Chromosomes, DNA and human evolution


The Yunis-Prakash diagram comparing the chromosomes of humans, chimpanzees, gorillas and orangutans

Evolution in general and human evolution in particular continue to be bones of contention, so to speak, as evidenced by the ongoing efforts by some groups to prohibit or downplay evolution, or to mandate “equal time” for “intelligent design,” in state and local high school curricula. At of the present date (May 2018), just in the U.S., campaigns are active in Indiana, Louisiana, Oklahoma, South Dakota, Tennessee, Texas and Utah.

Although the role of chromosomes in heredity and evolution was recognized in the 19th century, it was in the 1920s and 1930s that creationists argued that humans could not have evolved from apes, as scientists had asserted, because all present-day great apes have 24 pairs of chromosomes, whereas humans have only 23. In response, biologists hypothesized, without any convincing evidence at the time, that two of the 24 ape chromosome pairs must have fused together in some ancient ancestor of modern-day humans, after the human lineage had split off from ape lineages.

Real evidence for this hypothesis did not come until 1982, when Jorge Yunis and Om Prakash, researchers at the University of Minnesota Medical School, carefully stained and magnified each of the 23 pairs of human chromosomes and compared them with corresponding chromosomes of chimpanzees, gorillas and orangutans. They found a “remarkable similarity” between the banding patterns of the 23 human chromosomes and those of the corresponding chimpanzee chromosomes, provided that one compares human chromosome 2 with the end-to-end combination of two corresponding chimpanzee chromosomes (now known as chimp chromosomes 2A and 2B). See their original diagram at the right.

Modern DNA analysis of the human chromosome 2 fusion

Needless to say, Yunis’ and Prakash’s discovery largely confirmed the hopeful hypothesis that had been made decades earlier. Nonetheless, conclusive, molecular-level evidence had to wait until the completion of the human genome project in 2003.

Human chromosome 2 (white), compared with chimp chromosomes 2A (green) and 2B (pink)

As Brown University biologist Kenneth R. Miller pointed out in his new book The Human Instinct: How We Evolved to Have Reason, Consciousness, and Free Will, anyone with an internet browser can now view and compare the detailed DNA sequences of humans and numerous other species. Using the Ensembl facility, with the synteny tool to compare human chromosomes with those of chimpanzees, one can produce the graphic at the right, which shows in some detail how human chromosome 2 is very similar to an end-to-end union of chimp chromosomes 2A and 2B.

What’s more, one can now study the fusion site at the gene and DNA level (see Miller’s book or Daniel Fairbanks’ Relics of Eden: The Powerful Evidence of Evolution in Human DNA). The gene sequence is:

IL1RN – PSD4 – PAX8 – CBWD2 – Fusion – RABLA2A – SLC35F5 – ACTR3 – DPP10

The genes to the left of the fusion position are found on the end of chimp chromosome 2A, while those to the right match the start of chimp chromosome 2B.

Now we can analyze the fusion site itself. Chromosomes normally end with “telomeres,” consisting of long stretches with several hundred repeating copies of the base-pair sequence TTAGGG, which is paired with its complementary sequence AATCCC. At the actual point of fusion, which has been identified at base-pair position 113,602,928 in human chromosome 2, one finds this sequence:


Note that the sequence switches from the base-pair TTAGGG pattern to the complementary pattern CCCTAA right at the point of fusion. It is hard to imagine a more dramatic confirmation of the evolutionary hypothesis. Say it out loud: “Human chromosome 2 arose from the fusion of two chromosomes in an ancient ancestor of modern-day humans, after the lineage leading to modern-day humans had split off from the lineage leading to modern-day chimpanzees.”

Other DNA evidence for human evolution

“Transposons” or “jumping genes” are a highly unusual type of mutation where a section of DNA has been randomly copied from one part of an organism’s genome to another. Although most of the time these inserted genes do no damage, they do provide an excellent means to classify species into their phylogenetic (“family tree”) relationship. This is because it is exceedingly unlikely that the same random insertion of an entire gene would occur at the same spot in the genomes of two or more different species, unless, of course, each inherited this curious feature from a common ancestor.

