Humans, Chimps Not as Closely Related as Thought?

John Pickrell
for National Geographic News
September 24, 2002

For decades, scientists have agreed that human and chimpanzee DNA is 98.5 percent identical. A recent study suggests that number may need to be revised. Using a new, more sophisticated method to measure the similarities between human and chimp DNA, the two species may share only 95 percent genetic material.

The result is surprising, said David Nelson, a geneticist at Baylor College of Medicine in Houston, "in that it's more than twice as much difference as we thought" existed.

DNA is the nucleic acid found in all cells that stores and transmits genetic information from one generation to the next. By comparing the similarity of DNA between two species, scientists can determine how closely they are related.

Researchers at the University of California, Berkeley, first discovered the astonishing genetic similarity between the two species in the mid 1970s, when they compared proteins in humans and chimpanzees, (Pan troglodytes), and found that they were 99 percent identical.

Further experiments by the same team showed that 98.5 percent of DNA sequences are shared by humans and chimps. The same methods showed that two humans share 99.9 percent of their DNA. In contrast, the DNA of humans and mice is only around 60 percent similar.

Revisiting the Numbers

"People were initially very surprised by the close proximity," said Nelson. Ideas about the uniqueness of man led some people to expect that the chimpanzee would have quite different DNA, he said.

At the time, said Nelson, the results helped resolve a debate regarding the relationships among the great apes—a group which also includes the gorilla, (Gorilla gorilla), and the orangutan, (Pongo pygmaeus). The DNA data proved that humans and chimps are more closely related to one another than either is to the gorilla.

Using the more sophisticated methods that became available in the 1980s and 1990s, scientists revisited the question of how much DNA humans and chimps share, and came to similar conclusions.

However, these researchers may have been missing some crucial information, said Roy J. Britten, a geneticist at the California Institute of Technology in Corona del Mar. Britten is a co-developer of the method originally used to look for genetic similarities in the 1970s.

The early methods only take into account certain types of evolutionary change called substitutions, said Britten. Substitutions occur when one of the four molecules that join to form DNA—called a nucleotide—is replaced by one of the other three types.

Continued on Next Page >>




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