Chromosome 2

It has been proposed and well evidenced that human chromosome 2 is the result of a fusion event between two chromosomes in our evolutionary past. Briefly, here is the evidence:

All great apes except humans have 24 pairs of chromosomes. We only have 23. That means we need an explanation for such a difference that dates back only a relatively short period of time (5-7 million years). As it happens, human chromosome 2 shows strong evidence of being two fused chromosomes. The way we know this is that all chromosomes have telomeres and centromeres. Telomeres are repeating units of DNA that serve to protect the ends (and therefore middles) of chromosomes, sort of like a good pair of shoes and a strong helmet. Centromeres are DNA units located somewhere between the telomeres of chromosomes, generally relatively close to the center. Their function is to help assemble the two parts of a chromosome during cellular replication and reproduction. In human chromosome 2, we see that there are actually two telomeres fused together in the center. There are also telomeres on the end, but between each end and the center are centromeres. That means we have three telomeres (one of which is fused) and two centromeres.

I bring this up because I was recently reading yet another excellent post by The A-Unicornist and he was dealing with this stuff:

ID is really nothing but an argument from ignorance – it claims that certain things simply cannot be explained by science, so it must be ‘best explained’ by a designer instead. Take for example this post from The New Creationist. I often point creationists to the Ken Miller video where he explains the Chromosome-2 fusion in humans, because it’s a perfect example of the theory of evolution making a falsifiable prediction that ended up being powerful evidence that evolution is true – something that ID has never done and in principle cannot do, which is why it will never be a science. Now, this “new creationist”, who incidentally sounds just as credulous as the old ones, argues that such a fusion is impossible – that the chromosome should never have been able to fuse at all.

Being that I’m not a biologist, I have no idea how to directly refute what he’s arguing. But it’s conspicuously odd that rather than, I dunno, ask a biologist or two (like, golly I dunno, write a letter to Ken Miller?), he simply frames his argument as though the unanswered question itself creates a major problem for the theory of evolution.

Since I’ve used chromosome 2 as an argument for evolution, I am familiar with the creationist responses. As such, I want to address what the blogger known as The New Creationist is arguing:

If the fused chromosomes in an end-to-end fusion are ripped apart by the centromeres during cell division and cells must divide to produce an embryo then how does an embryo develop with two previously fused but now ripped apart chromosomes? We know that the loss of just one chromosome would be lethal and here we have the loss of both of the two
fused chromosomes. If fused chromosomes do not make it through cell division then how could a fused chromosomal configuration be a result of common descent since there would be no descendants by a biological pathway. Such would be miraculous. Indeed, I believe it is a miracle not only because it can not be explained by any natural pathway but also because it is contradicted by experimental data.

What he is trying to say (and what he later says a little more clearly) is that two centromeres would cause division and assembly to occur in two separate places. This would be an all around mess that would prevent not only mitosis, but meiosis as well. So what could the solution be? Well, he answers it himself:

Now, it has been proposed that the deactivation of one of the centromeres in the fused chromosome would prevent the rupture and subsequent loss of the newly formed fusion…

And that is the case. One of the centromeres has been deactivated. One possible reason for this could relate to the fact that the area near the deteriorated centromere (the pericentromeric sequences) has gone through a large number of duplication events, but this isn’t known and requires certain confirming evidence around other deactivated centromeres. I don’t know if any significant research has been done in this area since the 2006 paper about chromosome 2.

The New Creationist continues:

…but this poses another equally lethal problem during the pairing off of homologous chromosomes.

Let’s say that if C2A fused with C2B forming C2 (which has 2 centromeres) in the paternal germ line, the male’s sperm. Now, that sperm would have to fertilize an egg where both C2A and C2B not having been fused would have to pair off with the paternal C2 BUT if C2 has been prevented from being ripped apart because one of its centromeres has been deactivation then the corresponding maternal C2B (or C2A) will not combine with C2 in the mother’s egg because that centromere would have been deactivated.

In other words, he is saying that if two ancestral primates had offspring with the fused chromosome, then that offspring would have 23 chromosomes whereas the rest of the population still had 24. Mating between the two could not occur as a result, thus the fused chromosome could never make it beyond a single generation.

