Photo courtesy of Gary McLeod

If you find yourself sitting close to a chimpanzee, staring face to face and making sustained eye contact, something interesting happens, something that is alternately moving, bewildering, and kind of creepy. When you gaze at this beast, you suddenly realize that the face gazing back is that of a sentient individual, who is recognizably kin. You can't help but wonder, What's the matter with those intelligent design people?

Chimpanzees are close relatives to humans, but they're not identical to us. We are not chimps. Chimps excel at climbing trees, but we beat them hands down at balance-beam routines; they are covered in hair, while we have only the occasional guy with really hairy shoulders. The core differences, however, arise from how we use our brains. Chimps have complex social lives, play power politics, betray and murder each other, make tools, and teach tool use across generations in a way that qualifies as culture. They can even learn to do logic operations with symbols, and they have a relative sense of numbers. Yet those behaviors don't remotely approach the complexity and nuance of human behaviors, and in my opinion there's not the tiniest bit of scientific evidence that chimps have aesthetics, spirituality, or a capacity for irony or poignancy.


What accounts for those differences? A few years ago, the most ambitious project in the history of biology was carried out: the sequencing of the human genome. Then just four months ago, a team of researchers reported that they had likewise sequenced the complete chimpanzee genome. Scientists have long known that chimps and humans share about 98 percent of their DNA. At last, however, one can sit down with two scrolls of computer printout, march through the two genomes, and see exactly where our 2 percent difference lies.

Given the outward differences, it seems reasonable to expect to find fundamental differences in the portions of the genome that determine chimp and human brains—reasonable, at least, to a brainocentric neurobiologist like me. But as it turns out, the chimp brain and the human brain differ hardly at all in their genetic underpinnings. Indeed, a close look at the chimp genome reveals an important lesson in how genes and evolution work, and it suggests that chimps and humans are a lot more similar than even a neurobiologist might think.

DNA, or deoxyribonucleic acid, is made up of just four molecules, called nucleotides: adenine (A), cytosine (C), guanine (G), and thymine (T). The DNA codebook for every species consists of billions of these letters in a precise order. If, when DNA is being copied in a sperm or an egg, a nucleotide is mistakenly copied wrong, the result is a mutation. If the mutation persists from generation to generation, it becomes a DNA difference—one of the many genetic distinctions that separate one species (chimpanzees) from another (humans). In genomes involving billions of nucleotides, a tiny 2 percent difference translates into tens of millions of ACGT differences. And that 2 percent difference can be very broadly distributed. Humans and chimps each have somewhere between 20,000 and 30,000 genes, so there are likely to be nucleotide differences in every single gene.

To understand what distinguishes the DNA of chimps and humans, one must first ask: What is a gene? A gene is a string of nucleotides that specify how a single distinctive protein should be made. Even if the same gene in chimps and humans differs by an A here and a T there, the result may be of no consequence. Many nucleotide differences are neutral—both the mutation and the normal gene cause the same protein to be made. However, given the right nucleotide difference between the same gene in the two species, the resulting proteins may differ slightly in construction and function.

One might assume that the differences between chimp and human genes boil down to those sorts of typographical
errors: one nucleotide being swapped for a different one and altering the gene it sits in. But a close look at the two codebooks reveals very few such instances. And the typos that do occasionally occur follow a compelling pattern. It's important to note that genes don't act alone. Yes, each gene regulates the construction of a specific protein. But what tells that gene when and where to build that protein? Regulation is everything: It's important not to start up genes related to puberty during, say, infancy, or to activate genes that are related to eye color in the bladder.