Humans and chimpanzees share 96 percent of the same genetic material yet are profoundly different creatures, a mystery scientists hope to begin to unravel with Wednesday’s announcement that the genes of humanity’s closest cousin have been cataloged.
Publication of the complete chimp genome, marked by a celebratory issue of the journal Nature, is viewed by scientists as a landmark event that will lead to a better understanding of how the human genome has evolved and insight into diseases that people get but chimpanzees rarely suffer, including cancer and Alzheimer’s.
It long has been speculated that humans and chimpanzees diverged from a common ancestor between 6 and 7 million years ago, a relative eye-blink in evolutionary terms. Their genes are quite similar, posing a direct challenge to the uniqueness of humans.
The scientific value of the chimp genome, researchers say, lies in the subtle differences. Data are now at hand that can be used to figure out what makes us human, they said.
“This genomic comparison dramatically narrows the search for the key biological differences between the species,” said the study’s senior author, Dr. Robert Waterston, chair of the department of genome sciences at the University of Washington School of Medicine in Seattle.
Nature asked Wen-Hsiung Li, a pioneering genetic evolutionist at the University of Chicago, to write a commentary on the work.
“Because the genomes are huge – 3 billion base pairs of DNA – the differences in molecular terms are quite a lot,” Li said. “Most of them probably will not be biologically significant, but some may be crucial. Our task now is to identify those that are meaningful and prove it in the laboratory.”
Comparison of the human and chimp genomes reveals that their genetic sequences are directly comparable over 96 percent of their lengths, and these regions are 99 percent identical.
Out of the 3 billion base pairs of DNA in each genome, about 35 million differ, of which as many as 3 million may lie in crucial protein-coding genes.
The number of genetic differences between a human and a chimp is about 60 times less than that between a human and a mouse and about 10 times less than between a mouse and a rat – but it is about 10 times more than between any two humans.
Most of the work of sequencing and the assembling the chimp genome was done at the Broad Institute of the Massachusetts Institute of Technology and Harvard University and the Washington University School of Medicine in St. Louis.
The chimp DNA came from a male named Clint, who died last year of heart failure at the Yerkes National Primate Center in Atlanta.
Tarjei S. Mikkelsen, an MIT graduate student of such promise that his elders honored him by making him senior author of a major paper, said researchers will be studying genetic changes that may be related to such human-specific features as walking upright on two feet, a greatly enlarged brain and complex language skills.
Among the discoveries so far is that three key genes involved in inflammation appear to be deleted in the chimp genome, perhaps explaining some of the known differences with humans in respect to the immune inflammatory response.
On the other hand, humans appear to have lost function of the caspase-12 genes, which produce an enzyme that may help protect other animals from Alzheimer’s disease.
“The sequencing of the chimp genome is a historic achievement that is destined to lead to many more exciting discoveries with implications for human health,” said Dr. Francis Collins, director of the National Genome Research Institute, which funded the project at a cost he put at $20 million to $40 million.
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