HAR = lot in the non-protein-coding parts of the human genome (nonsense/dark/junk DNA) that shows marked differences when compared with the corresponding batch of the amniot genomes mapped so far (amniotes = the vertebrate compartment that includes mammals, birds and reptiles). Currently, some 50 such regions (HARs) have been identified.
The greatest difference is a DNA portion on chromosome number 20. The region is called HAR1F. It consists of 118 base pairs. Of these, 18 pairs differ from the corresponding region in studied amniotes. In these animal species, on the other hand, the differences between the HAR1F regions are negligible. From this it has been concluded that the corresponding DNA lot has been preserved unchanged during more than 300 million years of evolution. The HAR1F party began to change only in connection with the human and chimpanzee (which has an "untouched" HAR1F area) for approx. 3 million years ago, different paths of evolution went.
It now turns out that the HAR1F region has gene function and that it gives rise to (codes for) a specially shaped RNA molecule. The HAR1F gene appears to be active only during the foetal period, counting from the end of the second foetal month and 10 weeks onwards, and is then almost exclusively active in the predisposition for the cortex of the large brain. Here, the HAR1F gene is primarily switched on in the so-called Cajal-Retzius neurons. The Cajal-Retzius neurons are primitive, distinctive and early maturing nervelles located in the surface layer of the cerebrum.
The production of HAR1F RNA coincides with the Cajal-Retzius neuron forming the glycoprotein Reelin. Reelin directs the formation of new cortex neurons and guides the immature neurons to their proper locations in the growing cerebral cortex.
It is guessed that man's unique HAR1F RNA controls the reelin gene, which could then provide a reasonable explanation for the enormous evolutionary leap that the human brain has taken since it separated from the chimpanzee brain. Note that the human brain with a weight of approx. 1400 grams weighs four times as much as the chican brain (approx. 350 grams).
But then the unanswered question must be asked: What was it that a couple of million years ago triggered the unique changes in the future man's HAR1F region??????
Pollard, K.S. et al. An RNA gene expressed during cortical development evolved rapidly in humans. Nature 443:14,167-172 (2006)