Despite sharing much of their genetic identity with people, chimpanzees exhibit previously unappreciated DNA distinctions, according to the first rigorous comparisons of the two species’ complete genetic sequences.
The new research “dramatically narrows the search for the key biological differences between the species,” says geneticist Robert Waterston of the University of Washington School of Medicine in Seattle.
Waterston led an international consortium that analyzed the genetic sequence of a male common chimp and compared it with DNA data from people (SN: 4/19/03, p. 245: Available to subscribers at Moving On: Now the human genome is really done). Initial results from their study, and from four related studies, appear in the Sept. 16 Science and the Sept. 1 Nature.
Waterston’s group found that the roughly 3 billion base pairs in the genomes of the two species have the same sequence 96 percent of the time. Even so, as many as 3 million base pairs, or DNA building blocks, residing within protein-encoding and other functional areas of the genome differ between chimps and humans.
The new cross-species comparison identified six DNA segments in people that appear to have been strongly shaped by natural selection over just the past 250,000 years. Gene functions in these regions are largely unknown.
Differences in the evolutionary duplication of complete or partial genes, not of individual base pairs, primarily distinguish chimp DNA from that of people, report Washington’s Evan E. Eichler and his coworkers. Differing degrees of gene duplications account for 2.7 percent of chimp and human DNA, whereas single base pair differences represent 1.2 percent.
In both chimps and people, the tip regions of chromosomes appear most volatile, showing signs of frequent gene duplication and migration of various genes from one location to another, according to a group led by Washington’s Barbara J. Trask. Chromosome ends served as “hot spots” for generating DNA disparities among primate species, the scientists propose.
Intriguingly, mutations on the chimp Y chromosome have led to the inactivation of several genes, but no comparable mutations exist on the human Y chromosome, report David C. Page of the Massachusetts Institute of Technology and his colleagues. The researchers speculate that increasing inactivation of the chimp Y chromosome is linked to their high-volume sperm production and fierce competition to impregnate receptive females.
In yet another finding, Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his coworkers discovered that genes active in the brain have accumulated more changes in people than in chimps. Alterations of regulatory genes and protein-making genes have shaped human-brain evolution in equal measure, the scientists also conclude.
In related news, anthropologists have found in Kenya the first fossil of a chimp ancestor. The scientists unearthed three 500,000-year-old teeth that resemble those of common chimps today, report Sally McBrearty of the University of Connecticut in Storrs and Nina G. Jablonski of the California Academy of Sciences in San Francisco.
Fossils of a human ancestor, perhaps Homo erectus, come from the same ancient soil layer that the teeth did. Human and chimp ancestors apparently lived side by side, the scientists conclude.
That’s possible, but fossils from the same soil layer also could represent creatures that inhabited the area at different times, notes Jay Kelley of the University of Illinois at Chicago.