21 October, 2009

Human Chromosome 2 Fusion

This topic has been discussed many times over on the interwebz, but I thought I'd just add my own explanation of it anyway...

A valid question one might ask about the evolutionary relationship between humans and chimpanzees is this - why do the human and chimpanzee genomes have different numbers of chromosomes when evolutionary theory insists that we are so closely related?

The story of how this apparent stumbling block was resolved is one of the greatest triumphs for evolutionary theory over ID/creationism.

The Great Apes, as in chimps, gorillas and orangutans, have 24 pairs of chromosomes compared to 23 pairs in humans. This has been known to be the case since the 1970s. Back then, it was a potential problem for evolutionary theory, because it had to be explained how there was a difference between these related species.

Creationists, of course, were happy that the numbers were different. This confirmed for them that humans are unique and were designed with 23 pairs of chromosomes. Evolutionists, on the other hand, came up with a testable hypothesis (real science) to explain the differences...

One initial possibility was that humans may have lost a pair of chromosomes following divergence from the rest of the Great Apes, but this was widely ruled out as loss of whole chromosomes would almost certainly be fatal. So the generally accepted hypothesis was that two chromosomes must have fused together to form a new longer chromosome, thus reducing the number of pairs from 24 to 23. The implication of this, of course, was that if no evidence of a fusion event could be found, then evolutionary theory would be in big trouble.

Decades later, when the technology allowed for it, an undeniable fusion event was found on human chromosome 2 (the second longest in our genome). Look at the picture below. Every human chromosome (left-hand side) lines up with the corresponding chimpanzee chromosome (right-hand side) with a fairly high degree of similarity - except for chromosome 2. Here, we can see that there are two shorter chromosomes in the chimpanzee genome, which line up quite nicely with the single longer human chromosome 2. In fact, it is known that the genes on these two shorter chimpanzee chromosomes match the genes found on human chromosome 2, effectively putting the fusion hypothesis beyond all reasonable doubt.




But........just in case you're not convinced by all that, there's even more evidence to back it up......

Imagine a chromosome as being a shoelace. The plastic bits at the end of the shoelace keep the material which makes up the shoelace from unravelling. Chromosomes have similar 'plastic bits' at each end called telomeres. They protect the ends of the chromosome from degradation. In fact, the scientists who discovered this were recently awarded the 2009 Nobel Prize in Physiology or Medicine. Now imagine that a shoelace also has another plastic bit in the middle, and through this you can connect a pair of shoelaces together for storage. This is called a centromere on a chromosome, and it is where the chromosome pairs connect.


So all chromosomes have a centromere in the middle and two telomeres, one at each end.

However, when scientists looked at human chromosome 2, they found that it has not one but two centromeres. Not only that, but they also found remnants of two extra telomeres in between the two centromeres. This is indicative of a linear connection between two chromosomes and is exactly what would be expected if a fusion event had occurred. So at some stage in the evolution of humans, a fusion event occurred producing human chromosome 2 and leaving us with 23 pairs of chromosomes. This means that our ancestors had 24 pairs of chromosomes, just like the rest of the Great Apes.

Now, don't forget, a lack of this fusion evidence would have [Edit: been a major setback for the theory that humans shared an ancestor with apes], but it was found. The design hypothesis is essentially debunked, because it can't explain the presence of the two extra telomeres and one extra centromere. Why would an intelligent designer make human chromosome 2 with unnecessary extra telomeres and centromeres? That's just bad design, which certainly wouldn't happen with an intelligent designer.

ID creationists sometimes argue that this fusion event is unlikely, as the DI lawyer Casey Luskin does here, because the individual it occurred in would be unable to mate successfully due to different chromosome numbers. But this is completely false. The following comment from this post at Panda's Thumb sums it up nicely:

Reply Edit


Wild Horses 66, Domestic Horses 64, and Donkeys 62 Chromosomes

When I confront creationists on the issue of Human/Ape chromosome numbers, I use all the science mentioned in this blog, plus I add information from the genus Equus.

