Symbiosis drove cellular evolution: SSymbiosis fueled evolution of lineages of Foraminifera (eukaryotic cells) into exceptionally complex giant protists

Anthropocentric cultural bias led to conceptualizations of evolution as a tree with branches leading to a crown of vertebrates and higher plants. Gradually refined over the years it became part of common scientific culture to think of eukaryotic evolution as process by which cells, limited by surface to volume ratios and other factors, became specialized, leading to multi-cellularity and eventually to the crowned tree. Knowledge of other pathways of cellular evolution is available, but not broadly recognized. Molecular systematics, genetic analyses and ultrastructural comparisons have changed our outlook on evolution and the diversity of life. The discovery of a huge (average size 2.1 cm) unicellular, new, exceptionally complex, dinoflagellate-hosting soritid foraminiferan from the Heron-Wistori Channel, (GBR Australia) gave impetus to re-explore some old ideas on cellular evolution and place them in a contemporary context. In particular, it caused change in perspective of the evolution of the collective group known as larger foraminifera (LF). They exemplify the power by which symbiosis drove the evolution of a predisposed and malleable group of organisms. The factors that underlay foraminiferan predisposition to symbioses with algae are discussed. Each of the evolutionary lines of LF has developed, in its own way, amazing structural adaptations making them extremely complex giant cells.