The frequency of genetic variants associated with attention-deficit/hyperactivity disorder (ADHD) has decreased progressively in the evolutionary human lineage from the Palaeolithic to the present day, according to a study published in the journal Scientific Reports.
The new genomic analysis compares ADHD-associated genetic variations in current European population to assess its evolution within samples of the human species.Homo sapiens), modern and ancient, and in samples of Neanderthals (Homo neanderthalensis). The conclusions show that the low tendency in European populations cannot be explained by the genetic mix of African populations or the introduction of Neanderthal genomic segments into our genome.
The new genomic study isled by Professor Bru Cormand, from the Faculty of Biology and the Institute of Biomedicine of the University of Barcelona (IBUB), the Research Institute Sant Joan de Déu (IRSJD) and the Rare Diseases Networking Biomedical Research Centre (CIBERER), and the researcher Oscar Lao, from the Centro Nacional de Análisis Genómico (CNAG), part of the Centre for Genomic Regulation (CRG). Paula Esteller, a CNAG-CRG researcher, is the first author of the study. She is currently a doctoral student at the Institute of Evolutionary Biology, IBUB, and the Research Institute Sant Joan de Deu (IRSJD). Oscar Lao, from the Centro Nacional de Analisis Genomico (CNAG), part of the Centre for Genomic Regulation (CRG).
ADHD: An adaptive value in the evolutionary lineage for humans?
The attention deficit/hyperactivity disorder (ADHD) is an alteration of the neurodevelopment which can have a large impact on the life of the affected people. It can be characterized by hyperactivity and impulsiveness as well as attention deficit.
Evolutionarily, it would be expected that any negative effects would be eliminated from the population. There are several natural hypotheses that can explain this phenomenon. These hypotheses focus on the context in which the Neolithic transition took place, specifically the Mismatch Theory.
“This theory states that cultural and technological changes over the past thousands of years would have allowed humans to adapt their environment to meet our physiological needs in the near future. These changes would have caused an imbalance in the environment in which our hunter/gatherer ancestors evolved in the long-term,” the authors note.
In ancestral environments of nomadism, some traits such as hyperactivity or impulsiveness — which are typical for people with ADHD — might have been favoured. However, these same traits would not have been adaptive in environments that are more recent (mostly sedentary).
It is one reason why it is so common in children and teens.
Based on the study of 20,000 ADHD patients and 35,000 controls, the new study revealed that the genetic variants and alleles that are associated with ADHD are found in genes that are intolerant of mutations that cause loss function. This indicates the existence of a selective pressure.
According to the authors, ADHD’s current high prevalence could be due to a positive selection that occurred in the past. Although ADHD is an unfavourable phenotype given the new environment, its prevalence would still be high as it has not disappeared. However, the absence of genetic data for ADHD has prevented any empirical comparisons.
“The results of the analysis that we did guarantee that there were some selective pressures which would have been in place for many years against ADHD-associated variants. These results are compatible to the mismatch theory, but they also suggest that negative selective pressures may have begun before the transition between Palaeolithic & Neolithic, approximately 10,000 years ago,” the authors.