I first ask creationists if Horses, Donkeys, and Zebras are related. They often reply “yes, they are of the same created kind”, often noting that Horses, Zebras, and Donkeys can breed with each other and that they are all descendants of the 2 horses on Noah’s ark 4000 years ago. Then I reveal their chromosome numbers.

Wild Horses (Equus ferus przewalskii) have 66 chromosomes. Domestic horses (Equus caballus) have 64 chromosomes, and Donkeys have 62 chromosomes. The cross of a domestic horse and a donkey produces a mule or hinney with 63 chromosomes. Crossing a wild horse with a domestic horse produces a horse with 65 chromosomes.

If Equus species that range in chromosome number from 62 to 66 can all descend from a common ancestor, why can’t apes (48 chromosomes) and humans (46 chromosomes) descend from a common ancestor?

I also ask them if any humans have more than 46 chromosomes. Every time but once, they have said no. Then I inform them that people with Down’s Syndrom have 47 (due to 3 copies of chromosome 21).


Successful reproduction in individuals with different chromosome numbers is also known to occur in humans, as is seen with individuals with a Robertsonian translocation (a disorder where a full or partial chromosome fuses to another). More than 1 in 1000 people have Robertsonian translocations and can still reproduce successfully. Not all of their offspring will survive, but some will, which is all that matters. I'm not going to get into the Mendelian genetics of it, but its roughly a 1 in 4 survival chance. Now, the offspring with the same fusion (it will be passed on to 50% of the viable offspring) will have the same fertility as the parent, so will also be able to reproduce with a 1 in 4 chance. And so on...

Here comes the amazing bit...

Think back to our ancestor who had the original fusion event. Following generations of reproduction, the individuals with the fusion event would slowly accumulate in a population, although still be a minority. When two individuals with the fusion event mate, because they have identical chromosome numbers, they will have 100% fertility. This 4-fold increase in fertility will cause the population with the fusion event to continue to mate with each other. Similarly, the larger population without the fusion event will prefer to mate with each other, and not the new population.

As I hope you can see, this would potentially contribute to reproductive isolation and the beginnings of speciation...!

So the fact that we have different chromosome numbers to the Great Apes has been lucidly and elegantly explained by evolutionary theory. This was done by making a testable hypothesis and then doing the research. Contrast that to the complete lack of an explanation or research from ID creationism. Fusion events have also been observed in the genomes of other mammals, and always verify expected lineages. An excellent additional bonus is that it provides us with a potential reproductive explanation for speciation.

*Ahem*



Pwnage.

16 comments:

Ryk said...

Excellent post and a good argument to file away next time I find a creationist wanting to pretend they are using science.

Anonymous said...

tobacco has 48 chromosomes.
could you explain the relationship between apes and tobacco?
regards

rhiggs said...

Anonymous,

The last common ancestor between apes and tobacco was so long ago that it would be impossible to even speculate on what chromosomal events have happened since then.

The fact that apes (excluding humans) and tobacco have the same number of chromosomes does not mean anything.

It's just a coincidence.

A similar numbers of chromosomes doesn't automatically mean there is a close relationship. This is deduced by similarity in sequence.

Humans and the Great Apes have very similar sequence throughout the genome. What this post explains is how the discrepancy in chromosome numbers was worked out.

Can I assume you are an ID creationist? If so, how about you try and answer this...

If humans are the product of special creation and are far far more complex and brilliant than tobacco...

...then why does tobacco need 48 chromosomes worth of DNA?

...and why do some species of fern have 252 chromosomes worth of DNA?

Pretty dumb design if you ask me.

photosynthesis said...

Thanks Rhiggs,

I was trying to find the information in published articles. But of course I was doomed to fail because what the creationists thought about the chromosome disparity would not be there.

G.E.

Anonymous said...

Only one counterpoint. The whole thing was that, if evolution was true, maybe they would find evidence of fusion. Failure to find it would not have debunked evolution.

rhiggs said...

Anonymous,

Who knows? Perhaps you are right, but it certainly would have been a huge setback to common descent and very difficult to explain. The point is that they did find it, as predicted by the theory. It's neither here nor there to predict what might have happened if something that did happen hadn't happened.

