All of life is social

germ–soma differentiation as a model for the molecular biology of development. Certain slime mold genotypes get into the germline tissue more frequently than other neighboring genotypes in the aggregation [5]. An aggregation with a higher frequency of those that bias their reproductive success leads to shorter stalks and lower total reproductive output: competition for reproduction opposes the cooperative success of the group. Slime mold aggregations are relatively small, on the order of 10 4 cells. How did multicellularity overcome internal conflicts to allow larger, more complex aggregations to function in a unitary way [1]? Most multicellular organisms develop from a single-cell progenitor, usually a haploid reproductive cell or a diploid zygote. All tissues form a clone derived from the progenitor, thus reducing the chance of competition between different genotypes. By this view, the single-cell stage was a key solution to the social problems of multicellularity [6]. Even with a single-cell progenitor, mutations during development can create genetic variation and conflict within the organism [1]. Many attributes of cellular biology, which originated in single-celled organisms, may have subsequently evolved to repress such conflicts in multicellularity. For example, the DNA repair system, apoptotic controls and cell cycle checkpoint processes tightly regulate cellular birth and death, preventing renegade lineages from growing into uncontrolled tumors. Rigid developmental regulation of the germ–soma separation may reduce the potential for reproductive conflict. Complex systems of self versus nonself discrimination in marine organisms probably originated to protect the multicellular clone from genetic invasion by competing neighbors; those recognition systems may have been the evolutionary precursors of major components of the vertebrate immune system. Sociality means living in groups. Group living intensifies two opposing forces. On one side, proximity exacerbates conflict for local resources. On the other side, cooperation with neighbors may enhance group efficiency and aid in competition against other groups. Starting in the 1960s, biologists made many conceptual advances in understanding the forces of conflict and cooperation that shape social life. That new understanding was applied with great success to honey bees, macaques, and other obviously social, group-living species. Then biologists began to recognize the social history of other types of groups. For example, multicellularity originated through a complex evolutionary history of cellular aggregations, in which the opposing social forces of conflict and cooperation likely played a key role. Similarly, genomes arose through social histories of genetic aggregations and organelle symbioses. Several aspects of multicellularity, of genomes, …