The Mismeasure of Man: Why Popular Ideas about Human-Chimp Comparisons Are Misleading or Wrong
You have probably heard that our DNA, the stuff that makes us human, is only 1% different from chimps. The claim that we are little more than apes is now part of the Zeitgeist of our culture, having been propagated in the popular press for nearly forty years. However, that statement and the conclusions drawn from it are false.
Let's look at the first claim, that we are only 1% different from chimps. That measurement only compares base changes in human and chimp DNA. It doesn't include other kinds of changes to the DNA, like deletions and insertions or rearrangements. In addition, because of the sequencing methods used, repetitive DNA is not included.
Now that complete or nearly complete genome sequences for humans and chimps are available, a better picture of our differences and similarities is emerging. A 2007 essay in the journal Science, "Relative Differences: The Myth of 1%," says this (the pdf is here ):
Researchers are finding that on top of the 1% distinction, chunks of missing DNA, extra genes, altered connections in gene networks, and the very structure of chromosomes confound any quantification of "humanness" versus "chimpness."
To be specific, in addition to the 1% distinction already noted, entire genes are either duplicated or deleted between the two species, sometimes in long stretches called segmental duplications. Such duplications represent a 6.4% difference between chimps and humans. There are also insertions and deletions within genes, which affect the structure and function of the proteins they encode. That contributes another 3%, according to some estimates. And there are entirely new genes, specific to humans.
There are also changes that affect the timing and amount of gene expression. These changes include the insertion of new regulatory sequences upstream of genes. For example, some 6% of our genome is unique Alu insertions, as they are called. And Alu sequences are known to affect gene expression.
In addition, there are human-specific increases in DNA methylation that affect gene expression in the brain, and increased RNA modifications in the brain. These changes would not be detected by simply comparing DNA sequences. Yet they affect gene expression and interaction. Indeed, by one measure, 17.4% of gene regulatory networks in the brain are unique to humans.
Then there are DNA rearrangements. How genes are organized along chromosomes, and even the chromosomes structures themselves can be different. Our Y-chromosomes are strikingly different from those of chimps, for example. This was a surprise to researchers, given the relatively short time our species supposedly diverged from one another. Rearrangements are also not included in the 1% number, and are difficult to quantify.
It should be apparent that we are only beginning to discover important differences between chimps and us, so our numbers are incomplete. In fact, there is no clear way even to count the changes. Beyond that, we do not even know yet how many or which of these differences are functionally important. Perhaps not all are. However, it would be a mistake (one that has been made before) to assert that none of them are functionally significant, as the Encode project demonstrated.
From the same Science paper:
Could researchers combine all of what's known and come up with a precise percentage difference between humans and chimpanzees? "I don't think there's any way to calculate a number," says geneticist Svante Pääbo, a chimp consortium member based at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. "In the end, it's a political and social and cultural thing about how we see our differences." [Emphasis added.]
Indeed. In the end, despite our inability to quantify them, the differences matter. You can have two houses built of the same materials -- two by fours, pipes, wall board, nails, wires, plumbing, tile, bricks, and shingles -- but end up with very different floor plans and appearances, depending on how they are assembled. So it is with us. We may have almost the same genes as chimps, but the timing and distribution of their expression are different, and matter in significant ways.
It should be said that many scientists are aware of these distinctions. What I have said here about differences between chimps and humans is derived from the scientific literature. In fact, the NIH recently recommended that chimps not be used as a model for medical research, precisely because we are different in many ways. Yet somehow these differences do not seem to make it into popular literature or TV shows.
Here are some large-scale differences that get overlooked in the drive to assert our similarity. Our physiology differs from that of chimps. We do not get the same diseases, our brain development is different, even our reproductive processes are different. Our musculoskeletal systems are different, permitting us to run, to throw, to hold our heads erect. We have many more muscles in our hands and tongues that permit refined tool making and speech.
Going beyond the physical, we have language and culture. We are capable of sonnets and symphonies. We engage in scientific study and paint portraits. No chimp or dolphin or elephant does these things. Humans are a quantum leap beyond even the highest of animals. Some evolutionary biologists acknowledge this, though they differ in their explanations for how it happened.
And that brings me to another false assumption underlying the mismeasure of man -- that genes make us who we are. Many things beyond our genes contribute to making us who we are. Our genes do not control us. Certainly, they can influence our predisposition to disease, the shape of our nose, or the color of our eyes, but they do not specify how we will respond to the challenge of disease, or what spouse we will choose. Our experience and our moral character have something to contribute to those things. New studies in psychology indicate, for example, that we can rewire our own brains to think in new patterns; those new thoughts actually change the underlying neural connections. The choices we make matter. And this is a very non-Darwinian thought.
The documentary The War Against Humans and Wesley Smith's companion e-book show how some people seek to equate us with animals, based in part on the false assumptions described above. They see humanity as a destructive species, even a scourge upon the earth that needs to be destroyed.
In truth, though, we are a unique, valuable, and surprising species with the power to influence our own futures by the choices we make. If we imagine ourselves to be nothing more than animals, then we will descend to the level of animalism. It is by exercising our intellects, and our capacity for generosity, foresight, and innovation, all faculties that animals lack, that we can face the challenges of modern life.