The most obvious solution to this problem is that several members of a population experienced a fusion event. It could have been a completely chance event, or it could have been due to a particular mutation that had spread down the line. That is, my money is on a mutation existing in a population that caused the fusion between two specific chromosomes. Perhaps all the pericentromeric duplications (which pre-date the fusion event, incidentally) gave rise to a gene that was free to mutate neutrally in the population. After some time, it managed to survive the generations, and made a marked difference. (That’s what has happened, minus the specific duplication events, with Richard Lenski’s E. coli.) Or maybe a mutation popped up just out of completely random chance, as opposed to being connected to any particular type of event. It’s hard to say just how any of this happened, but there are good hypotheses to be had on the question.

To conclude, the first argument presented here was defeated before it was even made. One of the two centromeres was deactivated long ago, as stated in the original paper. Indeed, that very paper even suggested a correlating factor in centromere deactivation that could be useful for future research. As for the second argument, I’m going to give Mike the last word:

[T]he fact that an explanation is either unknown or not immediately apparent would not refute the fact that the theory of evolution made this falsifiable prediction, nor would it suggest that there cannot be a rational explanation at all. Our new creationist seems to think that because he does not know how to explain it that a rational explanation is not merely unknown, but in principle impossible. Ergo, Goddidit. That ain’t how science works, kids.


21 comments on “Chromosome 2

  1. I find it interesting that you say, “All primates except for humans have 24 pairs of chromosomes”. You should familiarize yourself with primate karyotypes to see why what you said is not true.

  2. For starters, you claimed that one organism with the new pairing sequence could not continue its lineage because there would be no other individuals with the same, new pairing. A common mutation solves this. There is copious demonstration of how this works, but one more recent piece of digestible evidence comes from the Lenski experiments I cited.

    Then, of course, there is also the issue of you claiming that there is no answer here other than some sort of god because there are gaps in our knowledge. That is a classic fallacy.

  3. How does Lenki’s experiments with E.coli say anything about end to end chromosomal fusions? E.coli only has one chromosome and it’s a circle chromosome at that. My claim is that end to end chromosomal fusions are ripped apart during cell division. This is demonstrated empirically and well documented. End to end chromosomal fusions being passed down to subsequent generations has not been observed because it is a genetic dead end.

  4. That wasn’t the point I made. Lenski’s experiment shows contingent mutation existing in a population. Those mutations, which are initially neutral, then lead to mutations that have impact that is culled by natural selection. This answers your notion that a population would have just a single member with fused chromosomes, thereby disallowing continued mating and the passing on of said chromosomes.

  5. You brought up Lenski’s work on E.coli and you said it showed a common mutation that explains something about fused chromosomes but E.coli only have one chromosome AND are asexual so Lenski’s research says nothing about what I said.

  6. You aren’t understanding what was said. You have proposed that an individual with a fused chromosome could not leave descendents because it would have fewer chromosomes than a perspective mate. It therefore, according to you, would be a genetic dead end. One solution here is a mutation that acts upon several members of a population simultaneously.

  7. I understand what I said perfectly. I do NOT propose what you say I do since an individual with a fused chromosome would never develop in the first place because end to end fused chromosomes tear apart during cell division and cells must divide billions of times to form an individual capable of mating.

  8. You have acknowledged the question and answer to the prospect of two centromeres causing issues: One of them is deactivated. You then separately and secondarily raised the issue of one individual not being able to mate with another due to differential chromosome count. However, I am no longer sure if you realize that that is what you proposed.

    At any rate, the first issue (double centromeres) was resolved before it began (deactivation), and the second issue (differential chromosome count) is resolved by a mutation that affects several members of a population in the same generation.

  9. Firstly, there is no actual evidence of a deactivated centromere on human chromosome 2. If there were once a centromere which is now deactivated then we would hope to see evidence of it’s remnants. When a car breaks down it is still there, it’s just broken, it doesn’t just disappear. But in the case of human chromosome 2 there is nothing to indicate that there ever was a second centromere. It’s just not there.

    Secondly, there is no evidence that the deactivation of one of the centromeres in an end-to-end fusion would prevent it from being ripped apart during cell division. It would still be ripped apart because 2a and 2b separate at different times but now there would be no counter balance on the fused chromosome for 2b. This would lead either to a mosaic or the rupture of the newly fused chromosomes both of which are lethal because one half of the mosaic would lack chromosome 2b in an embryo and a chromosomal rupture (ripped apart chromosome) results in the loss of the fused chromosomes.

