Ancient DNA (aDNA) from fossil remains continues to shed light on how neuropsychiatric disorders prevalent in modern-day human populations may have originated when modern humans split from neanderthals approximately 500 kya. Writing in Neuropsychopharmacology, Michael Gregory and Karen Berman from the National Institutes of Health bring together multiple comparisons between modern and ancient human genomes to show how evolutionary vestiges impact psychiatric disorder risk.
This relationship between the Neanderthal and Homo sapiens genomes has been well-known for a fairly long time. Indeed, research in the last decade has found – not very surprisingly – that remnants of the Neanderthal genetic material in modern humans are not merely vestiges sitting idle.
The interbreeding event between Neanderthals and Anatomically Modern Humans (AMH), which that took place fifty thousand years ago, has given us genetic variants that are very much functional and that continue to determine our hair and skin colour. The Neanderthal footprint in the modern human DNA is as high as 62-64%, depending on the populations examined.
More importantly, genetic variants derived from our Neanderthal ancestors have helped us adapt to climates outside of Africa.
These evolutionary hangovers from our ancestors are often visible in physical characteristics, such as skull shapes. The same holds true for the organ that the skull houses. In a previous study by Michael Gregory and his group, MRI scans of individuals with a significant Neanderthal genetic hangover had revealed differences in the manner their brains were wired. For instance, a brain mapping study conducted by Gregory et al. on the intraparietal sulcus (IPS) – a region in the brain that deals with processing visual stimuli, hand-eye coordination and numeric memory – found that those individuals with a higher Neanderthal genetic material had better functional connectivity with visual processing regions.
However, they exhibited a marked reduction in IPS connectivity with regions responsible for social cognition.
Quite unexpectedly, connections have been found for brain disorders as well. There is considerable overlap between the regions we have inherited from Neanderthals and those that are responsible for disorders such as schizophrenia; lending credence to the theory that disorders such as schizophrenia evolved only with the emergence of ‘higher order cognitive abilities.’ Moreover, a lab-based experiment observed that introduction of genetic variants from ancient humans in stem cells significantly slowed down the pace of their development. Prior work by Gregory and co-workers had shown that individuals with ‘less Neanderthal-derived genetic variation’ harbour a higher risk of acute schizophrenia. At the same time, those individuals with higher Neanderthal derived genetic material had shown ‘less severe psychotic symptoms.’
Gregory et al. supplement their study by also correlating Neanderthal introgression with the production of dopamine, a chemical that is produced in specific ‘dopaminergic’ regions of the brain and induces the feeling of motivation towards/away from something.
The study found that higher the Neanderthal admixture, the lower the dopamine production. The finding is significant because there exists a veritable link between high dopamine production and schizophrenia risk. This link has been well-accepted in the medical community since the 1950s, when chlorpromazine, a dopamine suppressant, was able to give symptomatic relief in patients of schizophrenia. The dopamine-schizophrenia hypothesis has stood the test of time to this date, albeit in different iterations.
While studies in ancient DNA are still in their incipient stages, Gregory and Berman argue that with the development in sequencing, repositories and new potential discoveries of ancient genetic material holds tremendous promise in giving us a peek into evolutionary processes that result in serious psychiatric conditions.
