Ricard Solé, Sergi Valverde

Paper #: 12-05-005

The emergence of complex multicellular systems and their associated developmental programs is one of the major problems of evolutionary biology. The advantages of cooperation over individuality seem well known but it not clear yet how such increase of complexity emerged from unicellular life forms. Current multicellular systems display a complex cell-cell communication machinery, often tied to large-scale controls of body size or tissue homeostasis. Some unicellular life forms are simpler and involve groups of cells cooperating in a tissue-like fashion, as it occurs with biofilms. However, before true gene regulatory interactions were widespread and allowed for controlled changes in cell phenotypes, simple cellular colonies displaying adhesion and interacting with their environments were in place. In this context, models often ignore the physical embedding of evolving cells, thus leaving aside a key component. The potential for evolving pre-developmental patterns is a relevant issue: how far a colony of evolving cells can go? Here we study these pre-conditions for morphogenesis by using CHIMERA, a physically embodied computational model of evolving virtual organisms. Starting from a population of single cells moving in a fluid, closed space, and exploiting one nutrient source from a given repertoire of food particles falling from the top of their environment, it is shown that cells undergo major transitions as they evolve their metabolism and adhesion properties in order to exploit resources and occupy space. At some point, some cells "discover" the energy-rich top of their world and the whole population flips towards the upper layer. At this point they act as ecosystem engineers, modulating the flow of nutrients and creating opportunities for new niches to form, as illustrated by the subsequent emergence of a specialized level of detritivores.