WASHINGTON, Feb 11 (Reuters) - An eruption of a poorly understood kind of genetic change set humans apart from great apes, and also sets chimps, gorillas and orangutans apart from monkeys, researchers reported on Wednesday.

Right before the great apes branched off from other apes and monkeys 10 million years ago, their DNA began to make explosive changes -- not classic mutations, but another change known as copy number variation, University of Washington geneticist Evan Eichler and colleagues found.

These changes in DNA sequences may help explain what makes humans -- and other apes -- unique, they report in the journal Nature.

"These are really like volcanoes in the genome, blowing out pieces of DNA," Eichler said in a statement.

Eichler, a Howard Hughes Medical Institute researcher, and his team analyzed the DNA of a human, a chimpanzee and an orangutan, as well as of a macaque monkey. Looking at species known to be at various distances on the evolutionary tree can act as a kind of time machine to track changes over millennia.

As expected, they found humans and chimps were very close in terms of overall sequence. The main differences lie in copy number variations -- repeats of the same genetic sequence over and over, deletions of the sequence, or even instances in which a sequence runs backward.

Among humans, these variations have been associated with diseases ranging from AIDS to autism. They may also underlie the differences between species, Eichler said.

The analysis suggested that in the ancestral branch of primates leading to humans and the African great apes, the number of these duplications began to increase at the same time that the more classic genetic mutations were slowing down.

Humans and chimps, in particular, tend to have extra copies of these sequences, they reported.

"There's a big burst of activity that happens where genomes are suddenly rearranged and changed," Eichler said.

"There is the possibility that these genes might be important for language or for aspects of cognition, though much more work has to be done before we'll be able to say that for sure." (Reporting by Maggie Fox; Editing by Xavier Briand)

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