Sunday, 16 June 2013

Genetic Evidence for Human Evolution - Chromosome 2 Fusion

The subject of evolution and the Flood have been recently  debated at the main Christadelphian Facebook page. Inevitably, human evolution has been raised, with one of the resident evolution denialists asking:
But your wholesale dodge of a really important question still requires address. I repeat: could you please clarify whether you believe Jesus (on his mother's side) is ultimately descended from the apes? [1]
Whether the question is meant in good faith or not, or simply meant to poison the well and make rational discussion impossible is beside the point. The question is one which is definitely on many people’s minds.
What do you mean when you say that all humans share common descent with apes? The only thing I can agree on is that God created both apes and humans but we do not come from apes. God formed man Adam from the dust of the earth. I don't know what genetic evidence you are referring to anyway. [2]
I am sorry but I believe in creation and that humans did not come from apes. If we did it would have been in the bible that we did. So I will never buy that. Oh and I do not believe that Jesus ever descended from apes. That is silly. I agree. [3]
OK. So if we are going for exact terms... Do you...believe that apes form part of our human ancestry. In otherwords if we go back far enough, do you believe that our great great great....... (n'th degree) grandfather was an ape...We share DNA with cabbages, but that doesn't mean we descended from them! Please clarify your response. Thankyou. [4]
Clarinda it is only when in creation that some say that we share some of the same DNA with apes when we clearly do not. To me that is the offensive part. In death humans and animals die the same death. We go back to the dust of the earth but the difference between humans and animals is that humans that have decided to follow Jesus have the hope of the resurrection as the animals do not. The only thing we have in common with animals is when we die and cease to breath and turn back to dust. [5]
Speaking as someone who actually knows something about human molecular genetics (that sort of knowledge comes with the turf when you're a medical doctor who has studied for post-graduate examinations) these responses betray a complete ignorance of the subject of human genetics and evolutionary biology by those making them. In the spirit of disabusing people of their ignorance, here is an outline of the evidence for human-ape common ancestry, which contrary to what many in our community think, is overwhelming.

Part 1. Human chromosome 2 is a fusion of two ape-like chromosomes

Humans have 23 pairs of chromosomes, but the great apes have 24 pairs. If humans and apes shared a common ancestor, then one of the chromosomes would need to be a fusion product. This prediction has been confirmed: chromosome 2 was produced by a fusion of two chromosomes in our remote evolutionary past. 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. [6]
Here's what our chromosomes look like when compared with ape chromosomes. Note the similarity between chromosome 2 and the ape chromosomes.

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. [7]
One year later, further research showed evidence [26] 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 2resulted from a telomere to telomere fusion. (Emphasis mine). [9]
The evidence is unambiguous and irrefutable. Human chromosome 2 is the product of an ancient fusion of two ape-like chromosomes, as evidenced by the telomeric and centromeric remnant in the chromosome. Cell biologist Kenneth Miller - who is also a Christian - summarises the reasons why the chromosomal fusion provides strong evidence of human-ape common ancestry:

One attempted rebuttal to this is that such chromosomal fusion events hinder fertility. This is not true. The molecular biologist Arthur Hunt points out that contrary to creationist assertions, such chromosomal translocations do not necessarily impact on fertility and quotes numerous examples of observed chromosomal translocations in mammals, and concludes that "it should be clear that [the creationist claims] are simply not reflective of the current thinking in mammalian cytogenetics." [10]

Genomic data from extinct hominids provides further support for the reality of human-ape common ancestry just from this line of evidence. Recently, DNA from a hominid bone found in the Denisova cave in Siberia was sequenced, showing that it came from a species closely related to the Neanderhals. It turns out that the Denisovan hominids also had the fused second chromosome. Palaeoanthropologist John Hawks quotes the researchers, noting that:
Sometime in our evolution, two separate chromosomes fused into one, giving us a karyotype of 46 chromosomes where chimpanzees, bonobos and gorillas have 48chromosomes. The high-coverage genome was sufficient to show that Denisova shared the human fusion: 
"Of more relevance may be examination of aspects of the Denisovan karyotype. The great apes have 24 pairs of chromosomes while humans have 23. This difference is caused by a fusion of two acrocentric chromosomes that formed the metacentric human chromosome 2 , and resulted in the unique head-to-head joining of the telomeric hexameric repeat GGGGTT. A difference in karyotype would likely have reduced the fertility of any offspring of Denisovans and modern humans. We searched all DNA fragments sequenced from the Denisovan individual and identified twelve fragments containing joined repeats. By contrast, reads from several chimpanzees and bonobos failed to yield any such fragments. We conclude that Denisovans and modern humans (and presumably Neandertals) shared a karyotype consisting of 46 chromosomes." 
We still have no idea whether this fusion made any difference to any phenotype in ancient humans.
Many, many people have written me over the years to ask whether this fusion of two ancestral chromosomes might have been important to our evolution. Perhaps, many suggested, if Neandertals had a chromosomal incompatibility with us, that would explain why they became extinct. I have always doubted this, but without information it was impossible to be certain.
It's nice to now have the information in hand: This fusion happened earlier in our evolution. [11]
This poses a huge problem for special creationists. Why would God not only create humans with chromosome 2 looking like the product of a fusion of two ape chromosomes, but do the same thing to an extinct species of humans who lived thouisands of years before Adam (assuming we place Adam around the time animals and plants were domesticated in the ANE)? Common descent solves this problem neatly - the Denisovans and Homo sapiens share a common ancestor in which he chromosomal fusion took place. Special creationists have no credible answer.

We make ourselves look like idiots when we blithely declare that there is no evidence for human-ape common ancestry when this material has been in the public doman for years And this is only one part of the genetic evuidence for human-ape common ancestry. More to come in later posts.

This article first appeared on my Facebook page here.


6. Yunis J.J. Prakash O, "The origin of man: a chromosomal pictorial legacy". Science (1982) 215:1525–1530.

7. 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

8. Avarello R et al "Evidence for an ancestral alphoid domain on the long arm of human chromosome 2" Hum Genet (1992) 89:247-9

9. 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

10. Hunt A "The Rise of Chromosome 2: The Fertility Problem" The Panda's Thumb February 8th 2009 

11. Hawks, J "The fused chromosome 2 was in Denisova" John Hawks Weblog 1st Sep 2012