Mitochondria are bacteria-like organelles inside our cells tasked with the production of chemical energy stores, among other tasks. Like all organelles their collection of intricate protein machinery is wrapped by a membrane - or rather two membranes in this case, inner and outer.
The membrane pacemaker hypothesis advances the idea that membrane resistance to oxidative damage is an important determinant of differences in life span between species: there are correlations in the data from various different species. Mitochondria generate damaging reactive oxygen species as a consequence of their packaging of chemical energy stores, and are vulnerable to self-damage as a result. Some forms of that damage can spiral out to cause further harm that contributes to degenerative aging.
It is not yet completely clear that all these dots can actually be joined in the obvious way - that long-lived animals are long-lived because their mitochondria are more resistant to self-harm, and thus they suffer little from this cause of degenerative aging. Efforts to repair mitochondrial damage to see what happens may overtake efforts to build a better understanding, but work proceeds nonetheless. This paper is open access, but the full text is PDF format only at this point:
Mitochondria play vital roles in metabolic energy transduction, intermediate molecule metabolism, metal ion homeostasis, programmed cell death and regulation of the production of reactive oxygen species. As a result of their broad range of functions, mitochondria have been strongly implicated in aging and longevity. Numerous studies show that aging and decreased lifespan are also associated with high reactive oxygen species production by mitochondria, increased mitochondrial DNA and protein damage, and with changes in the fatty acid composition of mitochondrial membranes.
It is possible that the extent of fatty acid unsaturation of the mitochondrial membrane determines susceptibility to lipid oxidative damage and downstream protein and genome toxicity, thereby acting as a determinant of aging and lifespan. Reviewing the vast number of comparative studies on mitochondrial membrane composition, metabolism and lifespan reveals some evidence that lipid unsaturation ratios may correlate with lifespan. However, we caution against simply relating these two traits. They may be correlative but have no functional relation.