  10. You haven’t read the paper. The deactivated centromere in humans which corresponds with the still present centromere in chimp chromosome 13 is buffered by a significant region of sequences which pre-date the fusion event. That is, the deactivated centromere is surrounded by a series of sequences which identify its location and its chimp origins.

    Secondly, there is no evidence that the deactivation of one of the centromeres in an end-to-end fusion would prevent it from being ripped apart during cell division.

    It’s deactivated. That means it is not active. That means it no longer functions.

  11. Except that I did read the paper years ago. I have also looked at the evidence and there are substantial problems for evolution.

    LOC744 and GPR39 are the genes surrounding the centromere on chimp 2b and the centromere is about 3.4 million base pairs long. So, we should find centromere repeats or there remnants in the corresponding region on the human chromosome 2. So, where are they ?

  12. Chromosome 2 is unique to the human lineage of evolution, having emerged as a result of head-to-head fusion of two acrocentric chromosomes that remained separate in other primates. The precise fusion site has been located in 2q13–2q14.1 (ref. 2; hg16:114455823–114455838), where our analysis confirmed the presence of multiple subtelomeric duplications to chromosomes 1, 5, 8, 9, 10, 12, 19, 21 and 22 (Fig. 3; Supplementary Fig. 3a, region A). During the formation of human chromosome 2, one of the two centromeres became inactivated (2q21, which corresponds to the centromere from chimp chromosome 13) and the centromeric structure quickly deterioriated42. A search of genome sequence for the presence of vestigial centromere and pericentromeric sequences identified a 2.6-Mb region in 2q21.1–2q21.2 that is enriched for pericentromeric duplications to chromosomes 1, 7, 9, 10, 13, 14, 15, 18, 21 and 22 as well as a variety of centromeric satellite repeat sequence motifs (HSAT5, GSATII, ACRO1). The degree of sequence identity of the interchromosomal duplications (< 98%) suggests that these pericentromeric segmental duplications existed before the formation of this chromosome. Within this 2.6-Mb interval, we identified a relatively large tract of satellite sequence (three tracts totalling 31,198 bp of alpha-satellite sequence over 36,696 bp), which likely demarcates the position of the ancestral centromere (Supplementary Fig. 3a, region B).

  13. I’m not convinced you do know. If you did, you would be acknowledging that there is a high degree of matching sequences that only make sense if this is the location of a fusion event.

  14. Alpha and beta satellites are present yes, but the patterns are very peculiar and certainly not what one would expect to find as a result of common ancestry so the question for me is, why are these satellites there? But you didn’t ask. In fact, you didn’t ask me anything before, during or after misrepresenting my argument nor did you contact me so it would seem that you really don’t care what I think. None the less, there is a way to check to see one way or another.

  15. The analogous chromosome sequence matches exactly the analogous chromosomes from our ancestors, the telomeric sequences match in great detail, and the surrounding sequences not only match but they match in a way that is exactly what we would expect due to a fusion event from a common ancestor. You haven’t a grasp of this material; I recommend you stay clear of creationist websites.

  16. Since there is no sample of DNA from a common ancestor to run comparisons against what you say above, “The analogous chromosome sequence matches exactly the analogous chromosomes from our ancestors”, is complete bullshit. The rest of what you say simply isn’t true. For starters, of the two protein coding sequences surrounding the centromere on Chimp C2b, one of those genes is not on human chromosome 2 at all. Then there are thousands of missing centromere repeats and the chimp satellites don’t align at all between humans and chimps. I know because I ran my own alignments, not because I read it on a website.

  17. The sequence matches with our ape cousins; our ape cousins’ sequences trace backwards in time in a way which allows us to safely extrapolate that it existed in our ancestors. The sequences are just what we would expect.

    As for the telomeric and surrounding sequences, they match prior to the telomeres, in the telomeres, when the telomeres are looked at in reverse, and again after the telomeres. You’re free to say that isn’t true, but, well, it is.

    You need to actually read the papers surrounding this topic. The sequences match just as one would expect, no matter how much you deny it. But hey, feel free to show us all your alignments.

  18. I might be a LITTLE late here, but it’s my understanding that our chromosome 2 has it’s identical sequences on chimp chromosomes 12 and 13. So…wouldn’t comparing our chromosome 2 to chimp chromosome 2 be looking at the wrong chromosome entirely?

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