Scientists have sequenced the genome of the chimpanzee and found that humans are 96 percent similar to the great ape species.
"Darwin wasn't just provocative in saying that we descend from the apes—he didn't go far enough," said Frans de Waal, a primate scientist at Emory University in Atlanta, Georgia. "We are apes in every way, from our long arms and tailless bodies to our habits and temperament."
Because chimpanzees are our closest living relatives, the chimp genome is the most useful key to understanding human biology and evolution, next to the human genome itself. The breakthrough will aid scientists in their mission to learn what sets us apart from other animals.
By comparing human and chimpanzee genomes, the researchers have identified several sequences of genetic code that differ between human and chimp. These sequences may hold the most promise for determining what creates human-specific traits such as speech.
"If people are asking what makes us human, they're not going to find a smoking gun [in this study]," said Evan Eichler, a genome scientist at the University of Washington in Seattle who was part of the research team. "But they're going to find suggestions for where to look."
The project was conducted by an international group of scientists called the Chimp Sequencing and Analysis Consortium. Sixty-seven researchers co-authored the study, which is detailed tomorrow in the journal Nature.
To map the chimp genome, researchers used DNA from the blood of a male common chimpanzee (Pan troglodytes) named Clint, who lived at the Yerkes National Primate Research Center in Atlanta. Clint died last year from heart failure at the relatively young age of 24.
A comparison of Clint's genetic blueprints with that of the human genome shows that our closest living relatives share 96 percent of our DNA. The number of genetic differences between humans and chimps is ten times smaller than that between mice and rats.
Scientists also discovered that some classes of genes are changing unusually quickly in both humans and chimpanzees, as compared with other mammals. These classes include genes involved in the perception of sound, transmission of nerve signals, and the production of sperm.
Despite the similarities in human and chimp genomes, the scientists identified some 40 million differences among the three billion DNA molecules, or nucleotides, in each genome.
The vast majority of those differences are not biologically significant, but researchers were able to identify a couple thousand differences that are potentially important to the evolution of the human lineage.
"The goal is to answer the basic question: What makes us humans?" said Eichler.
Eichler and his colleagues found that the human and chimp sequences differ by only 1.2 percent in terms of single-nucleotide changes to the genetic code.
But 2.7 percent of the genetic difference between humans and chimps are duplications, in which segments of genetic code are copied many times in the genome.
"If genetic code is a book, what we found is that entire pages of the book duplicated in one species but not the other," said Eichler. "This gives us some insight into the genetic diversity that's going on between chimp and human and identifies regions that contain genes that have undergone very rapid genomic changes."
Humans and chimps originate from a common ancestor, and scientists believe they diverged some six million years ago.
Given this relatively short time since the split, it's likely that a few important mutations are responsible for the differences between the two species, according to Wen-Hsiung Li, a molecular evolutionist at the University of Chicago in Illinois.
"If you look at two species of frogs over 10 million years, you probably won't see a lot of the morphological or behavioral differences that you see between humans and chimps," said Li, who wrote an accompanying commentary on the chimp genome sequencing for Nature.
There are several hypotheses that account for the evolution of human traits. Li believes these traits come from changes in the parts of the genome that regulate other gene activity.
Scientists agree that many questions remain unanswered but the chimp genome provides important clues to understanding what makes us human.
"We're in a very nice intermediate stage of understanding human-chimp differences," said Eichler. "We can't say, This is the difference that makes us human, but we can say, These are the regions of the genome that show a lot of potential and are excellent candidates to do further work on."