NEW YORK: Scientists working on human genes believe they have identified the gene sequence that is central in giving humans their unique brainpower. While the DNA sequences of humans and chimpanzees are 98 per cent identical, the 2 per cent difference represents some 15 million changes in human genes since the time of the common ancestor for humans and the chimps some six million years ago, the scientists feel.
The scientists, using a new computational technique, searched for segments in the DNA that showed the most changes between humans and chimps and identified 49 regions that have changed particularly quickly between the two and at least one gene critical to the development of larger brains in humans.
They call these areas as human accelerated regions, or HAR, and the most radical one HAR1. And it has been found that in HAR1 there were 18 major changes in humans and only two in chimps. Within the region, they found a gene that turns on during the 12 weeks when a key brain structure, responsible for language and data processing, forms within the human embryo.
Details of the research, carried out at the University of California-Santa Cruz, are published in the journal Nature. One of the scientists involved in the study, David Haussler, director of the Center for Biomolecular Science and Engineering at the university and a specialist in computer-based genetic analysis, said the question why humans are brainier than chimpanzees has not been definitively answered, but there are clues now.
The scientists first studied animal genomes to make out areas that are highly similar, indicating they had changed little over millions of years. They then compared these areas in humans and found one area, HAR1, where significant changes had occurred. The HAR1 stretch is designated using 118 letters of code and all but two were the same in chickens and chimpanzees, Haussler said. However, in humans, there are 18 changes and this could not have happened by chance, he added.
He says the region was virtually identical in humans and other animals for 300 million years, until the changes began in humans about 2 million years ago.
The scientists established that a piece of RNA existed within HAR1 that turns on from the seventh to the 19th week during the development of human embryos -- precisely the time when the cerebral cortex begins to form. Cerebral cortex, known commonly as the brain's grey matter, plays a central role in human capabilities like planning, emotion and judgement and is equally important as the human skill of language and memory.
The lead investigator in the study Svante Paabo said comparing the genetic sequences will help in understanding the evolution of characteristics specific to each group and even cognitive function.
The scientists' team was also involved in the Human Genome Project, building the program that assembled the first working draft of the human genome sequence from information produced by sequencing centers worldwide. Other members of the team came from the University of Brussels and Universite Claude Bernard in France.
