Posted by jlwile on March 18, 2010
I recently got an E-MAIL from a student who heard a “university professor” say that the human and chimpanzee DNA are 99% similar. She asked whether or not the professor was correct and, if not, how similar is human DNA to chimpanzee DNA?
Well, the answer to her first question is quite easy. The professor was horribly wrong. The nonsensical idea that human and chimp DNA are 99% similar comes from misinterpreting a 1975 paper by Mary-Claire King and A. C. Wilson. 1 This groundbreaking (for its time) article compared several proteins in chimpanzees to their equivalent proteins in humans.
In case you don’t know, proteins are complex molecules that are composed of many smaller molecules (called amino acids) linked together. The primary structure of a protein is simply the order in which its amino acids link up. King and Wilson showed that in many, many proteins, the difference in the primary structures of chimpanzee and human proteins was about 1%. Since DNA determines the order of amino acids in each protein an organism makes for itself, they made the reasonable inference that for the portions of DNA that code for those proteins humans and chimpanzees are 99% similar.
However, the genes that code for these proteins make up a tiny, tiny fraction of the human or chimp genome, and only SOME of those proteins were studied. Thus, the idea that one can extend that number to the entire genome and say that human and chimp DNA are 99% similar is just absurd.
As time went on, of course, we started being able to directly analyze the DNA of an organism and directly compare one genome to another. DNA has several components, but the information-coding part of DNA is composed of nucleotide bases that come in one of four forms: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases link together to hold the DNA in its familiar double-helix structure. However, C can link only to G (or vice-versa) and A can link only to T (or vice-versa). Two nucleotide bases linked together are called a “base pair,” and the order of its base pairs is the way DNA codes its information.
In 2002, a study analyzed roughly 1 million base pairs of the human genome and the chimp genome, and it found that those sections of DNA were about 95% similar.2 Given that there are roughly 3.2 billion base pairs in the human genome, it is clear that such a comparison, while better than that done by King and Wilson, is still such a tiny sample that you really can’t reasonably say anything about how the human and chimp genomes compare.
In 2003, another study looked at 1.9 million base pairs on another part of the chimp and human genome, and it found only 87% similarity. 3 Once again, compared to the total size of the genome, this is a pretty small chunk.
Then things became a lot clearer…sort of. In 2005, the complete draft sequence of the chimpanzee genome was published.4 As a result, the entire chimpanzee genome could be compared to the entire human genome. What was learned? Well, that depends on exactly how things are compared.
The most logical way to compare two genomes is to find the parts that match nearly perfectly. When that is done, we find that 2.4 billion of the base pairs of the human genome line up “nearly” perfectly with 2.4 billion of the base pairs of the chimp genome. As the 2005 paper on the draft sequence says:
Best reciprocal nucleotide-level alignments of the chimpanzee and human genomes cover 2.4 gigabases (Gb) of high-quality sequence, including 89 Mb from chromosome X and 7.5 Mb from chromosome Y. 5
So, out of the 3.2 billion base pairs in the human genome (and about the same in the chimp genome), 2.4 billion of them line up nearly perfectly. It turns out that there are some differences within these 2.4 billion base pairs, and they account for about 3% of those 2.4 billion base pairs.
So…if those 2.4 billion base pairs lined up perfectly, the chimp and human genome would be about 75% similar. However, given that 3% of those base pairs don’t line up perfectly, human and chimp DNA are about 72% similar. Several geneticists have obviously looked at these data, and there are those who think the number will eventually drop below 72% once all the data are in. In fact, Dr. Richard Buggs (geneticist at the University of Florida) says
I predict that when we have a reliable, complete chimpanzee genome, the overall similarity of the human genome will prove to be close to 70% (and very far from 99%).6
In spite of what the data say, PBS asserts the following:
Today, many a schoolchild can cite the figure perhaps most often called forth in support of [a common ancestor for apes and humans]—namely, that we share almost 99 percent of our DNA with our closest living relative, the chimpanzee. 7
I think PBS needs to stop listening to schoolchildren when it comes to evidence for evolution. Of course, the big question is: If 99% similarity was such strong evidence for a common ancestor between chimpanzees and humans, will 70% similarity be considered evidence against a common ancestor? Of course not! Evolution can use special pleading to accommodate any data. It does so with the fossil record, homology, etc. Why not do it with genome similarities as well?
1. Mary-Claire King and A. C. Wilson, “Evolution at Two Levels in Humans and Chimpanzees,” Science 188:107-16, 1975
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2. Brittin, R., “Divergence between Samples of Chimpanzee and Human DNA Sequences is 5%, Counting Indels,” Proceedings of the National Academy of Sciences, USA 99:13633-35, 2002
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3. Tatsuya, A., et al. , “Comparative Sequencing of Human and Chimpanzee MHC Class I Regions Unveils Insertions/Deletions As the Major Path to Genomic Divergence,” PNAS USA 100:7708-13, 2003
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4. The Chimpanzee Sequencing and Analysis Consortium, “Initial sequence of the chimpanzee genome and comparison with the human genome,” Nature 437:69-87, 2005
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5. Ibid, p 71
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