The Latest on Human-Chimpanzee Genetic Comparisons, Part 1 of 2

The Latest on Human-Chimpanzee Genetic Comparisons, Part 1 of 2

The 1% Myth

In 1891 Harley Procter commissioned a laboratory to conduct a chemical analysis that would have far-reaching impact. The laboratory determined that only 0.56% of the measured ingredients in a bar of soap failed to fit into the category of “pure soap.” This result led to one of the best-known advertising slogans of all time: Ivory Soap™ “99 44/100% Pure.”

In 1975 Mary-Claire King performed an analysis that also had far-reaching impact. She determined that several human and chimpanzee proteins display a 99% agreement in amino acid sequence. King’s work showed that humans and chimpanzees possess a closer genetic relatedness than anyone at that time had thought. This result has led to one of the best-known cultural icons of our time: “Humans are 99% chimpanzee.”

For many, this genetic similarity represents one of the most compelling arguments for humanity’s evolutionary origin. Presumably, the 99% sequence overlap observed for proteins and DNA indicates that humans and chimpanzees arose from a common ancestor some time in the relatively recent past. Accordingly, the small genetic differences arose after the human and chimpanzee lineages split as a consequence of mutational changes to each species’ genetic material.

(By the way, a high degree of genetic similarity between humans and chimpanzees does not controvert a biblical view of human origins. In fact, Scripture seems to imply that biological similarity exists between humans and other animals, including the Great Apes. (See a recent article I wrote in Connections for a full explanation.)

Even though many evolutionary biologists consider the 99% genetic similarity between humans and chimps as a profound truth, it turns out to be largely useless information. The genetic similarity between humans and chimps doesn’t explain why there is such a fundamental biological and behavioral difference between us and these creatures.

Anthropologist Jonathan Marks addresses this point in his book, What It Means To Be 98% Chimpanzee. Marks maintains that comparisons based on the percent similarity (or difference) of DNA sequences is largely meaningless. As a case in point, he highlights the fact that humans and daffodils possess a 35% genetic similarity. According to Marks,

In the context of a 35% similarity to a daffodil, the 99.44% of the DNA of human to chimp doesn’t seem so remarkable. After all, humans are obviously a heck of a lot more similar to chimpanzees than to daffodils. More than that, to say that humans are over one-third daffodil is more ludicrous than profound. There are hardly any comparisons you can make to a daffodil in which humans are 33% similar.

A recent news focus piece written by John Cohen for Science makes the same point as Marks. The 99% genetic similarity provides limited biological insight, at best. University of California, San Diego zoologist Pascal Gagneux states in the article that

Now it’s totally clear that it’s (the 99% genetic similarity) more a hindrance for understanding than a help.

It turns out that there are numerous features of the human and chimpanzee genomes that differ significantly. And these genetic differences have the potential to account for the biological and cognitive disparities between our species and chimpanzees.

Cohen identifies several key differences between human and chimp genomes that went unnoticed until recently because of the fixation on the 1% genetic difference. For example:

  1. The true genetic similarity between humans and chimps is not 99% (which is based on substitution mutations). Instead it’s about 90% when indels (insertions and deletions in the DNA sequences) are considered.
  2. Researchers have also discovered that the gene copy number differs for human and chimp genomes. In fact, preliminary data suggests that gene copy number may differ by 6.4% between the two genomes.

Both indels and gene copy number are biologically significant, affecting gene expression.

Thus, the research reveals that humans and chimpanzees are not as genetically similar as popular myth would have us believe. If a 99% genetic similarity implies a close evolutionary relationship, what does a 90% similarity mean?

Nobody would have bought Ivory Soap if it was only 90% pure. Nobody should buy that humans are 99% chimpanzee.

Next week I will discuss another important genetic difference between humans and chimpanzees.

For a detailed discussion of the genetic similarities and differences between humans and chimpanzees and what it means for a human evolutionary and for RTB’s creation model see Who Was Adam?

Part 1 | Part 2