An interesting view on the evolutionary depths of longevity in mammals, achieved through analysis of presently available genomes: "It is widely assumed that our mammalian ancestors, which lived in the Cretaceous era, were tiny animals that survived massive asteroid impacts in shelters, and evolved into modern forms after dinosaurs went extinct, 65 Mya. The small size of most Mesozoic mammalian fossils essentially supports this view. Paleontology, however, is not conclusive regarding the ancestry of extant mammals, because Cretaceous and Paleocene fossils are not easily linked to modern lineages. Here we use full-genome data to estimate the longevity and body mass of early placental mammals. Analysing 36 fully-sequenced mammalian genomes, we reconstruct two aspects of the ancestral genome dynamics ... Linking these molecular evolutionary processes to life history traits in modern species, we estimate that early placental mammals had a life-span above 25 years, and a body mass above one kilogram. This is similar to current primates, cetartiodactyls or carnivores, but markedly different from mice or shrews, challenging the dominant view about mammalian origin and evolution. Our results imply that long-lived mammals existed in the Cretaceous era, and were the most successful in evolution, opening new perspectives about the conditions for survival to the Cretaceous-Tertiary crisis."

Link: http://www.ncbi.nlm.nih.gov/pubmed/22949523