WORLD SCIENCE

"Long before it's in the papers"
December 19, 2005

Aug. 31, 2005
Special to World Science

Scientists have for the first time published the draft DNA sequence of the chimpanzee, the closest living relatives to humans.

Tammy the chimpanzee at the Saint Louis Zoo in St. Louis, Missouri. (Photo by Carol Weerts, Saint Louis Zoo)

Comparing the the two species’ genomes could reveal what genetic changes resulted in the evolution of Homo sapiens, researchers say, possibly unlocking some of the oldest mysteries about what it means to be human. 

But for now, the data is providing few answers, and some new puzzles. 

The evolutionary lineages leading to chimps and humans are believed to have separated about 6 millions years ago, a short time in evolutionary terms.

“As our closest living evolutionary relatives, chimpanzees are especially suited to teach us about ourselves,” said Robert Waterston of the University of Washington School of Medicine in Seattle, Wash. Waterston is senior author of the paper comparing the two genomes, published in tomorrow’s issue of the research journal Nature. 

But sequencing a genome, like piecing together a book in an unfamiliar language, only means finding out what all the letters are, and in what order. It doesn’t mean you understand the book. Although biologists have made major strides in deciphering the language of genetic code to gain such an understanding, they’re a long way from full comprehension.

To start exploring the chimp-human difference through the genome, researchers are examining where the letters are changed between the genomes. Using that, they can figure out what types of genes these changes affect, if they know where the genes are and what they do. 

A preliminary examination of these changes has revealed that, confusingly, most of the changes between humans and chimps seem to affect genes unrelated to language, intelligence or any of the special things we associate with humanity. 

Rather, most of the changes involve genes affecting our immune and reproductive systems. This actually makes sense, some researchers say, because these are among the fastest-evolving parts of the genome in many animals. 

Immune systems evolve rapidly because of evolutionary arms races between animals and the pathogens that attack them. The two sexes also undergo arms races of a sort in sexual organisms, leading to speedy reproductive system evolution.

The genetic changes that account for what we think of as our “humanness” seem to be fewer, wrote Wen-Hsiung Li and Matthew A. Saunders of the University of Chicago in the same issue of the journal, commenting on the genome findings.

For instance, as is already known, a relatively paltry two “letters” of genetic code differ between humans and chimps in the only gene scientists have directly linked to language, called FOXP2.

Cluttering the picture further is that most the genetic changes between humans and chimps aren’t single-letter changes of that type, the pair wrote. Rather, they are changes known as insertions, deletions or duplications, which roughly correspond to duplicated or deleted stretches of text in a book. The effects of such changes are often unclear. Nor is it apparent whether this type of change or the single-letter change has been more important, Saunders and Li wrote. 

What does seem clear, some researchers said, is that these genetic rearrangements contribute to making the chimp and human genomes more different overall than scientists traditionally thought.

Although the two sequences differ by only 1.2 percent in terms of single-letter changes, duplications and rearrangements of larger stretches of DNA add a further difference of 2.7 percent, wrote Evan Eichler of the University of Washington School of Medicine in the same issue of Nature.

An additional reason that all these changes could be hard to understand, researchers said, is that is that many of them may affect genes with no direct influence on bodily functioning.

Rather, these genes could simply serve to influence other genes, such as by activating or deactivating them. It is “notoriously difficult” to identify such genes, called regulatory genes, according to Saunders and Li.

The chimp sequence was published in the journal by an international group of more than 70 researchers, called the Chimpanzee Sequencing and Analysis Consortium. Several other studies analyzing the data also appear in the journal, as well as the Sept. 2 issue of the research journal Science.

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