by Sharvari Apte
Credits: Bill Sanderson/Science Source
Human beings are undeniably a unique species on this planet, with no other life form able to rival the self-awareness and innovativeness which we have come to possess. This attributes to the prominently developed ventrolateral frontal cortex [1] of our brains, only having evolved in primates.
Even if we look at our closest relatives, chimpanzees and bonobos, there is still a 1.2% difference between the human genome and that of the chimpanzee. This equates to approximately 35,000,000 differences in the DNA base pairs in each cell [2]; the big question is, why? Why is the human DNA sequence so different from any other known organism?
The presence of HARs, or Human Accelerated Regions, has been a long-standing cause of speculation. These are sections of the human genome which have an inexplicably high number of base substitutions, deletions and duplications [3] - mutations - when compared to the genome sequence homo sapiens would have inherited from the last chimpanzee-human common ancestor.
Extensive research has been conducted into HARs since their discovery in 2006, and scientists have gathered that the majority of the genes that constitute HARs are non-coding[4]. However, these regions have been shown to be strongly related to the neurodevelopment of individuals, being responsible for genetic expression in the cerebral cortex and neural maturation [5].
Specific mutations within HARs also correlate with human-specific conditions, such as autism spectrum disorder, schizophrenia and epilepsy, since the non-coding genes are responsible for the regulation of the localised expression of genes. For instance, mutations in the AUT2S gene during embryonic development shows a strong correlation in numerous surveyed individuals with the presence of Autism Spectrum Disorder. Since this gene is expressed highly in the neocortex and prefrontal cortex of the brain, it is considered that it is responsible for an individual’s sensory processing and cognition, however, as with many such genes constituting HARs, their exact function remains obscure, and research is ongoing.
Whilst there is still a lot left to be learnt about these HARs, such as the reason for their abrupt, unprecedented prevalence through the evolution of the homo sapiens species in the genome, they help researchers to further understand the primarily neurodevelopmental differences between humans and other primates, as well as to delve deeper into matters such as the true cause of neurocognitive divergence and disorders.
They provide another opportunity for us to get to know more about the ongoing evolutionary process in the world around us today, as well as about the genes which are responsible for making us all unique individuals of the same species.
Citations
[1] “Brain area unique to humans linked to cognitive powers,”, University of Oxford, January 2014, https://www.ox.ac.uk/news/2014-01-28-brain-area-unique-humans-linked-cognitive-powers#:~:text=MRI%20image%20of%20the%20human,in%20humans%20and%20other%20primates.
[2] “Comparing chimp, bonobo and human DNA: AMNH,” American Museum of Natural History, n.d.
[3] Anastasia Levchenko, Alexander Kanapin, Anastasia Samsonova, Raul R Gainetdinov, “Human Accelerated Regions and Other Human-Specific Sequence Variations in the Context of Evolution and Their Relevance for Brain Development,”, National Library of Medicine, November 2017,
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5767953/#
[4] Melissa J Hubisz, Katherine S Pollard, “Exploring the genesis and functions of Human Accelerated Regions sheds light on their role in human evolution,”, Science Direct, December 2014,
[5] Nancy Fliesler, “‘Human accelerated regions’: How they make our brains uniquely human,”, Boston Children’s Hospital, October 2021,
https://answers.childrenshospital.org/human-accelerated-regions/
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