Chromosome Fusion: Chance or Design?
Human and chimpanzee chromosomes are very similar.
Note that human chromosome 2 is very similar to a
fusion of two chimpanzee chromosomes.
For the last few weeks I have been corresponding with someone about intelligent design (ID). More specifically, we have been chatting about why humans have 46 chromosomes and most of the great apes have 48.
To me, this is great evidence for evolution. Why? Because if you look at the chromosomes closely, you can see that human chromosome 2 is really just a fusion of two great ape chromosomes.
The idea is that a few million years ago, a common human-chimpanzee ancestor of ours had two of his or her chromosomes fused together. This sort of thing happens all the time even today. Around 1 in 1000 live births has one of these kinds of fusions.
Then, probably through chance,this ancestor with the fused chromosomes went on to found the human race. Now people have 46 chromosomes and chimpanzees have 48.
An alternative explanation is that the designers fused the two chromosomes together when they created humans. The idea would be that the designer wouldn’t create every plant, animal, bacteria, and virus from scratch–why reinvent the wheel every time? Instead the designers would mix and match parts that worked.
So as part of the process of designing a human, the designer fused two ape chromosomes together. This would presumably be simpler than creating a human chromosome 2 the way the other chromosomes were made.
The difficulty with this idea is that there is no obvious advantage to having 46 chromosomes instead of 48. What matters is our DNA, not how it happens to be packaged.
It is possible that there was some advantage to fusing the chromosomes together. For example, maybe a new gene was created at the fusion point. Or maybe genes that were shut off before were now turned on in the new fused chromosomes.
There isn’t any evidence of these kinds of things. And even if there were, a designer who can easily put in the 60 million or so differences between humans and chimpanzees should be able to accomplish whatever results a chromosome fusion gives more elegantly than sticking two ape chromosomes together.
Also, when you look at the fusion point, you can see that the DNA isn’t exactly what you would expect if a fusion happened in the last 10,000 or even 100,000 years. The results look more like an event that happened millions of years ago.
The ends of a chromosome have a defined sequence of DNA repeats called a telomere. The DNA at the fusion point looks very similar to a string of telomeres (as we would expect from a fusion) but it isn’t perfect. This is just what you would expect if the fusion happened millions of years ago. Because our DNA gets changed a little all of the time.
The environment or even our own cells can cause the wrong letter to end up in our DNA. Our cells are pretty good at fixing these mistakes but they don’t catch them all. What this means is that our DNA builds up mutations over time.
When an unfixed change happens in a sperm or egg, then it is passed down to the next generation. If the changes that aren’t fixed happen somewhere important, then they are selected for or against. But if they’re neutral, then they just build up over time. Scientists can even use these sorts of errors to predict how long ago something happened. Or to trace human migration patterns.
These DNA changes at the fusion point do not fit with ID if they don’t serve a purpose. Otherwise, why put them there? It will be interesting to see the results of experiments that might show if these sequences matter or not.
Dr. Barry Starr is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA.
Hey Dr. Starr,
(That sounds like a bad Dr. Pepper knock-off–sorry, couldn’t look past it.) Anyway, thanks for the info on chromosomal fusion; I’ve learned some about it in recent weeks, but this helped shine more light on the subject.
Before I continue, I have to admit that I do hold to the assumption that there is a Designer–I’m a Christian, no two ways around it. So if I get at all defensive or biting, please point it out and forgive me–I really don’t intend to.
I had a quick question, though, in light of one remark you made here. You say at one point, “What this means is that our DNA builds up mutations over time.” Over what amount of time have scientists actually seen these mutations pile up? I guess it doesn’t seem to me like we’ve been doing this kind of work for very long–in the grand scheme of things. And have those mutations that have slipped past our cellular defenses had any kind of an impact?
Thanks again. Have a good one.
Sorry I didn’t respond sooner…I usually get an email telling me my blog got a comment but didn’t this time (Craig has some splaining to do).
