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Virtually the entire genomes of both chimpanzees and humans has been mapped out in some detail. At the level of the whole genome, about 98.7 percent of the nucleotides in human and chimp DNA are exactly the same. Another way to say this, of course, is that we only differ from chimps at 1.3 percent of our DNA base pairs. Yet chimps and humans look and act quite differently from one another. How can that be? Let's consider two hypotheses that have been put forth to address this question: 1) the "variation in coding regions" hypothesis, and 2) the regulatory genes hypothesis. Both of these hypotheses rely on the power of natural selection, but they do so in very different ways.
The variation in coding regions hypothesis works like this: the human and chimp genome are both composed of about 3 billion base pairs. If we apply the 1.3 percent difference between chimps and humans at the nucleotide level, we get about 40 million base pair differences. If only a small proportion of those basepair differences translate into important functional differences between chimps and humans, than there is plenty of variation for natural selection to operate, and we can begin to understand why chimps and humans act and look so different, but are so similar at the DNA level.
The second hypothesis to address the apparent lack of fit between the levels of molecular genetic variance and the morphological/behavioral differences between species is the regulatory genes hypothesis. To understand the power of gene regulation, remember that every cell in your body has the same set of genes, but skin cells look, feel, and do very different things than cells in muscles, cells in the liver and so on. This is because expression of genes ¾ which genes are turned on and off, and the timing of when they are turned on and of f¾ differs in these cells.
Modern molecular biology has shown that a good deal of DNA is not directly involved in coding for phenotypically visible traits, but rather produces proteins that then regulate the expression of other genes. These regulatory enhancers code for proteins whose function is to determine whether or not other (target) genes will be transcribed; that is, they "turn on" switches that lead to target genes producing products. What this means is that changing specific switches at specific times during development can lead to much in the way of variation, even if genes coding for traits such as body size, pigmentation, time of reproduction and so on remain unchanged. These switches, and when and how they are flipped, may help explain why two species, like humans and chimps can share 99 percent similarity at the DNA level and yet look and act so different.
It is still too early to tell if either of these two hypotheses, or perhaps even a different hypotheses yet-to-be-developed, will help us understand both the similarities and differences between two species that are so remarkably similar at the level of DNA base pairs. I, for one, am waiting with baited breath for the next major breakthrough on this front.
My latest book is The Prince of Evolution: Peter Kropotkin's Adventures in Science and Politics.
Speaking engagements: I have had the opportunity to speak about my work on both the evolution of social behavior and the history of science at hundreds of venues, including 70+ universities, The Smithsonian Institute, The American Museum of Natural History, Colonial Williamsburg, Poptech! and The Idea Festival. You can see more about my prior and upcoming speaking engagements at: louisville.edu. I love visiting with interesting groups of people, hearing about their work, and talking about my own research. Unlike a number of my colleagues, I do not charge exorbitant speakers fees.
