The organisational complexity of life has increased over evolutionary time: from prokaryotes to unicellular eukaryotes, and from there to multicellular organisms and superorganismal insect colonies. Each of these domains is characterised by previously independently reproducing units becoming life-time-committed to reproduce jointly: nuclear and organelle genomes in cells, gametes forming zygotes that produce somatically differentiated bodies, and monogamous parents founding social insect colonies with differentiated queen and worker castes.
For more than 50 years now, the dynamics of cooperation between co-replicating units belonging to the same gene pool have been studied with resounding success using the inclusive fitness (kin selection) paradigm developed by William D. Hamilton. Hamilton’s rule captures that genes coding for cooperative traits will increase in frequency when the benefits (b) of passing on genes by helping relatives reproduce exceed the cost© in units of foregone personal reproduction after scaling the b/c ratio for relatedness to relatives versus own offspring (rx/ro).
Extensive testing of this principle has shown that there are unifying principles for the expression of cooperation and reproductive altruism that apply across the complexity domains of life. However, what drives the irreversible passages from one complexity domain to another, also known as the Major Transitions in Evolution, has remained opaque. I will summarise the history of Inclusive Fitness and Major Transition theory and show that the origins of the Major Transitions that have shaped eukaryote complexity can all be explained by a simplified version of Hamilton’s rule, where the relatedness term cancels out because rx remains at its maximal possible level, so rx/ro invariably equals one.