What Makes Us Human? 

Comparisons of the genomes of humans and chimpanzees are revealing those rare stretches of DNA that are ours alone

By Katherine S. Pollard  | April 20, 2009 | 69

The 1 percent difference: Humans are distinct from chimpanzees in a number of important respects, despite sharing nearly 99 percent of their DNA. New analyses are revealing which parts of the genome set our species apart. Image: James Balog Getty Images

In Brief

• Chimpanzees are the closest living relatives of humans and share nearly 99 percent of our DNA.
• Efforts to identify those regions of the human genome that have changed the most since chimps and humans diverged from a common ancestor have helped pinpoint the DNA sequences that make us human.
•  The findings have also provided vital insights into how chimps and humans can differ so profoundly, despite having nearly identical DNA blueprints.

Six years ago I jumped at an opportunity to join the international team that was identifying the sequence of DNA bases, or “letters,” in the genome of the common chimpanzee (Pan troglodytes). As a biostatistician with a long-standing interest in human origins, I was eager to line up the human DNA sequence next to that of our closest living relative and take stock. A humbling truth emerged: our DNA blueprints are nearly 99 percent identical to theirs. That is, of the three billion letters that make up the human genome, only 15 million of them—less than 1 percent—have changed in the six million years or so since the human and chimp lineages diverged.
Evolutionary theory holds that the vast majority of these changes had little or no effect on our biology. But somewhere among those roughly 15 million bases lay the differences that made us human. I was determined to find them. Since then, I and others have made tantalizing progress in identifying a number of DNA sequences that set us apart from chimps.