Early Multicellular Organisms Co-opt Cell-Level Characteristics into Group-Level Properties via the Principle of Maximum Entropy

In the earliest stages of the evolution of multicellularity, genetic changes occur at the individual cell level yet selection acts at the group level. New group level traits emerge when mutations affecting cell-level properties are co-opted into consistent group-level traits. However, it is unclear how readily coherent group-level properties emerge absent a regulatory developmental plan. It even seems likely that small fluctuations at the cellular level may elicit large fluctuations at the group level, destroying the chance for survival. Here we demonstrate that lab-evolved simple multicellular groups with permanent intercellular bonds follow the principle of maximum entropy. As a result, a large space of microstates (e.g. specific cell configurations) correspond to a smaller space of macrostates (e.g. cluster volume), thereby achieving robust, consistent macroscopic properties. We derive an equation of state that relates these macroscopic properties together and experimentally verify predictions from the equation of state, demonstrating that robust group-level properties readily emerge from individual cell traits. Finally, we speculate that the emergence of group-level properties is possible within any cluster with fixed bonds between mother and daughter cells.