AT THE DNA level, humans and chimps are almost 99 percent identical, but recent microarray studies show that it is probably gene expression in the brain that makes all the difference.
An international team of researchers led by Svante Pääbo from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, reported in last week’s Science the results of study in which they compared gene expression in liver and brain tissues from humans, chimpanzees, and orangutans using Affymetrix U95A arrays carrying approximately 12,000 human genes.
From the data, the researchers calculated pairwise distances between samples in order to build distance trees that signaled the evolutionary relationship of the three species. Although they might have missed some differences because the oligo probes on the chip were human-specific, the human gene expression patterns differed more in the brain than the liver samples, suggesting faster evolution in the brain. These findings contrasted with those in a comparative study of three mouse species the scientists conducted using 12,000 gene- Affymetrix murine arrays.
In a second experiment, the researchers used membrane-based cDNA arrays carrying about 21,500 DNA sequences with an average length of one kb to compare brain, liver and blood samples from humans, chimpanzees, and macaques. Again, they found that the rate of change of gene expression levels in the brain was accelerated in humans compared to chimpanzees.
Finally, the scientists assessed differences in protein expression between human and chimp brains using two-dimensional gel electrophoresis, and found a lot of quantitative differences.