  Understanding the emergence and evolution of multicellularity and cellular
differentiation is a core problem in biology. We develop a quantitative model
that shows that a multicellular form emerges from genetically identical
unicellular ancestors when the compartmentalization of poorly compatible
physiological processes into component cells of an aggregate produces a fitness
advantage. This division of labour between the cells in the aggregate occurs
spontaneously at the regulatory level due to mechanisms present in unicellular
ancestors and does not require any genetic pre-disposition for a particular
role in the aggregate or any orchestrated cooperative behaviour of aggregate
cells. Mathematically, aggregation implies an increase in the dimensionality of
phenotype space that generates a fitness landscape with new fitness maxima, and
in which the unicellular states of optimized metabolism become fitness saddle
points. Evolution of multicellularity is modeled as evolution of a hereditary
parameter, the propensity of cells to stick together, which determines the
fraction of time a cell spends in the aggregate form. Stickiness can increase
evolutionarily due to the fitness advantage generated by the division of labour
between cells in an aggregate.
