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Evolution

You stated in an earlier answer that human chromosome 2 is made up of two fused chromosomes from the common human-chimp ancestor. This fused chromosome then spread throughout the human population. Could common design be an alternative explanation? What specifically RULES out common design for the fused chromosome?

-A curious adult from New Jersey

May 08, 2008

Common design is the idea that humans and apes (and all species) were created separately but were designed with similar features. And this designer would not reinvent the wheel for every one of its designs. Once the designer found an idea that worked, this idea would be used in many different situations.

For example, an architect doesn't redesign how a light switch works for every building he or she draws up. Instead each building uses light switches to turn on lights. Some may dim and every plate won't look the same but basically they all use the same principles.

Fusing two chromosomes to make one does not fit with this idea very well. Having 46 chromosomes (23 pairs) is not better than having 48 (24 pairs).

What this means is that there is no obvious reason why a designer would fuse these two chromosomes in humans and leave them unfused in other great apes. The only possible advantage would be if the fusion event created some new gene or activity at the junction point.

And let's say that the fusion did give some advantage. The advantage would come from some new gene being created. Or by genes being turned up or down to different levels.

There are lots of easier ways to do this than by fusing two chromosomes! Particularly for a designer who could incorporate the millions of DNA differences we see between human and ape genomes. What we'd need to say is that the designer designed millions of DNA differences but then for no obvious reason stuck two chromosomes together to make human chromosome 2.

This would certainly not be the most elegant way to design in the changes needed to make a human (see below). And as I said in my previous article, there is no evidence of any advantage in the fusion anyway.

All of this discussion hinges on the evidence that human chromosome 2 resulted from a fusion of two ancestral chromosomes. What evidence do we have that two chromosomes fused to create human chromosome 2?

The very strong evidence to back up the fusion comes from the weird architecture of chromosome 2. Chromosomes usually have a centromere in the middle and a telomere at each end. But human chromosome 2 has telomeres both in the middle and the ends. And it has two centromeres too. The easiest (and possibly only) explanation is that this happened because of the fusion of two chromosomes.

As I said before, chromosomes have a centromere and two telomeres. The centromere is a DNA sequence that is found near the middle of the chromosome. It is used to separate chromosomes during mitosis (replication). Telomeres are DNA sequences that are found at the ends of chromosomes. They protect the chromosomes from losing sequence during DNA replication.

Now let's predict what would happen to our chromosomes if there was a fusion event. We would expect to find telomeres (end sequences) where they don't belong (in the middle of a chromosome). We would also expect to find two centromeres in a single chromosome.

And that is exactly what we find in human chromosome 2. As I said before, this chromosome has two centromeres and telomeres where they don't belong.

Now let's go a little deeper into the evidence. Remember that telomeres are DNA sequences found at the ends of chromosomes. They are made up of many small DNA repeats that run toward the end of the chromosome. There is also a unique pattern of DNA sequence called the pre-telomeric region. This lies just before the telomere. And every chromosome has these sequences.

At the proposed fusion point we find both telomeric and pre-telomeric sequences. In fact, we first see pre-telomeric sequence, then telomeric sequence. Then we see the telomeres inverted and the pretelomeres too (see the figure below). This is exactly what we would predict for a chromosome fusion.

But these sequences don't look exactly like the telomere sequences we see at the ends of the chromosomes. These fusion telomere sequences have collected many DNA changes over time. Biologists can still recognize these sequences at the fusion as old telomeres because even though they aren't exactly the same, they are still pretty similar. At some places they are about 80-90% similar!

And the fact that the fusion telomere sequences have changed over time is even more evidence in support of common ancestry. Evolution predicts that DNA collects random mutations over time. Sometimes these mutations lead to changes we can see. And sometimes these changes don't seem to do anything. Common design predicts that DNA mutations have a purpose. But these fusion sequences don't seem to give humans any advantage.

Biologists also find a second centromere in chromosome 2. This centromere is in the exact location we would expect. It is also no longer functional--it has been inactivated over time.

Now it is possible that the designer fused two chromosomes together and inactivated the extra centromere in order to create a human. But this just isn't likely because there is no design advantage in having 46 instead of 48 chromosomes.

And as we've talked about, there are much simpler ways to get the effects you want without a chromosome fusion. Particularly for a designer who can design in so many differences.

Again, this fusion is not the only difference between people and apes. There are millions of other changes sprinkled throughout all of the chromosomes. It is difficult to see why a designer would make all of those changes and fuse two chromosomes together too.


The DNA sequence of the fusion point on human chromosome 2. Click here to make the image larger.

Compare the telomere sequence found at the ends of the chromosome (dark yellow) to the fused telomere sequence (light yellow). Notice how the letters change after the fusion. This is actually even stronger evidence for the fusion. Remember that DNA has 2 strands but we are only showing one. The match for TTTAGG on the other strand would be CCTAAA the way we have written it.

Monica Rodriguez, Stanford University


Chimpanzee and human chromosomes are very similar.