Let's just focus on what we know, that there is overwhelming evidence for a fusion event. Meaning our ancestors had 24 pairs of chromosomes, just like the rest of the Great Apes.

How do you explain this? How do creationists explain the extra telomeres and centromere?

Anonymous said...

"It's neither here nor there to predict what might have happened if something that did happen hadn't happened."

And yet you did. You predicted that, if it hadn't happened, evolution would have been debunked. ("Now, don't forget, a lack of this fusion evidence would have debunked evolution...")

"How do you explain this? How do creationists explain the extra telomeres and centromere?"

I don't know about creationists. And I don't try to explain it at all. I do not consider myself qualified to determine the origins of life or the cause for life's diversity. I can, however, determine when people are claiming a high standard and not actually meeting it.

The idea of positive data sounds impressive. But every pseudo-science claims positive data (according to its interpretation.) I can't assume that scientists are immune to the possibility of defending a pseudo-science as though it were a science.

rhiggs said...

Anon,

Fair point - I had actually forgotten that I said that. You are right that I can't say for sure that evolution would have been debunked, but I do maintain that it would have been a major setback, possibly fatal to the theory.

I've edited the text according to your comment.

Allopatrik said...

I enjoyed reading your article. As a graduate student in biology with an interest how human chromosome 2 became fixed (at a frequency of 100%) in the population, I have a couple of comments/clarifications.

1.HC2 is the result of a "centric fusion", which is a kind of Robertsonian translocation. Robertsonian translocations (RT’s) by definition involve exchanges between two acrocentric chromosomes (those with one short arm and one long arm). The breaks occur near the centromeres. Most RT’s occur when one break is in the long arm and the other break is in the short arm. When the exchange occurs, the result is a metacentric chromosome (one with roughly equal arms) with one centromere, and a small fragment with the other centromere which is often lost (because it has little essential genetic information). A centric fusion, on the other hand, involves breaks in both short arms, and results in a dicentric chromosome (one with two centromeres). The centromeres are usually in close proximity. IDers and creationists are fond of saying that a dicentric chromosome would be torn apart at meiosis, but the fact is, in humans, over 90% of the dicentric chromosomes formed by Robertsonian translocations are mitotically and meiotically stable (we’re not sure why).

2. The fusion is most likely not involved in our reproductive isolation from the chimpanzee. At most, it may have reduced fertility in heterozygotes by ~10% (based on human population cytogenetic studies for Robertsonian translocations).

Keep up the good blog!

Dave Wisker

rhiggs said...

Dave,

Thanks for your comments.

Just to clarify, I didn't mean to suggest that the HC2 fusion was involved in our reproductive isolation from the chimpanzee per se, but rather that when the fusion event happened, it might have eventually contributed to a local reproductive isolation from the 'non-fusion' members of the same community, thus promoting its stability.

In other words, it might have contributed somewhat to speciation at some point in hominid evolution, long after the original split between modern humans and modern chimps.

Allopatrik said...

There is good evidence that the population structure of early hominids consisted of small, semi-isolated breeding groups called demes. In fact this is true for most populations that occupy any significant geographic space. The reason is, individuals tend to find mates that live in close proximity to them. This results in localized clusters of gene frequencies. In all likelihood the fusion occurred after the chimp-human split in one individual in a singke deme, and became fixed in one deme before spreading (via colonization and emigration) throughout the rest of the population.

I wrote a series of essays on this at the blog "The Panda's Thumb" called "The Rise of Human Chromosome 2". Here is a link to the last one, and in it you'll find links to the rest:

http://pandasthumb.org/archives/2009/09/the-rise-of-hum-3.html

Allopatrik said...

Oops. I noticed you linked to my stuff already. Never mind :)

rhiggs said...

Hi Dave,

Thanks - I agree with your synopsis of what probably happened.

I didn't realise you were the author of that article at PT. It says the author's name is Arthur?? Anyway, I'm glad I had already linked to it! ;)

Allopatrik said...

Art Hunt is a friend of mine who sponsored the articles. My name appears below his.

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