Understanding the nature of human behavior and its genetic underpinnings is a crucial issue in neuropsychology and evolutionary genetics. High cognitive and social abilities in humans, as represented by language, empathy, and altruism, are considered unique to our species. Brain neurochemicals such as serotonin and dopamine are known to underlie many important cognitive and emotional functions [1,2,3,4]. A recently proposed neurochemical hypothesis suggests that humans possess a dopamine-dominated striatum, i.e., a striatum with an elevated level of dopamine and a reduced level of acetylcholine that reflects our externally motivated behaviors [5,6,7,8]. Although monoamine neurotransmitters are produced in relatively few neurons in small areas of the brainstem, they are released widely and diffuse throughout the cerebral cortex [9]. Monoaminergic neurotransmitter systems have been implicated in various psychiatric disorders including depression, schizophrenia, attention-deficit hyperactivity disorder, and autism spectrum disorders [10,11,12,13]. As a result, regulatory monoaminergic pathways in the central nervous system have become targets for psychopharmacological interventions [14, 15].
The evolutionary origin of monoaminergic systems is thought to date back at least 600 million years to the stem metazoan (i.e., the common ancestor of Cnidaria and Bilateria) [16,17,18,19]. Social behavior regulated by monoaminergic systems is evolutionarily conserved in both vertebrates and invertebrates [20], suggesting that flexible behavioral control toward various environmental stimuli via monoamine neurotransmitters is advantageous for the survival of a wide range of taxa. Genetic differentiation in monoaminergic neuronal systems has been reported in several groups—especially among primates—and could be linked to the evolution of diverse emotional and/or social behaviors [21,22,23,24,25,26,27,28].
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