14 DAY TRIAL // A new study comparing the genomes of humans, chimps, monkeys, rats and mice found an unexpectedly high degree of genetic difference in the human DNA regions that influence nerve cell adhesion, compared with the DNA of the other animals.
The researchers compared the genomes investigating a set of 202 carefully chosen "highly accelerated regions" (HARs) in order to discover any evidence of rapid evolution in the human lineage. The team suspects that human neurons connect among themselves with greater complexity, enabling the growth of a bigger brain.
The assembly of the ten billion neurons in the human brain relies on precise expression of adhesion molecules that allow for thousands of connections between neurons and the matrix of proteins around them. "Cell adhesion controls many aspects of brain development, including growth and structure," says Shyam Prabhakar at Lawrence Berkeley National Laboratory in Berkeley, California, US.
The scientists looked for highly conserved DNA sequences between chimpanzees and rodents, comparing them to those from human genome in order to see the fastest evolving areas in human and thus revealing the genes behind the rapid evolution of the human brain. "Most of the differences between chimps and humans are not in our proteins, but in how we use them." explained Katherine Pollard, at the UC Davis Genome Center.
In fact, scientists found only three HARs placed inside the genes, thus encoding proteins. The rest are located in DNA zones which do not encode proteins, and many HARs are close to genes involved in growth and development. However, HAR1, a dramatically changed DNA sequence, appears to encode a RNA sequence that may have a function in brain development. RNA transports the genetic code from DNA to the ribosomes particles that read it and bind the amino-acids in proteins. "They're not in genes, but they're near genes that do some very important stuff," Pollard said.
Non-coding regions of DNA suffer more rapid and diverse mutations than regions carrying genes, because they are not stopped by natural selection from accumulating. The human HARs are highly conserved across the other groups of examined mammals, suggesting that they possess important functions that impede them from great variation.
Scientists want to see now if and in which way differs the levels of proteins involved in nerve adhesion between the brains of monkeys, chimps and humans, as DNA analysis suggested. "We don't have any [physical] changes to link the genetic changes to," explains Todd Preuss at Yerkes National Primate Research Center in Atlanta, Georgia, US.