Human accelerated regions (HARs), first described in August 2006,[1][2] are a set of 49 segments of the
human genome that are conserved throughout
vertebrate evolution but are strikingly different in
humans. They are named according to their degree of difference between humans and
chimpanzees (HAR1 showing the largest degree of human-chimpanzee differences). Found by scanning through genomic databases of multiple species, some of these highly
mutated areas may contribute to human-specific traits. Others may represent loss of functional mutations, possibly due to the action of biased
gene conversion[2][3] rather than
adaptive evolution.[4][5][6]
Several of the HARs encompass genes known to produce proteins important in neurodevelopment. HAR1 is a 106-base pair stretch found on the long arm of
chromosome 20 overlapping with part of the
RNA genesHAR1F and HAR1R. HAR1F is active in the developing human brain. The HAR1 sequence is found (and conserved) in chickens and chimpanzees but is not present in fish or frogs that have been studied. There are 18 base pair mutations different between humans and chimpanzees, far more than expected by its history of conservation.[1]
HAR2 includes
HACNS1 a
gene enhancer "that may have contributed to the evolution of the uniquely opposable human
thumb, and possibly also modifications in the
ankle or
foot that allow humans to
walk on two legs". Evidence to date shows that of the 110,000 gene enhancer sequences identified in the human
genome, HACNS1 has undergone the most change during the
evolution of humans following the split with the ancestors of
chimpanzees.[7] The substitutions in HAR2 may have resulted in loss of binding sites for a repressor, possibly due to biased gene conversion.[8][9]