Hi GL, I agree with everything in your post.
Hi Anita,
It is asking way too much to expect someone without a science background to follow this completely, but here is my paraphrase of the below article. There was a nice picture of comparative chromosomes that didn't make it through the scan.
“Comparison of the Human and Great Ape Chromosomes as Evidence for Common Ancestry” by Robert
Williams, from The Evolution Evidence Page at
http://www.gate.net/~rwms/EvoEvidence.html
Here is striking evidence for the common ancestry of humans and the great apes independently of the usual paleontological, morphological, and molecular phylogenetic data that we usually see.
Looking at the chromosomes of humans and the living great apes (orangutan, gorilla, and chimpanzee), it is immediately apparent that there is a great deal of similarity between the number and overall appearance of the chromosomes across the four different species. The four species have a similar number of chromosomes, with the apes all having 24 pairs, and humans having 23 pairs. The fact that humans have 23 chromosomes and the other three have 24 — tells us the fusion of chromosome two and three occurred in the future homo species after chimps split off from the common ancestor of chimps and humans.
Most of the chromosomal differences among the four species involve inversions - localities on the chromosome that have been inverted, or swapped end for end. This is a relatively common occurrence among many species, and has been documented in humans. An inversion usually does not reduce fertility.
There are two potential explanations for the difference in chromosome numbers - either a fusion of two separate chromosomes occurred in the human line, or a fission of a chromosome occurred among the apes. The evidence favors a fusion event in the human line. This presents two predictions. Since the chromosomes were apparently joined end to end, and the ends of chromosomes (called the telomere ) have a distinctive structure from the rest of the chromosome, there may be evidence of this structure in the middle of human chromosome 2 where the fusion apparently occurred. Also, since both of the chromosomes that hypothetically were fused had a centromere (the distinctive central part of the chromosome), we should see some evidence of two centromeres.
The first prediction (evidence of a telomere at the fusion point) is shown to be true. Telomeres in humans have been shown to consist of head to tail repeats of the bases 5'TTAGGG running toward the end of the chromosome. Furthermore, there is a characteristic pattern of the base pairs in what is called the pre-telomeric region, the region just before the telomere. When the vicinity of chromosome 2 where the fusion is expected to occur (based on comparison to chimp chromosomes 2p and 2q) is examined, we see first sequences that are characteristic of the pre-telomeric region, then a section of telomeric sequences, and then another section of pre-telomeric sequences. Furthermore, in the telomeric section, it is observed that there is a point where instead of being arranged head to tail, the telomeric repeats suddenly reverse direction - becoming (CCCTAA)3' instead of 5'(TTAGGG), and the second pre-telomeric section is also the reverse of the first telomeric section. This pattern is precisely as predicted by a telomere to telomere fusion of the chimpanzee (ancestor) 2p and 2q chromosomes, and in precisely the expected location. Note that the CCCTAA sequence is the reversed complement of TTAGGG (C pairs with G, and T pairs with A).
The second prediction: The normal centromere found on human chromosome 2 lines up with the 2p chimp chromosome, and the remnants of the 2q chromosome is found at the expected location based upon the banding pattern.
Some may raise the objection, how could the first human ancestor with the fusion have successfully reproduced? The last remaining species of wild horse, Przewalski's (sha-val-skis) Wild Horse has 66 chromosomes while the domesticated horse has 64 chromosomes. Despite this difference in chromosome number, Przewalski's Wild Horse and the domesticated horse can be crossed and do produce fertile offspring.
Now, the question has to be asked - if the similarities of the chromosomes are due only to common design rather than common ancestry, why are the remnants of a telomere and centromere (that should never have existed) found at exactly the positions predicted by a naturalistic fusion of the chimp ancestor chromosomes 2p and 2q?
Another chromosomal rearrangement has recently been discovered, this one shared both by humans and chimpanzees, but not found in any of the other monkeys or apes that were tested. This rearrangement was the movement of about 100,000 DNA pairs from human chromosome 1 to the Y chromosome10. See "The Promise of Comparative Genomics in Mammals" Science, Oct. 1999 to learn how similar chromosomal comparisons are being used to map the evolutionary relationships of all living mammals.
DB