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.

However, this isn't the only type of change, or mutation, that can occur through evolution, said Britten. Single nucleotides or whole sections of DNA can end up being deleted or inserted into the existing sequence. These kinds of changes are known as indels, he said.

Due to the paucity of long strings of accurately sequenced DNA data, it hasn't been possible until recently to compare the number of indels between sequences, said Nelson.

Britten decided to re-examine the question of genetic similarity looking at both indels and nucleotide substitutions. He compared long DNA sequences—735,000 nucleotides in length—taken from both the human and chimpanzee genome databases. Britten reports his findings in the upcoming issue of the journal Proceedings of the National Academy of Sciences. The study became available online September 23.

Filling the Gaps

While the results confirmed that single nucleotide substitutions did account for roughly 1.4 percent of the differences, in accordance with previous estimates, Britten also found that indels account for a further 3.9 percent of divergence. This gives a rough estimate of five percent difference, he said.

"There seems to be a deep interest in this question," of how genetically similar we are with chimpanzees, said Britten. "Increasing the number is mostly a technical matter though; we are still the same distance away as we were before, and that is about five million years," he said.

The new estimate could be a little misleading, said Saitou Naruya, an evolutionary geneticist at the National Institute of Genetics in Mishima, Japan. "There is no consensus about how to count numbers or proportion of nucleotide insertions and deletions," he said.

Indels are common in the non-functional sections of the genome, said Peter Oefner, a researcher at Stanford's Genome Technology Center in Palo Alto, California. Scientists estimate that up to 97 percent of DNA in the human genome has no known function. However, he added, indels are extremely rare in gene sequences.

"We haven't observed a single indel in a [gene] to date between human and chimp," said Oefner. Therefore, the revised estimate doesn't alter the amount of DNA that holds information about our species. Humans and chimps still differ by about one percent in gene sequences, he said.

Nevertheless, "5 percent is probably closer to what people thought [the difference would be] a priori," said Nelson. Even the smaller figure of 1.5 percent is quite large across the three billion or so nucleotides that make up the human genome, he said.

Researchers hope that studying the differences between the human and chimpanzee genomes could provide insight into language, intelligence, and other factors that define our species. To this end researchers are now in the process of deciphering the chimpanzee genome.

Despite the small genetic differences between our species, the chimpanzee doesn't suffer from many afflictions that regularly affect people—illnesses ranging from malaria to some types of cancer. Studying the genetic differences between chimps and humans may provide insight into some of these human diseases.

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