Did Human Culture Emerge in a Cultural Evolutionary Transition in Individuality?

Evolutionary Transitions in Individuality (ETI) have been responsible for the major transitions in levels of selection and individuality in natural history, such as the origins of prokaryotic and eukaryotic cells, multicellular organisms, and eusocial insects.\ua0The integrated hierarchical organization of life thereby emerged as groups of individuals repeatedly evolved into new and more complex kinds of individuals. The Social Protocell Hypothesis (SPH) proposes that the integrated hierarchical organization of human culture can also be understood as the outcome of an ETI—one that produced a\ua0“cultural organism” (a “sociont”) from a substrate of socially learned traditions that were contained in growing and dividing social communities. The SPH predicts that a threshold\ua0degree of evolutionary individuality would have been achieved by 2.0–2.5 Mya, followed by an increasing degree of evolutionary individuality as the ETI unfolded. We here assess the SPH by applying a battery of criteria—developed to assess evolutionary individuality in biological units—to cultural units across the evolutionary history of Homo. We find an increasing agreement with these criteria, which buttresses the claim that an ETI occurred in the cultural realm

Evolution of Individuality: A Case Study in the Volvocine Green Algae

While numerous criteria have been proposed in definitions of biological individuality, it is not clear whether these criteria reflect the evolutionary processes that underlie transitions in individuality. We consider the evolution of individuality during the transition from unicellular to multicellular life. We assume that “individuality” (however it is defined) has changed in the volvocine green algae lineage during the transition from single cells, to simple multicellular colonies with four to one hundred cells, to more complex multicellular organisms with thousands of differentiated cells. We map traits associated with the various proposed individuality criteria onto volvocine algae species thought to be similar to ancestral forms arising during this transition in individuality. We find that the fulfillment of some criteria, such as genetic homogeneity and genetic uniqueness, do not change across species, while traits underpinning other aspects of individuality, including degrees of integration, group-level fitness and adaptation, and group indivisibility, change dramatically. We observe that different kinds of individuals likely exist at different levels of organization (cell and group) in the same species of algae. Future research should focus on the causes and consequences of variation in individuality

The Origin and Evolution of Cellular Differentiation in the Volvocine Green Algae

During the evolution of multicellularity, the unit of selection transitions from that of the single cell to that of the integrated multicellular organism. Cellular specialization mediates this transition, as the evolution of differentiated cell types leads to fitness becoming a property of the multicellular organism. How does cellular specialization evolve? While we know that genetic changes result in the development of new cell types, it remains unclear whether the development of a new cell type can be ancestrally plastic prior to coming under developmental-genetic control via genetic assimilation. We use the volvocine green algae as a model system to address the overarching question of whether the plastic development of a new cell type preceded its fixation via genetic assimilation. Previous research on the evolution of cellular differentiation in this clade has determined that a gene necessary for somatic cell development is present in species with and without soma and that an ancestral stress response was co-opted during the evolution of somatic differentiation. This raises the possibility that somatic cells may have originated as a plastic trait prior to coming under developmental-genetic control. Here, I show that Eudorina species previously characterized as undifferentiated develop a small proportion of cells resembling soma following exposure to cold shock, an environmental stressor. We also find that the offspring of cold-shocked colonies (but not the grand-offspring) also develop somatic-like cells. We show that these cells are morphologically consistent with the somatic cells seen in closely related species that are obligately differentiated. We find that somatic-like cells in cold-shocked colonies are controlled by cell-level, temporal regulation while the production of somatic-like cells in the offspring of cold-shocked colonies are controlled by group-level, developmental regulation induced by the maternal environment. We propose that cell-level control of cellular differentiation, a cross-level byproduct, could be an intermediate step in the evolution of a group-level trait. We then repeatedly exposed a different Eudorina species that has been classified as undifferentiated to two repeated rounds of cold shock to determine if obligate somatic cells can evolve by genetic assimilation. We assessed the lines dozens of generations after treatment and found that repeatedly cold-shocked lines evolved an increased proportion of differentiated colonies. Moreover, we found that the proportion of differentiated cells also increased over the course of the study. These results provide evidence that plasticity followed by genetic assimilation likely played an important role in the evolution of somatic differentiation in the volvocine green algae