  The evolutionary transition to multicellularity transformed life on earth,
allowing for the evolution of large, complex organisms. While multicellularity
can be strongly advantageous, its earliest stages bring unique physical
challenges. Nascent multicellular organisms must contend with a novel
constraint: intercellular stresses arising from cell-cell interactions that can
limit multicellular size. Among the possible evolutionary routes to overcoming
this size limit, two appear obvious: multicellular organisms can increase
intercellular bond strength, allowing them to tolerate larger stresses, or,
they can slow the rate of stress accumulation by altering their internal
structure. Recent experiments demonstrated that multicellular 'snowflake yeast'
readily find a solution to this problem via the latter route. By evolving more
elongated cells, which decreases cellular packing fraction and thus the rate of
internal stress accumulation during growth, snowflake yeast evolve to delay
fracture and grow larger. However, it is unclear if snowflake yeast evolved
large size along an optimal path, or if the observed path to large size was
taken due to proximate biological reasons. Here, we examine the geometric
efficiency of both strategies using a minimal model that was previously
demonstrated to replicate many experimentally observed structural properties of
snowflake yeast. We find that changing geometry is a far more efficient route
to large size than evolving increased intercellular adhesion. In fact,
increasing cellular aspect ratio is on average ~13 times more effective at
increasing the number of cells in a cluster than increasing bond strength.
These results suggest that geometrically-imposed physical constraints may have
been a key early selective force guiding the emergence of multicellular
complexity.
