Hypothesis

Aging is an actively maintained, population‑level trait that limits individual lifespan after reproduction to free resources for kin, mediated by a conserved senescence‑associated secretory phenotype (SASP) that signals nutritional status to relatives.

Mechanistic Basis

Recent work shows that extrinsic mortality shapes aging rates, supporting selection gradients rather than programmed traits[3]. Yet the quasi‑program concept highlights that pathways like mTOR remain active post‑maturity, creating a developmental shadow that could be co‑opted for signaling[2]. We propose that, in addition to cell‑autonomous damage, senescent cells release specific SASP factors (e.g., IGF‑BP3, TGF‑β1) packaged in extracellular vesicles that travel through the bloodstream to offspring or close relatives, transiently suppressing mTOR activity in their tissues. This inter‑organismal cue reduces somatic growth and reproductive effort in recipients, thereby conserving nutrients for the senescent individual's kin. The machinery is conserved because the same vesicles also carry microRNAs that modulate epigenetic marks in germ cells, providing a transgenerational inheritance route that reinforces the trait[5].

Predictions and Experiments

1. SASP vesicle transfer – Isolate plasma from old (≥20 mo) mice and young (3 mo) mice, label vesicles with a fluorescent dye, and inject into young recipients. We predict a significant reduction in phospho‑S6K (mTOR readout) in liver and muscle of recipients within 6 h, absent when vesicles are depleted of IGF‑BP3/TGF‑β1 via immunoprecipitation.
2. Kin‑benefit assay – In a semi‑natural mouse colony, manipulate SASP secretion by CRISPR‑knocking out IGF‑BP3 specifically in p16^high^ senescent cells after first litter. Measure litter size, pup survival, and food consumption of siblings over three generations. The hypothesis predicts that loss of the SASP factor will increase individual lifespan but decrease relative fitness of kin (lower pup weight, higher sibling competition) without altering overall colony productivity.
3. Falsification – If pharmacological blockade of mTOR (rapamycin) in young mice replicates the lifespan extension seen in SASP‑deficient old mice without affecting kin fitness, then the observed longevity is due to cell‑autonomous mTOR inhibition rather than a programmed signal, refuting the group‑selection component.

Potential Implications

Longevity interventions that merely dampen senescence may inadvertently disrupt an evolved kin‑support system, leading to unforeseen ecological costs. Instead, therapies could aim to modulate the SASP signal—preserving its beneficial kin effect while uncoupling the detrimental tissue‑damage arm—or to mimic the signal in younger individuals to allocate resources adaptively during periods of scarcity.