When it comes to the source of longevity in mammals, the finger is pointing to the mitochondria. Either the mitochondria are more efficient, or as is shown to the be the case in the mole-rat and now the echidna, the surrounding biochemistry is more resistant to damage. The echidna "is exceptionally long-living. Its documented maximum lifespan of 50 years is 3.7 times that predicted from its body mass. Other exceptionally long-living mammals (naked mole-rats and humans) are known to have peroxidation-resistant membrane composition .... The peroxidation index (=peroxidation susceptibility) calculated from this membrane composition was lower-than-expected for their body size, indicating that the cellular membranes of echidnas would be peroxidation-resistant. Additionally when the calculated peroxidation index was plotted against maximum lifespan, the echidna values conformed to the relationship for mammals in general. These findings support the membrane pacemaker theory of aging and emphasise the potential importance of membrane fatty acid composition in aging and in the determination of maximum longevity." Mitochondrial damage is the first domino in a long and spreading line of follow-on processes of damage. Ongoing research into mitochondrial repair, replacement and damage resistance deserves far more funding, given the potential payoff for human healthy longevity.

Link: http://www.ncbi.nlm.nih.gov/pubmed/18586080