why would we not if we had limited other clues to go on. Differing chromosome numbers usually indicates infertility. In the case of the 44 chromosome man, we have many other clues that he is still of the human race. With the limited amount of ancestral human remains we have, any significant differences would give reason to separate them into different species. And we would also not expect to find the first or only person who had such a rearrangement, but a representative of the population.

The split would have to occur at the point where the populations diverge and interbreeding ends (there are nuances, but that's close enough). Did the chromosomal arrangement cause that? I don't think that there is a strong case that it did - and even if it did, it wouldn't happen immediately.

No, of course it wouldn't happen immediately. Is there a more plausible explanation for the break between humans and chimps? Chromosome 2 is presented as evidence of human - chimp evolution (and I agree it's quite a strong piece of evidence) but if it is not a plausible explanation for the evolution of chimp - human, doesn't that still leave us without a plausible explanation?

HBD


Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for. But until the end of the present exile has come and terminated this our imperfection by which "we know in part," I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca

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jar
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Posts: 29622
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.1


Message 15 of 56 (668290)
07-19-2012 12:00 PM
Reply to: Message 14 by herebedragons
07-19-2012 11:29 AM


HBD writes:

The split would have to occur at the point where the populations diverge and interbreeding ends (there are nuances, but that's close enough). Did the chromosomal arrangement cause that? I don't think that there is a strong case that it did - and even if it did, it wouldn't happen immediately.

No, of course it wouldn't happen immediately. Is there a more plausible explanation for the break between humans and chimps? Chromosome 2 is presented as evidence of human - chimp evolution (and I agree it's quite a strong piece of evidence) but if it is not a plausible explanation for the evolution of chimp - human, doesn't that still leave us without a plausible explanation?

Remember that there most likely is no single cause of the split, but rather a whole host of unrelated events. Also many and probably the most important, may not be directly genetic changes in nature.


Anyone so limited that they can only spell a word one way is severely handicapped!

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