On day 21 of adulthood, three quarters of the worms are already dead, and the survivors barely twitch. That is exactly when the scientists stepped in and gave them another 26 days, even though the control animals lived only four. Aging, it turns out, is not the accumulation of breakdowns but a slide down an unstable slope: you don't have to clear away the rocks, you just have to make the grade gentler.
The experiment was built on the nematode C. elegans, whose insulin receptor DAF-2 was tagged with a special label (a degron). Adding auxin destroyed the protein within 30 minutes, and removing the substance restored it. This let the researchers switch the signaling pathway off and on at any age, like a toggle. In 21-day-old worms, the intervention halted cuticle breakdown, restored resistance to heat and osmotic stress, and partly dissolved protein aggregates, although age-related pathologies of the pharynx, gonads and uterus remained.
To understand why a late intervention works just as well as an early one, the scientists built a mathematical model. They described the state of the organism with a single variable that changes under the influence of random noise. Death occurs when this variable reaches a critical threshold for the first time. The model reproduced the experimental survival curves if the intervention lowered the instability parameter without changing the noise level and without resetting the accumulated state.
For humans, this means a change of strategy in the search for anti-aging therapies. Instead of trying to erase damage, we should look for ways to stabilize regulatory networks. In long-lived species such as humans, the dynamics stay stable for most of life, and instability grows only in old age, so interventions that alter the dynamics may produce only a temporary effect. To extend maximum lifespan, it will be necessary to slow the accumulation of entropic damage.
The worm at death's door got a second life not because it was repaired, but because the rules of its fall were changed. The slope became gentler, and the rocks that used to trip it up now simply lie in its path.
The nematode C. elegans has a stable life form as well: the dauer larva, which can wait out unfavorable conditions for months with almost no aging. Intervention in the DAF-2 signaling pathway apparently partially activates this stability program even in adult worms.
The idea that aging can be 'switched off' by changing the parameters of a system rather than repairing it echoes Greg Egan's novel Permutation City, where people manage their biological state through software settings.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.