A French version of this release is available by clicking here.
A German version of this release is available by clicking here.
Full size image available through contact |
The human brain shows strikingly different patterns of gene expression compared to the chimpanzee brain, a difference that isn't seen in other parts of the body like the liver and white blood cells, an international research team reports.
Their study in the 12 April issue of the journal Science, published by the American Association for the Advancement of Science, may shed light on why chimps and humans are so genetically similar--humans and chimpanzees share 98.7 percent of their DNA sequences--yet so mentally and physically different.
Zeroing in on these differences could also help scientists learn more about the genetics underlying medical traits such as susceptibility to AIDS, malaria, and Alzheimer's disease, say the Science researchers, led by Svante Pääbo of the Max-Planck-Institute of Evolutionary Anthropology in Leipzig, Germany.
The differences between humans and their closest relatives are more a matter of quantity than quality, according to the report. Differences in the amount of gene and protein expression, rather than differences in the structure of the genes or proteins themselves, distinguish the two species. The overall pattern suggests that the human brain may have experienced accelerated evolutionary change.
Pääbo and colleagues searched for these differences using gene chips carrying tiny dabs of human DNA, representing about 18,000 genes. The chip DNA interacted with genetic material purified from brain and liver tissue collected from humans, chimpanzees, and macaque monkeys, allowing the researchers to measure levels of gene expression in each of the species.
Gene expression proved to be a very individual thing, with some humans appearing more closely related to chimpanzees than to other humans in overall expression patterns. When the researchers pooled the data for each species, however, they uncovered an intriguing pattern.
For blood and liver tissues, levels of gene expression in humans were more like those in the chimp than in the macaque. Differences in gene expression from the macaque pattern appear to have accumulated at roughly the same rate in humans and chimpanzees, which might be expected given their close evolutionary ties.
But the brain tissue told a different story: This time, chimpanzee and macaque gene expression patterns were more similar to each other than to the human pattern. The data suggest that humans may have sped up the rate of change of gene expression in their brains, accumulating expression differences at least five times faster than chimpanzees and distancing themselves from their nearest cousin, say the Science researchers.
A similar trend appeared when the scientists went beyond the genes to examine differences in protein levels in the brain. Once again, Pääbo and colleagues discovered a significant difference in protein expression levels between humans and chimps, supporting the idea that the human brain was subject to accelerated evolutionary change.
To determine if the differences between humans and chimps were common among closely related species, the researchers also analyzed gene and protein expression in two mouse species that are as genetically similar to each other as humans are to chimps. They found fewer differences in expression levels among the mice, further suggesting that the human/chimp discrepancy marks a special evolutionary event.
But if the change was significant, what were the specific genes or proteins involved in the change? The researchers don't know yet whether the boosted expression levels in the human brain are shared generally across many genes, or whether changes in a few genes or gene families are behind the big difference between humans and chimps. The team is collecting more data to see if certain genes have changed "in a particularly consistent and profound way during recent human evolution," says Pääbo.
The Science study authors are also looking into the underlying reasons for different gene and protein expression in the human brain. Gene duplication, deletion, changes in the proteins that jump-start gene expression, and differences in the cellular makeup of brain tissue are all possible candidates, according to Pääbo.
"I believe gene duplications and deletions are likely to contribute a lot to what we find," says Pääbo, adding that comparisons between the human genome and a completed chimpanzee genome may help resolve the issue.
The other members of the research team include Wolfgang Enard, Sebastian Zöllner, Florian Heissig and Philipp Khaitovich at Max-Planck-Institute for Evolutionary Anthropology in Leipzig, Joachim Klose at Institut fuer Humangenetik Charité in Berlin, Germany, Patrick Giavalisco at Max-Planck-Institute for Molecular Genetics in Berlin, Kay Nieselt-Struwe at Max-Planck-Institute of Biophysical Chemistry, Göttingen, Germany, Elaine Muchmore, at University of California San Diego in La Jolla and VA Medical Center, Ajit Varki at University of California San Diego in La Jolla, Rivka Ravid at The Netherlands Brain Bank in Amsterdam, and Gaby M. Doxiadis and Ronald E. Bontrop at Biomedical Primate Researcher Centre in Rijswijkm, the Netherlands.
This researcher was supported in part by the Bundesministerium für Bildung und Forschung and the Max Planck Gesellschaft.
Journal
Science