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A remarkable treasure buried in the mundane is a captivating tale for children and scientists alike. As one case in point, an international team of researchers has deciphered the genetic code for chromosome 8, uncovering some extraordinary things about a seemingly ordinary cog in the wheel of the human genome.

As they report in the January 19 issue of Nature, chromosome 8 appears at first to have few embellishments: It is of moderate length and measures as the ninth largest chromosome in humans, outsized by eight of its nuclear siblings. And this slice of the genome, which constitutes more than 5% of the genomic workforce, is neither gene-rich nor gene-poor. According to a handful of other benchmarks used to evaluate chromosomes, such as the nature and number of repetitive sequences, chromosome 8 seems just another face in the crowd.

One feature, however, stands out against this lackluster backdrop. Lying near the outermost edge of the chromosome’s short arm, researchers found a region that is accumulating mutations at an incredible pace. In fact, only sections of the Y chromosome may be changing more quickly than this exceptional fragment.

“The region at the top of chromosome 8 is extraordinary in that it has been evolving twice as rapidly as the rest of the human genome since humans and chimps diverged,” said Chad Nusbaum, lead author of the Nature paper and co-director of the Genome Sequencing and Analysis Program at the ӳ����ý. “Our work on this region is shedding light on the evolutionary forces that have helped to uniquely define the human species.”

Scientists surveyed the terrain — comprising about one-tenth of the entire chromosome — and noted several genes that play important developmental roles or that help to convey signals in the brain. In addition, the peak of evolutionary change across the region centers on CSMD1, a sprawling gene that is highly expressed in the brain, where it may regulate cell growth.

The researchers also noticed at least two sites that have been preserved with intent through evolution’s diligence. The first houses a group of defensin genes, which encode short, antimicrobial proteins that are among the first lines of defense for the body’s immune system. Genetic variation in these genes is thought to contribute to the differences in disease susceptibility in humans.

The second site contains the gene MCPH1. Mutations in this gene cause microcephaly, a condition characterized by an abnormally small head and brain. The evolutionary signatures carried in this gene are known to correlate with the dramatic increase in brain size that distinguishes the primate lineage, including apes and humans. Thus, scientists are left to ponder whether the rapid pace of change at the chromosome’s tip had a hand in driving the course of human evolution, particularly with respect to the brain.

Among its other special attributes, chromosome 8 is the first human autosome to have the DNA near its centromere decoded, which marks a milestone that will pique the interest of chromosome aficionados. Centromeres — the compact zones that separate the two arms of a chromosome and ensure its proper inheritance — and their neighboring regions have been largely refractory to analysis using current technologies. Now, with these first snapshots, researchers may begin to track the history of centromeres, a task that has been hindered by a lack of sequence information.