Common descent and the curious origin of human chromosome 2

By Ken Gilmore   Source: Click here

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By Ken Gilmore

Humans have 23 pairs of chromosomes, while the great apes have 24 pairs of chromosomes. Common descent would predict that one of the human chromosomes would owe its formation to a fusion event. This is indeed the case.

Human chromosome 2 owes its origin to the fusion of two chromosomes homologous to ape chromosome. Special creationists are unable to deny this fact, and resort to quote mining technical papers on cell biology in a desperate attempt to cloud the issue.

Our understanding of the origin of chromosome 2 in a fusion event is not new. We've known about the close similarity between human and ape chromosomes as long as thirty years ago, with the researchers behind the seminal paper in Science in which this data was published noting:
"The telomeric fusion of chromosomes 2p and 2q accounts for the reduction of the 24 pairs of chromosomes of the great apes to 23 in modern man. [1]"
As one can see below, the similarities are striking:


 
Over the following 30 years, further work has pinpointed the exact location in human chromosome 2 where the fusion event occurred. Twenty years ago, careful examination of human chromosome 2 showed that it was the result of an ancient telomere to telomere fusion:
“The inverted arrangement of the 1TAGGG array and the adjacent sequences, which are similar to sequences found at present-day human telomeres, is precisely that predicted for a head-to-head telomeric fusion of two chromosomes...These data provide strong evidence that the inverted repeats in c8.1 arose from the head-to-head fusion of ancestral telomeres.” [2]
One year later, further research showed evidence [3] of an ancient centromere in human chromosome 2, giving us evidence of both centromeric and telomeric remnant DNA which is what one would expect if human chromosome 2 was the product of a fusion event. This is no longer controversial in molecular biology. For example, a decade ago, researchers investigating the structure and evolution of human chromosome two noted in passing:
"Humans have 46 chromosomes, whereas chimpanzee, gorilla, and orangutan have 48. This major karyotypic difference was caused by the fusion of two ancestral chromosomes to form human chromosome 2 and subsequent inactivation of one of the two original centromeres (Yunis and Prakash 1982). As a result of this fusion, sequences that once resided near the ends of the ancestral chromosomes are now located in the middle of chromosome 2, near the borders of bands 2q13 and 2q14.1. For brevity, we refer henceforth to the region surrounding the fusion as 2qFus. Two head-to-head arrays of degenerate telomere repeats are found at this site; their head-to-head orientation indicates that chromosome 2 resulted from a telomere to telomere fusion. (Emphasis mine). [4]

That chromosome 2 owes its origin to a fusion event is no longer in doubt. Recent papers on the subject tend to concentrate the specific details of when and how the fusion event took place. For example, Ventura et al in a 2012 paper in Genome Research proposed a model for how the fusion event occurred, and a scenario for the evolutionary history not only of human chromosome 2, but the two chromosomes that in chimps and gorillas did not fuse. The model proposed by Ventura et al is shown at the link below:


Ventura et al comment on their model:
"(C Anc) Catarrhine ancestor, (HCG Anc) human-chimpanzee-gorilla ancestor, (HC Anc) human-chimpanzee ancestor, (H Anc) human ancestor.
(A) A pericentric inversion of chromosome IIq in the gorilla-chimpanzee ancestor (Yunis and Prakash 1982; Roberto et al. 2008; Ventura et al. 2011).
(B) A segment of chromosome 10 was duplicatively transposed to the short arm of chromosome IIq in the common ancestor of human-chimpanzee-gorilla, placing it in close proximity to centromeric satellite sequences.
(C ) A pericentric inversion that occurred on chromosome IIp in the human-chimpanzee ancestor (Wienberg et al. 1994; Roberto et al. 2008) internalizing the SatIII sequences.
(D) A smaller local inversion in ancestral human-chimpanzee IIq placed the chromosome 2 segment adjacent to the StSat; StSat and potentially duplicated sequences spread to the distal end of the short arm of chromosome IIp in the human and chimpanzee ancestor.
(E ) A fusion of IIp and IIq human ancestor created human chromosome 2 (Yunis and Prakash 1982; Wienberg et al. 1994) and squelched the spreading of StSat to human subtelomeric regions. [5]
Do we know the exact details of the fusion event and the evolutionary history of chromosome 2 and the unfused homologous chromosomes in chimps and apes? No, but we have a good idea of how it could have happened. Are we in doubt that a fusion event occurred? No. Anyone who claims otherwise is uninformed, mindlessly parroting creationist mendacity, or lying.

 
References

1. Yunis J.J. Prakash O, "The origin of man: a chromosomal pictorial legacy". Science (1982) 215:1525–1530.
2. IJdo JW, Baldini A, Ward DC, Reeders ST, Wells RA, Origin of human chromosome 2: an ancestral telomere-telomere fusion. Proc Natl Acad Sci USA (1991) 88:9051-5
3. Avarello R et al "Evidence for an ancestral alphoid domain on the long arm of human chromosome 2" Hum Genet (1992) 89:247-9
4. Fan Y, Linardopoulou E, Friedman C, et al "Genomic Structure and Evolution of the Ancestral Chromosome Fusion Site in 2q13–2q14.1 and Paralogous Regions on Other Human Chromosomes" Genome Res. (2002) 12:1651-1662
5. Ventura M et al. “The evolution of African great ape subtelomeric heterochromatin and the fusion of human chromosome 2 Genome Res. (2012) 22: 1036-1049

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