Here is an example of how transposon data can be used to determine the phylogenetic relationships (i.e., “family tree”) of various primates including humans. In the table below, the columns labeled A through E denote five transposon blocks, and x and o respectively denote that the transposon block is present or absent in the genome of the given species:

						Transposon blocks
			Species		A	B	C	D	E
        /---------	Human		o	x	x	x	x
       /----------	Bonobo		x	x	x	x	x
      / \---------	Chimp		x	x	x	x	x
     /------------	Gorilla		o	o	x	x	x
-----|------------	Orangutan	o	o	o	x	x
     \------------	Gibbon		o	o	o	o	o

Needless to say, it is abundantly clear from this table that our closest primate relatives are chimpanzees and bonobos, with gorillas and orangutans somewhat more distant, yet still closely related, and all six species deriving from a common ancestor. See Alan Rogers’ The Evidence for Evolution for details.

Creationist objections

Some creationist-intelligent design writers, recognizing the strength of DNA evidence, have tried to mount arguments against it. As Kenneth Miller notes in The Human Instinct, the Institute for Creation Research recently claimed that an active gene is located at the chromosome 2 fusion site, and therefore the whole chromosome 2 fusion theory is falsified.

Indeed, the pseudogene (not a regular active gene) DDX11L2 is located adjacent to the fusion site. But it is one of a collection of 18 copies of DDX11L2 scattered around the human genome. Sure enough, all but one of these are found adjacent to telomeres (ends) of chromosomes. Only the one adjacent to the fusion site mentioned above is in the middle of a chromosome. Further, right next to the copy of DDX11L2 adjacent to the fusion site is the gene WASH, which also is adjacent to copies of DDX11L2 wherever they appear, on the opposite side of the telomere sequence, near the ends of chromosomes. Ironically, the observation mentioned by the creationists strengthens rather than falsifies the case for chromosome fusion.

Creationists have also argued that there aren’t as many telomeres, or copies of the TTAGGG sequence, on either side of the fusion site as on the ends of most chromosomes. But this too is actually consistent with the fusion hypothesis, because it has long been noted that chromosomes that have lost telomeres in the process of cell divisions are actually more prone to fusion. Creationists have further argued that the TTAGGG sequences and their complements spanning the fusion site have a few errors, insertions and deletions (note the insertion of an extra G in the sequence above, for instance). But this again is consistent with mutations in the several million years since the fusion event, and certainly not with meticulous molecular-level design.

Finally, some creationist-intelligent design writers have claimed that if such a fusion had occurred in an individual, that individual would not survive, or, at the least, would be completely infertile. But researchers have long noted fused chromosomes in both animals and humans. As Miller points out, a 2013 Chinese report described a 25-year-old male, otherwise completely healthy, with only 22 pairs of chromosomes instead of 23, the result of a fusion of chromosome pairs 14 and 15. Similarly, a Spanish study described a case in which both parents of a family had by chance carried a fusion between chromosome pairs 13 and 14. Three of their six children inherited the same fused chromosome pairs.

In short, none of these objections stand up to scrutiny. Fusion happened, and humans and chimpanzees share a common biological ancestor.


It is sad that in this day and age so many continue to resist the obvious conclusion that evolution has occurred and is occurring, over all the biological kingdom including humans.

Why? It cannot be a religious issue, at least not at a fundamental level. Most major Judeo-Christian denominations have long since made peace with science in general and evolution in particular, noting that the Bible and other works of scripture surely were not intended to be read primarily as scientific textbooks on how the creation transpired. The biologist Kenneth Miller, mentioned above, is a Roman Catholic. The biologist Francis Collins, head of the National Institutes of Health, is an evangelical Christian. The Spanish-American biologist Francisco Ayala, of U.C. Irvine, has been a Dominican Priest. The American biologist Daniel Fairbanks, whose book was mentioned above, has served as a Mormon missionary.

One way or the other, as Kenneth Miller laments, and in spite of 150 years of scientific research since Darwin, many in society are still uneasy with evolution. And yet, as Francisco Ayala and John Avise among others have pointed out, evolution has numerous philosophical and even theological advantages, among them a potential solution to the “problem of evil” — evil exists as a consequence of our long evolutionary heritage, not as a consequence of specific “design.” Further, as Avise wrote,

The evolutionary-genetic sciences thus can help religion to escape from the profound conundrums of Intelligent Design and thereby return religion to its rightful realm — not as the secular interpreter of the biological minutiae of our physical existence but rather as a respectable philosophical counselor on grander matters including ethics and morality, the soul, spiritual-ness, sacredness, and other such matters that have always been of ultimate concern to humanity.

Fortunately, in the U.S. and elsewhere, the public is slowly becoming more accepting of evolution. Let’s hope so — the scientific evidence certainly leaves no doubt.

[This also appeared at the Math Scholar blog.]

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