The common ancestor had 24 chromosomes.
If you look at the gene sequences, you'll find that Chromosome 2 in humans is pretty much just 2 shorter chimpanzee chromosomes pasted end-to-end, with perhaps a slight bit of lost overlap:
(H=Human, C=Chimpanzee, G=Gorilla, O=Orangutan)
Somewhere along the line, after humans split off from the other great apes, or during the split itself, there was an accidental fusion of two chromosomes, end-to-end. Where there used to be 24 chromosomes, now there were 23, but containing the same total genes, so other than a "repackaging", the DNA "instructions" remained the same.
If a chimpanzee gives birth to a creature with 23 chromosomes, that offspring isn't going to be a well-formed chimpanzee able to survive well.
It is if the same genes are present, which they would be in the case of a chromosome fusion.
Evolve would imply the genetic material changes little by little --not some big loss of two chromosomes at once but I don't see how they'd go away gene by gene.
Tacking two chromosomes together end-to-end is not a "big loss" of genes, and it really is a "little by little" change in the total genetic code. It's just been "regrouped" a bit. Instead of coming in 24 "packages", it's now contained in 23, but the contents are the same.
So how, you might ask, would the chromosomes from the first 23-chromosome "fused" individual match up with the 24 chromosomes from its mate when it tried to produce offspring? Very well, thanks for asking. The "top half" of the new extra-long Chromosome 2 would adhere to the original chromosome (call it "2p") from which it was formed, and likewise for the "bottom half" which would adhere to the other original shorter chromosome (call it "2q"). In the picture above, imagine the two chimp chromosomes sliding over to "match up" against the human chromosome. The chimp chromosomes would end up butting ends with each other, or slightly overlapping in a "kink", but chromosomes have overcome worse mismatches (just consider the XY pair in every human male -- the X and the Y chromosome are *very* different in shape, length, and structure, but they still pair up).
In fact, the "rubbing ends" of the matched-up chimp chromosomes, adhering to the double-long human-type chromosome, would be more likely to become fused together themselves.
For studies in which recent chromosome fusions have been discovered and found not to cause infertility, see:
Chromosomal heterozygosity and fertility in house mice (Mus musculus domesticus) from Northern Italy. Hauffe HC, Searle JB Department of Zoology, University of Oxford, Oxford OX1 3PS, United Kingdom. hauffe@novanet.itIn that last reference, the Przewalski horse, which has 33 chromosomes, and the domestic horse, with 32 chromosomes (due to a fusion), are able to mate and produce fertile offspring.An observed chromosome fusion: Hereditas 1998;129(2):177-80 A new centric fusion translocation in cattle: rob (13;19). Molteni L, De Giovanni-Macchi A, Succi G, Cremonesi F, Stacchezzini S, Di Meo GP, Iannuzzi L Institute of Animal Husbandry, Faculty of Agricultural Science, Milan, Italy.
J Reprod Fertil 1979 Nov;57(2):363-75 Cytogenetics and reproduction of sheep with multiple centric fusions (Robertsonian translocations). Bruere AN, Ellis PM
J Reprod Fertil Suppl 1975 Oct;(23):356-70 Cytogenetic studies of three equine hybrids. Chandley AC, Short RV, Allen WR.
Meanwhile, the question may be asked, how do we know that the human Chromosome 2 is actually the result of a chromsome fusion at/since a common ancestor, and not simply a matter of "different design"?
Well, if two chromsomes accidentally merged, there should be molecular remnants of the original chromosomal structures (while a chromosome designed from scratch would have no need for such leftover "train-wreck" pieces).
Ends of chromosomes have characteristic DNA base-pair sequences called "telomeres". And there are indeed remnants of telomeres at the point of presumed fusion on human Chromosome 2 (i.e., where the two ancestral ape chromosomes merged end-to-end). If I may crib from a web page:
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).Another piece of evidence is that if human Chromosome 2 had formed by chromosome fusion in an ancestor instead of being designed "as is", it should have evidence of 2 centromeres (the "pinched waist" in the picture above -- chromosomes have centromeres to aid in cell division). A "designed" chromosome would need only 1 centromere. An accidentally "merged" chromosome would show evidence of the 2 centromeres from the two chromosomes it merged from (one from each). And indeed, as documented in (Avarello R, Pedicini A, Caiulo A, Zuffardi O, Fraccaro M, Evidence for an ancestral alphoid domain on the long arm of human chromosome 2. Hum Genet 1992 May;89(2):247-9), the functional centromere found on human Chromosome 2 lines up with the centromere of the chimp 2p chromosome, while there are non-functional remnants of the chimp 2q centromere at the expected location on the human chromosome.
As an aside, the next time some creationist claims that there is "no evidence" for common ancestry or evolution, keep in mind that the sort of detailed "detective story" discussed above is repeated literally COUNTLESS times in the ordinary pursuit of scientific research and examination of biological and other types of evidence. Common ancestry and evolution is confirmed in bit and little ways over and over and over again. It's not just something that a couple of whacky anti-religionists dream up out of thin air and promulgate for no reason, as the creationists would have you believe.