A new comparison of chimpanzee and human genomes has offered an early but tantalizing look into what makes the two species, nearly identical at the DNA level, so different.

Scientists found key differences in areas linked to cell differentiation and immune response — and that could be just the beginning.

"By looking at all the variations, we will get a catalog, and when we find a variation in a person with a disease, it will help us understand the function of that variation," said study co-author Richard Redon, a geneticist at the Wellcome Trust Sanger Institute. "It will help us understand better how our species emerged."

Researchers already know that humans and chimpanzees share about 98 percent of the same genes. But rather than searching for mutations, Redon's team looked at a relatively unstudied phenomenon known as copy number variation, or CNV, in which genes are redundantly duplicated.

These variations were ignored in the early days of genetics, but have recently been recognized as important: Mutations are more likely to accumulate in a given type of gene when multiple copies exist, and the simultaneous application of multiple genes can provide a functional boost.

Redon's team is the first to assemble maps of CNV similarities across both chimpanzees and humans, and then compare the maps to each other.

What roles those cell differentiation and immune response variants might play isn't yet known — but the details of the findings, said

Redon, are less important than the example set by the study, published today in

Genome Research.

"It's just a start," he said. "We used a platform that isn't very high-resolution. We found the largest variations, but the smaller ones, maybe we missed. And the biggest aren't the most important — it's just a matter of size. Some of the littlest changes can have the largest effect."

Melanie Babcock, an Albert Einstein College of Medicine geneticist who has studied copy number variations in primates, called the work

"fascinating."

But Babcock, who was not involved in the study, cautioned that simple comparisons couldn't indicate when and where duplicate genes are active. That information could prove necessary in understanding the function of CNVs.

"Without looking at expression levels, it's hard to say. Yes, there might be a CNV, and it might mean that there is an amazing difference between humans and chimps — or that they don't use the genes very much, and so they accumulate and are not important," said Babcock.

Redon stopped short of saying that CNVs are more important than other types of genetic variation, instead calling them complementary.

"To have a better view, it's good to have a global picture with all types of variation," he said.

Copy number variation and evolution in humans and chimpanzees [Genome Research]

Image: Areas of CNV similarity between humans (top bar in each pair) and chimpanzees in two genes/Genome Research

Note: An eloquent evaluation of the study by molecular anthropologist Jonathan Marks arrived after the article was posted. The author of What it Means to Be 98% Chimpanzee noted that scientists don't fully understand the links between genes and physiology. The chimp-human CNV comparison "is reasonable," said Marks, and "calls attention to a generally overlooked mode of mutation that seems to produce a non-trivial amount of difference" between our genomes — but we shouldn't "interpret that to mean that they have discovered why we are not sleeping in trees and eating the placentas of our newborns."

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