<p>We review theoretical principles and empirical evidence concerning selection dynamics operating at different levels of biological aggregation in human evolution. The present paper discusses the evolution of leveling mechanisms leading to the rise of institutions that prescribe cooperative behaviors and proscribe social parasitism between individuals that are distantly related at a genetic level within a group (<i>ultrasociality</i>), and anti-hierarchical structures in human small-scale societies. We also propose that biocultural and ecological factors are potential culprits for the observed variation in between-group competition in non-state sociopolitical systems. Previous cliodynamic studies have reported a sizeable contribution of between-group competition to the evolution of complex sociopolitical systems, wherein geopolitical expansion and the influx of resources initially lead to increased social stratification and specialism. We also integrate and interpret the literature on the collapse and regeneration of complex societies through the lens of Multilevel Selection Theory, wherein complex sociopolitical systems reach points of diminishing returns, compromising their cohesion and functionality. The erosion or dismantling of the latter sociopolitical mechanisms increases the risk of internecine conflict and potential conquest by rival factions featuring greater within-group cooperation. Finally, as a logical expansion of MLS theory, we explore how the rise of symbiotic associations between human communities and plants and other animals during the Mesolithic and Neolithic paved the way for domestication and <i>extended ultrasociality</i> towards plants and other animals. Consequently, across human history, competition among large-scale human societies often occurred as clashes between entire species assemblages facilitated by human niche construction.</p><p><b>Clinical Trial Number</b> Not Applicable.</p>