Human accelerated regions (Hars) are a collection of segments of the human genome. which, despite being shared with other vertebrates, are observed remarkably differently from our species.
What sets us apart from a chimpanzee? Behaviors, cognitive expressions and the ability to generate languages and civilizations reflect human neurological development at two different levels: one genetic and the other cultural. So, to unlock the secrets of these characteristics that make us so different from other animal species, we have to go to our evolutionary history and genetic mapping.
Human Accelerated Regions or Hars attempt to answer this impressive question, because variation in loci (fixed positions on a chromosome) between similar species, such as humans and chimpanzees, could be part of the answer to the evolutionary engine that drives us. has led to a “dominant position” as a species on Earth.
Human-accelerated regions: the key to behavior
Comparative genomics is responsible for studying the similarities and differences between the set of genes on the chromosomes of organisms on the planet.
This scientific discipline seeks to discover what characteristics have been set by natural selection over time, in order to understand the different evolutionary pressures to which living things have been subjected during their generations.
understand these underlying mechanisms that cause living things to vary over timeIt should be noted that there is a phenomenon of “genetic purification” in the natural world.
What happens when we deviate from natural selection?
It should be noted that negative selection is an evolutionary mechanism by which deleterious alleles (each of two or more versions of a gene) for a species are eliminated over time, “purifying” the population’s gene pool.
For example, an animal that has a mutation that is not beneficial for the community in which it lives will have fewer descendants or die faster (population genetic mechanisms), which will eliminate this harmful allele over generations. If a bird is born without an eye due to a mutation, it is to be expected that it will reproduce less or be hunted faster than others, right?
But … what about humans? that we have rejected this mechanism of negative selection, because in a world from a Western point of view, the survival rate of the individual is not influenced by his mutational fetters, as long as medicine allows it (auto- immunity or absence of any limb, for example). This, among many other factors produced by a purely anthropogenic society, could lead to three mechanisms:
- Accumulation of neutral mutations in gene sequences that have lost their essential function.
- Biased gene conversion by not responding to adaptive evolution.
- Change of a negative selection influence by a positive selection mechanism.
We are operating on confusing terrain that includes very complex genetic terminology, but one idea needs to be clear: human accelerated regions suffer from relatively rapid mutation rates compared to the rest of the genome, and due to a lack of selective pressure and adaptive responses, these areas are very divergent from other hominids.
Coding and conclusion, or not?
That much, it is essential to emphasize that 99% of human DNA is not codingIn other words, it does not present information on the production of proteins and therefore does not act directly on the metabolism of the individual.
Although these DNA segments were initially thought of as ‘waste’, it is increasingly common that they play an essential role in regulating the activation of essential genes in various ways, as it has been shown that certain regions can promote the activation or repression of the transcription of certain proteins.
This is one of the major problems with human-accelerated regions, since 92% of them are found in non-coding regions. Therefore, most of these genetic elements are found in uncharacterized areas of the genome and their evolutionary conservation should not predict any specific differential function in humans.
Despite this, this does not mean that these highly mutated areas do not meet human characteristics. Many of them are present in “intergenic” regions, that is to say regulatory sequences which could modulate the expression or the deletion of certain genes if coded. Of course, these ideas need to be investigated further to reach reliable conclusions.
A practical example
To understand all this mutagenic and evolutionary conglomerate, it is better to turn to an example. We have before us in the HAR1 region, a DNA sequence composed of 118 nucleotides; commonly called bases, by the nitrogenous base which contains each of them, adenine, cytosine, thymine and guanine. Let’s see some revealing realities about this segment:
- When we compare the HAR1 region between humans and chimpanzees, we see that there are 18 different bases.
- If we compare the same region between a chicken and a chimpanzee, we only find a difference of two bases.
- The chimpanzee lineage diverged from that of humans 6 million years ago, while chickens separated from it 300 million years ago.
- This sequence is not present in fish and frogs.
This data must mean something, right? If not, what is the point of showing a greater variation between two relatively differentiated lines recently? This fact suggests that this rapid rate of mutation may be correlated with certain characteristics that make us define ourselves as “humans”.
To put it more interestingly, other studies have shown that the five fastest accelerated human mutated regions show 26 times more substitutions (mutations) than their analogues in chimpanzees.
But are there any differences between the Hars in the history of human evolution? According to other sources, the differences in these regions between archaic (Neanderthal) hominins and modern humans are around 8%, illustrating that this evolutionary divergence that characterizes us must have accelerated around 500,000 ago. years, and could have been decisive for the characterization of the genus Homo. Of course, variations in the human genome throughout our evolutionary history can close much of the answer to our characteristics as a species.
Harassment and mental disorders
Even more surprising, if at all, is which studies have observed that some mutated genes are found in the vicinity of these accelerated regions in patients with mental disorders like schizophrenia, and it is therefore postulated that they can be influenced by them.
Beyond that, other research has documented that several genetic variations in autistic patients are found in accelerated regions. This could lead to a specific modulation of the production of proteins that interact with the brain, which would condition a “normal” functioning of the behavior of the individual.
As we have seen, human regions accelerated these are segments of DNA that could play an essential role in human developmentIn other words, those very special characteristics that define us as a species.
In addition, studies have shown that they can modulate the expression of certain genes, which would condition the metabolism of an individual and therefore his behavior, especially in disorders such as schizophrenia or autism.
As much as the research has laid promising foundations, it is essential to underline that at no time have we ceased to evolve within theoretical and experimental frameworks. Nothing that is exhibited here should be interpreted as dogma or absolute reality, as a long period of research is still needed to understand the uniqueness of these genetic segments.
- Allele, National Institute for Human Genome Research. Retrieved August 31, from https://www.genome.gov/es/genetics-glossary/Alelo
- Capra, JA, Erwin, GD, McKinsey, G., Rubenstein, JL and Pollard, KS (2013). Many accelerated human regions enhance development. Philosophical Transactions of the Royal Society B: Biological Sciences, 368 (1632), 20130025.
- Doan, RN, Bae, BI, Cubelos, B., Chang, C., Hossain, AA, Al-Saad, S., … and Gascon, GG (2016). Mutations in human-accelerated regions disrupt cognition and social behavior. Cell, 167 (2), 341-354.
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- Levchenko, A., Kanapin, A., Samsonova, A. and Gainetdinov, RR (2018). Human-accelerated regions and other human-specific sequence variations in the context of evolution and their relevance to brain development. Biology and evolution of the genome, 10 (1), 166-188.
- What is non-coding DNA. Reference to the house of genetics. Retrieved August 31, from https://ghr.nlm.nih.gov/primer/basics/noncodingdna