Theory 023/Life & Evolution /Major transitions
Multicellularity Needs Adhesion
Clusters form easily. Staying together under shear is the hard part.
The transition to multicellularity requires a threshold in cell-cell adhesion energy rather than a threshold in cell number, so lineages that fail to cross it cannot become multicellular regardless of how often they form clusters.
01The evidence
Multicellularity has evolved many times, and many lineages form temporary clusters without becoming multicellular, which suggests a barrier beyond aggregation.
02The tension
The transition is modelled as a cooperation problem, where cells must forgo reproduction. That predicts cheaters as the barrier, not adhesion, and it does not explain why some aggregating lineages never make the transition.
03The mechanism
A cluster must hold together under the mechanical and hydrodynamic forces it experiences. Adhesion energy per cell sets whether it does. Below a threshold, clusters shear apart before differentiation can evolve.
That makes adhesion the rate-limiting step, and predicts that experimental selection for adhesion should precede the evolution of division of labour rather than follow it.
04What this is not
Those models focus on conflict. This focuses on mechanical integrity, which predicts adhesion evolves first.
05Predictions
- P1Adhesion should increase before division of labour in experimental transitions.
- P2Selection for adhesion should enable multicellularity.
- P3Clusters below the threshold should disperse under shear.
06Refutation
Division of labour evolving without increased adhesion would displace the mechanical account.
07What it changes
It would change the experimental route to multicellularity.