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Name: Roger
Status: Other
Grade:  Other
Location: N/A
Country: United States
Date: January 2006


Question:
Please could you advise me about the 48 chromosomes in chips/apes and the 46 chromosomes in humans that it is now being suggested is because humans split off from Chimps/apes at a point in the evolutionary tree. Is it possible that the apparent match and combination could be accounted for by any ohter reason than that it was a split in a common ancestor? Is this the only possible explanation?



Replies:
There is indeed good genetic evidence to support this. The other explanation is that the starting number is 46 and that the chromosomes split in two, but that would mean that it happened independently many times in all the other primates that have a chromosome number of 48. The probability of that is very low, so it makes more mathematical sense that primates other than humans have 48 and that in humans two chromosomes came together to make our #2. One of the ways that geneticists pair chromosomes in a picture is to look at the banding pattern. If you line up a human chromosome #2, and the two chromosomes from chimpanzees that match it, it appears that these chromosomes attached head to head (tails pointing out). Now that both the human and chimpanzee genomes have been sequenced, comparing the sequences of these chromosomes provides even better evidence of this.

The ends of chromosomes have stretches of repetetive DNA called telomeres. These seem to be "caps" on the ends of the chromosomes that prevent the DNA from "unraveling". This telomeric DNA has a specific sequence. When studying the middle of chromosome number 2 of humans, there is DNA that has the signature of telomeric DNA on both sides of the centromere, which the "pinched" portion of a chromosome. Why would telomeric DNA be found here? The explanation that makes the most sense is that two shorter chromosomes joined head to head. The fact that chimpanzees and humans have different numbers of chromosomes immediately causes a reproductive barrier and would be an immediate speciation event.

vanhoeck


Evidence suggests that humans evolved from a common ancestor of apes and human by the fusion of two pairs of chromosomes that reduced the chromosome number from 48 to 46. How this happened is not known.

Ron Baker, Ph.D.


We could always come up with all kinds of fantastic explanations. As a rule, scientists work with the simplest explanations first, because simple explanations require us to make few assumptions. This principle is known as "Occam's Razor". For the same reasons, scientists always look for explanations that are based on natural, observable, testable phenomena.

In the case of the striking similarities between human and chimp genomes, by far the simplest explanation anybody has ever proposed is that humans and chimps share a common ancestor, and a fairly recent one at that. Because nobody has ever produced compelling evidence that this explanation is wrong, it is the one that scientists tentatively accept.

If such evidence were to emerge, then scientists would look to modify the explanation.

C. Perkins



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