Why does a monogamous ancestor make helping pay off?
Eusociality is the extreme form of social living in which some individuals give up their own reproduction to help raise others' young. Kin selection explains how this can evolve: a helper can pass on copies of its genes indirectly by helping relatives that share those genes. The catch is that this only works well when the helpers are closely related to the young they rear. Ancestral monogamy creates exactly that situation, because a mother mating with a single partner means her offspring are full siblings and share a large fraction of their genes [2][4].
The haplodiploid sex-determination system of ants, bees and wasps adds a twist. In these insects, females develop from fertilized eggs and are diploid, while males develop from unfertilized eggs and are haploid. Under monogamy, this makes sisters unusually closely related to one another, which strengthens the indirect benefits of helping [2]. That genetic quirk may help explain why eusociality is so common in the Hymenoptera, though the paper presents this as a consistent explanation rather than a proven cause [2].
What do comparisons across species show?
The strongest support comes from reconstructing the family trees of eusocial groups and asking what their ancestors looked like. In the Hymenoptera, ancestral state reconstruction on published phylogenies identified the independent transitions to eusociality in each family that has it, and the route often runs through semisociality, where a single nest co-foundress monopolizes reproduction. That route is consistent with groups made up of a mother and her daughters, and with ancestral monogamy [2].
A similar pattern appears in vertebrates. In African mole-rats, a time-calibrated phylogeny combined with mating-system and sociality data inferred an ancestral monogamous state for social and eusocial species. One of the fastest-changing transitions was from monogamy plus solitary living to monogamy plus social living, which fits monogamy as a prerequisite for obligate eusociality [4]. The same study, however, found that monogamy alone is not enough: additional ecological and life-history factors are needed for eusociality to evolve and intensify [4].
Comparative work on sponge-dwelling shrimp points in the same direction. Eusociality there has been thought to arise as a response to ecological challenges, mediated by kin selection, and the analysis tested how eusociality associates with ecological performance and genetic structure shaped by non-dispersing larvae [3]. This extends the kin-selection picture beyond insects and mammals, though the shrimp evidence speaks to ecology and kin structure rather than to ancestral monogamy specifically [3].
What else has to line up besides monogamy?
Monogamy sets the stage, but the Hymenoptera evidence shows that life-history traits matter too. A loss of larval diapause, the dormant resting stage that lets larvae wait out unfavorable seasons, precedes or coincides with all but one transition to eusociality, a pattern confirmed by phylogenetic tests in sweat bees and apid bees [2]. The proposed link is that losing larval diapause and gaining subsociality may go with an extended breeding season that allows at least two broods, which is necessary before helpers can evolve [2].
Kin selection is also not a complete explanation on its own. A review of the field argues that kin selection models explain why individuals can evolve distinct reproductive or non-reproductive roles, but that division-of-labour models and evolutionary developmental biology are needed to explain how specialization and helper traits arise and become fixed. None of the three approaches is sufficient alone, and the authors call for unifying explanations and formal models [1].
The mole-rat result sharpens the point: monogamy looks like a necessary prerequisite for obligate eusociality, but not a sufficient one [4]. So the honest summary is that ancestral monogamy is a strong and repeatedly supported precondition, consistent with kin selection being central, while the full transition depends on ecology, life history and developmental mechanisms that monogamy alone does not supply [1][2][4].
Sources used in this answer
The evolution of eusociality: Kin selection theory, division of labour models, and evo-devo explanations
Kin selection, division-of-labour and evo-devo approaches are each insufficient alone to explain eusociality, and the authors call for unifying models linking reproductive altruism, task specialization and phenotypic plasticity.
Life History and the Transitions to Eusociality in the Hymenoptera
Ancestral state reconstruction in Hymenoptera supports a semisocial route to eusociality consistent with mother-daughter groups and ancestral monogamy, and finds that loss of larval diapause precedes or coincides with nearly all transitions.
Kin structure, ecology and the evolution of social organization in shrimp: a comparative analysis
Comparative analysis of sponge-dwelling shrimp tests the association of eusociality with ecological performance and genetic structure, supporting a role for kin selection mediated by non-dispersing larvae.
Monogamy is necessary but not sufficient for eusociality in African mole-rats (Rodentia: Bathyergidae).
In African mole-rats, ancestral monogamy is inferred for social and eusocial species and is consistent with monogamy being a necessary but not sufficient prerequisite for obligate eusociality.
Haplodiploidy and the evolution of eusociality: split sex ratios
Theoretical work on haplodiploidy and split sex ratios treats monogamy as a driver of eusociality through kin selection, assuming it because eusociality has evolved only in monogamous species.