The following clarification was printed in the Guardian's Corrections and clarifications column, Saturday August 20 2006

The source of the report below is the journal Nature.

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Scientists have identified perhaps the most crucial genetic region that makes us human. By comparing human DNA with that of chimpanzees and other animals they have found the region of the genome subjected to the strongest natural selection since we shared a common ancestor with chimps.

The 108-letter stretch of DNA contains two genes that appear to control brain development. The researchers speculate that the blistering pace of evolution indicates that they may have been crucial in the rapid increase in brain size and complexity that occurred in the human lineage. Our brains are three times larger than our closest relatives, the chimps.

"It's evolving incredibly rapidly," said Katherine Pollard at the University of California in Davis. "It's really an extreme case." Most of the 15m or so differences between the chimp genome and our own are random, inconsequential changes that make no difference to our appearance or abilities. To sort the interesting changes from the less influential ones, Professor Pollard looked further down the evolutionary tree to find regions of DNA that really are useful. She and her colleagues first looked for bits of the genome that are nearly identical in the mouse, rat and chimpanzee. These shared a common ancestor around 80m years ago, so the scientists reasoned that any DNA region that had not changed much in this time must be crucial for survival and that changes in its sequence would lead to problems rapidly weeded out by natural selection.

They then trawled these conserved regions for instances where the human equivalent had changed a lot. The beauty of the technique, according to experts, is that they did not have to know what the DNA was actually doing. Top of the list is a 108-letter sequence called HAR1 (Human Accelerated Region 1) which contains two genes. This region differs by just two changes between chimps and chickens, which shared a common ancestor around 310m years ago. But since humans and chimps split 5m years ago there have been 18 changes.

"There has been tremendous pressure for millions of years to keep the sequence as it was. Then something happened in our lineage," said Pierre Vanderhaueghen at the Free University of Brussels in Belgium.

He was able to get clues by adding colour labelled chemicals that would stick to the RNA product produced by the genes to slices of brain tissue from human foetuses. The brains were obtained from aborted or miscarried foetuses and were used with the consent of the mothers.

His results showed that one of the genes is expressed strongly in the developing neocortex during weeks seven to nine of pregnancy. "It's a very exciting finding because it is expressed in cells that have a fundamental role in the design and development of the mammalian cortex," said co-author David Haussler, also at the University of California.