The Limits of Longevity

Do all animals age? Although aging seems to be a widespread phenomenon, some demographic studies have failed to find evidence of aging in certain species, including some highly regenerative species of planarians and Hydra that reproduce through asexual fission. However, all demographic studies have limits on observation times and sample sizes, so it is unknown if these failures were because of an actual absence of aging or these inherent study limitations. Some argue that these species must be ageless. Because of pressures that result from the lack of a clean division between the germ line and the soma in fissiparous organisms, agelessness becomes necessary as a prerequisite of this kind of reproductive strategy. Others argue that fundamental theories of the evolutionary biology of aging absolutely preclude agelessness. Even putting evolutionary arguments aside, some mathematical models of cellular competition and senescence argue that agelessness is impossible mechanistically in multicellular organisms. In this work, I address evolutionary and mechanistic arguments for and against agelessness. I develop mathematical models of the Disposable Soma Theory that incorporate facets of the arguments for agelessness in asexual fissioning organisms. I construct models of mutation accumulation and drift within an individual and explore how this genetic decay could manifest in the mortality rates. I use these models to understand if aging is inevitable generally and apply them to planarians and Hydra to seek to estimate the likelihood of aging more narrowly in those specific cases. Contrary to other work, I find that agelessness (defined as non-increasing mortality rates in a population) is indeed possible as the optimal evolutionary strategy for multicellular organisms. However, the evolution and mechanistic realization of agelessness requires conditions that are unlikely to be met in any existing species. In the case of planarians and Hydra, they likely do not face the right kind of evolutionary pressure to completely avoid aging. Even if they do face necessary evolutionary pressure, intraindividual genetic decay will almost certainly induce increasing mortality on the population with little recourse. Therefore, these species likely do age, although they could have median lifespans on the order of hundreds or perhaps even thousands of years, which would make detecting aging in any given population study quite difficult indeed.

Name thesis:degree_name

Doctoral

Department dc:contributor.department

Massachusetts Institute of Technology. Computational and Systems Biology Program

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2025

Author dc:creator

• Rodriguez, Christopher W.

Advisor dc:contributor.advisor

• Reddien, Peter W.

Statement dc:rights

• In Copyright - Educational Use Permitted
• Copyright retained by author(s)

Licence dc:rights.uri

• https://rightsstatements.org/page/InC-EDU/1.0/

Handle dc:identifier.uri

https://hdl.handle.net/1721.1/163534

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/163534