A neurochemical hypothesis for the origin of hominids

Abstract

It has always been difficult to account for the evolution of certain human characters such as language, empathy, and altruism via individual reproductive success. However, the striatum, a subcortical region originally thought to be exclusively motor, is now known to contribute to social behaviors and "personality styles" that may link such complexities with natural selection. We here report that the human striatum exhibits a unique neurochemical profile that differs dramatically from those of other primates. The human signature of elevated striatal dopamine, serotonin, and neuropeptide Y, coupled with lowered acetylcholine, systematically favors externally driven behavior and greatly amplifies sensitivity to social cues that promote social conformity, empathy, and altruism. We propose that selection induced an initial form of this profile in early hominids, which increased their affiliative behavior, and that this shift either preceded or accompanied the adoption of bipedality and elimination of the sectorial canine. We further hypothesize that these changes were critical for increased individual fitness and promoted the adoption of social monogamy, which progressively increased cooperation as well as a dependence on tradition-based cultural transmission. These eventually facilitated the acquisition of language by elevating the reproductive advantage afforded those most sensitive to social cues.

Positive and balancing selection onSLC18A1gene associated with psychiatric disorders and human-unique personality traits

“…Raghanti et al. () demonstrated that the human striatum, a region of the brain modulating social behavior, exhibits unique neurochemical profile compared to other primates. Sousa et al.…”

“…Stimpson et al (2016) has identified an evolutionary change in the serotonergic innervation of the amygdala between chimpanzees and bonobos, and suggested that it has led to differences in the aggressive behaviors, cautious temperaments, risk preferences, and performances on "theory of mind" tasks even between these closely related species. Raghanti et al (2018) demonstrated that the human striatum, a region of the brain modulating social behavior, exhibits unique neurochemical profile compared to other primates. Sousa et al (2017) have reported that a type of dopaminergic interneuron exclusively exists in the human neocortex.…”

Oxytocin and vasotocin receptor variation sheds light into the evolution of human prosociality

“…Modern humans are characterized by great intrasocial compassion, are motivated by concern about the welfare of out-group individuals, and display a clear tendency to act in concert, to the extent that Homo Sapiens has been labeled as ‘ultra-social’ 7 . This trait is of special relevance, as it has been argued to underlie other singular traits of humans, such as their enhanced verbal communicative skill 6-10 .…”

“…The sequencing of two Neanderthal genomes from Altai (Siberia) 11 and Vindija (Croatia) 12 and a Denisovan from Alta 13 has made available genomic data to provide new insights into the discussion of the evolution of social cognition, complementing the archaeological evidence. Today, various hypothese 6, 10, 14, 15 still offer different explanations and timelines for the emergence of prosociality, ranging from the Pan-Homo split to later stages of human evolution, such as the split between Neanderthals and Denisovans on the one hand, and Modern Humans on the other. The critical effect of OXT and AVP on pair-bonding has led some of the authors of the aforementioned theories, most prominently, 14 , to ascribe to them a key role in the emergence of human social behavior, while others have challenged the centrality of OXT and AVP in this shift in favor of other hormones, such as P-endorphines and dopamin 10, 16 .…”

“…Today, various hypothese 6, 10, 14, 15 still offer different explanations and timelines for the emergence of prosociality, ranging from the Pan-Homo split to later stages of human evolution, such as the split between Neanderthals and Denisovans on the one hand, and Modern Humans on the other. The critical effect of OXT and AVP on pair-bonding has led some of the authors of the aforementioned theories, most prominently, 14 , to ascribe to them a key role in the emergence of human social behavior, while others have challenged the centrality of OXT and AVP in this shift in favor of other hormones, such as P-endorphines and dopamin 10, 16 . By examining the evolutionary variation in human OXT and AVP receptors, we aim to shed light onto the timing of the transition towards the current status of human prosociality, as well as determine more clearly the specific role that OXT and AVP could have played in this regard.…”