No abstract

Evolutionary transitions in individuality (ETIs) underlie the watershed events in the history of life on Earth, including the origins of cells, eukaryotes, plants, animals, and fungi. Each of these events constitutes an increase in the level of complexity, as groups of individuals become individuals in their own right. Among the best-studied ETIs is the origin of multicellularity in the green alga Volvox, a model system for the evolution of multicellularity and cellular differentiation. Since its divergence from unicellular ancestors, Volvox has evolved into a highly integrated multicellular organism with cellular specialization, a complex developmental program, and a high degree of coordination among cells. Remarkably, all of these changes were previously thought to have occurred in the last 50 -75 million years. Here we estimate divergence times using a multigene data set with multiple fossil calibrations and use these estimates to infer the times of developmental changes relevant to the evolution of multicellularity. Our results show that Volvox diverged from unicellular ancestors at least 200 million years ago. Two key innovations resulting from an early cycle of cooperation, conflict and conflict mediation led to a rapid integration and radiation of multicellular forms in this group. This is the only ETI for which a detailed timeline has been established, but multilevel selection theory predicts that similar changes must have occurred during other ETIs.evolution ͉ multicellularity ͉ multilevel selection ͉ transitions in individuality ͉ Volvox T he history of life on Earth has involved a number of evolutionary transitions in individuality (ETIs), in which groups of once-autonomous individuals became new individuals. Through the transfer of fitness from the individuals making up the group to the group itself, a new entity was formed with a single fitness and a single evolutionary fate. In this way, groups of interacting molecular replicators became single-celled organisms, prokaryotic cells became a primitive eukaryote, groups of single-celled organisms became multicellular organisms, and groups of multicellular organisms became social individuals (as in the social insects). In many cases such transitions have opened up entire new adaptive landscapes leading to vast radiations as completely new ways of being alive became available (e.g., cellular life, eukaryotes, plants, and animals). Understanding how and why groups of individuals become new kinds of individuals is a major challenge in explaining the history of life.The transition from unicellular to multicellular life is the paradigm case of the integration of lower-level individuals (cells) into a new higher-level individual-the multicellular organism. This transition has occurred dozens of times independently, for example in the red algae, brown algae, land plants, animals, and fungi (reviewed in ref. 1). Among the best-studied ETIs is the origin of multicellularity in the green alga Volvox and its relatives (the volvocine algae), which have been develop...

A second cycle, in which the defection of somatic cells must be prevented, may still be in progress.

The divergence of Escherichia coli bacteria into metabolically distinct ecotypes has a similar genetic basis and similar evolutionary dynamics across independently evolved populations.

The transition to multicellularity enabled the evolution of large, complex organisms, but early steps in this transition remain poorly understood. Here we show that multicellular complexity, including development from a single cell, can evolve rapidly in a unicellular organism that has never had a multicellular ancestor. We subject the alga Chlamydomonas reinhardtii to conditions that favour multicellularity, resulting in the evolution of a multicellular life cycle in which clusters reproduce via motile unicellular propagules. While a single-cell genetic bottleneck during ontogeny is widely regarded as an adaptation to limit among-cell conflict, its appearance very early in this transition suggests that it did not evolve for this purpose. Instead, we find that unicellular propagules are adaptive even in the absence of intercellular conflict, maximizing cluster-level fecundity. These results demonstrate that the unicellular bottleneck, a trait essential for evolving multicellular complexity, can arise rapidly via co-option of the ancestral unicellular form.