Certainly mutations do slip through and have an effect. For example, 7/8 of cases of dwarfism happen spontaneously–from a mutation in dad’s or mom’s gametes (see http://www.thetech.org/genetics/ask.php?id=179 for the details). Also, almost all cancers come from DNA changes (although these aren’t usually passed down). Fragile X comes from an instability in part of the X chromosome. Some cases of autism come from DNA that goes missing or is duplicated between generations. And I could keep going on…
I recognize that these aren’t good mutations–bad ones are just easier to see in a short period and are very obvious when they happen. A good mutation is trickier because there has to be a reason for it to be good. For example, some change in the environment that now favors lighter skin or drinking milk. We can see evidence in our DNA that these traits started out rare but quickly became common in certain populations but none have happened recently in humans that I know of.
There are certainly the obvious cases of penicillin resistance in bacteria or species creation in yeast (see http://www.thetech.org/genetics/ask.php?id=143 for details). Or cave fish losing their eyes the same way through different mutations in the same gene in isolated colonies (see http://www.thetech.org/genetics/ask.php?id=185).
Probably enough for now. Thanks for the response…I didn’t find it defensive or biting.
I see what you’re saying, and it’s definitely interesting, though I’ve never really questioned the microev- things–ya know, finch beaks and lactose tolerance. Now that piece you have on the yeast breeding and apparent speciation: that’s interesting stuff that I’ve never heard before. Would you say that that example carries enough weight to validate the concept of macroevolution, or is this perhaps just a strange trait of yeast? Why or why not?
Thanks for this discussion. I appreciate your cordiality and even keel.
I don’t think it is just a weird trait in yeast. We are able to see this happening in yeast because they have such a short generation time. In 20 years, you go through 1 human generation. In the same amount of time, you can go through around tens of thousands of yeast generations.
This sort of thing definitely happens in butterflies too (see http://www.sciencedaily.com/releases/2006/06/060616135623.htm and http://www.thetech.org/genetics/news.php?id=26 for a discussion of this). There are also cases of mules that are fertile. Get two of these together and you have a new species!
Weird stuff. So what definition of “species” are we working with to determine that butterfly A is different than butterfly B is different than hybrid butterfly C? Obviously it’s not some kind of sexual incompatability–because, well, they’re doin’ it just fine; is their chromosomal layout different or something?
Species is a tricky thing to define. It isn’t simply that two types of animals cannot create fertile offspring. For example, lions and tigers can create fertile offspring but I think everyone would agree they are different species.
Is the definition of “species” so fluid that it’s hypothetically possible–and at least mildly reasonable–for someone to hold that lions and tigers aren’t different species?
No, it isn’t that fluid. I just wanted to give an idea about how hard it can be to define a species. This difficulty in defining a species is what you might expect in a world undergoing evolution–there are going to be many gray areas where groups of animals are just diverging from one another. This URL has a nice concise description of the subject:
http://wiki.cotch.net/index.php/Science_can‘t_define_species
In order for this fusion be passed on, wouldn’t it have to happen in more than one animal? In other words, if we assume that fusion took place in one ape, wouldn’t it have to take place in another ape who would have to be it’s mate? Likewise for the children, grandchildren, and great grandchildren? I could be wrong, but it seems like it would have to happen at exactly the same place in the genome as well.
Because of something called a balanced translocation, it is possible for the change to happen in a single animal. That animal could then pass on a single change until two of its descendants meet up and have children. These children could then have the final change that results in the new chromosomal arrangement. See the following links for more information:
http://www.thetech.org/genetics/ask.php?id=229
http://www.thetech.org/genetics/ask.php?id=12
But they are BOTH still ‘cats.”
However, the burning issue might well be, “Exactly what are we seeing in this “fusion?”
And, has it been positively determined that the 2 “chimp” chromosomes are indeed what they appear, i.e., really “chimp” chromosomes. The only definitive evidence of that to date are the banding patterns.
We also might ask, “does this fusion create any benefit,” which may indicate design rather than “common ancestry.”