Today's open access paper makes for an interesting companion piece to the recently proposed adaptive-hitchhike model for the evolution of longevity. Here also, the length of species life spans and the degree to which age-related degeneration (senescence) takes place over time is suggested to be a side-effect of adaptations to specific ecological niches. The authors of this paper observe that a greater proportion of marine clades have a long life span and lesser degrees of senescence than is the case for land-dwelling clades. Further, a greater fraction of marine species continually grow throughout life, a capability that has implications for the processes of regeneration and tissue maintenance. In addition, species that evolved a return to marine life, such as aquatic mammals, tend to be longer lived than their near relative species that remained land-dwelling.
The paper provides a great deal of data and sorting of that data, but is light on detailed conclusions. It is interesting to see the clear advantage in life span enjoyed, on average, by marine species. It is already the case that many in the research community look at the existence of multiple unusually long-lived species in specific niches, such as naked mole-rats and near relative species in their oxygen-poor underground environment, and hypothesize that longevity arises as a side-effect of the evolutionary adaptations required to thrive in that environment. The work here provides much more food for thought on this topic, to be taken up by molecular biologists in search of compelling mechanisms to explain the observed data on life span, senescence, and ecological niche.
Our analysis of the maximum life expectancy of species belonging to the groups of animals revealed distinct groups. The first group was insects, with a 99% share of short-lived species. The second group consisted of amphibians and terrestrial reptiles, with an average maximum life expectancy of about 10 years, though some animals, such as olm (Proteus anguinus), show extremely high life expectancy. The third group consisted of land mammals and some birds. The average life expectancy of animals in these groups is approximately 15 years. Only humans (Homo sapiens) and few other species can be classified as long-lived. The next group comprised of about 20% long-lived species that live for over 35 years. This group included fish with short-lived species, but also a large number of very long-lived species, such as Greenland shark (Somniosus microcephalus). High average maximum life expectancy was also observed in soaring birds. However, aquatic mammals were completely different from the other groups, with over 40% of them being long-lived.
Terrestrial habitats have been dominated mainly by animals that show a clear senescence phenotype. Since the emergence of insects, mammals, and birds occurred at different points of time, senescence must have evolved independently. Consequently, senescence in these groups of animals is likely to be the result of convergent evolution. Thus, we considered the original questions regarding why these three diverse terrestrial clades, show symptoms of senescence and whether senescence could be adaptive. The arguments presented herein suggest that the appearance of senescence in the three major groups of terrestrial animals was a consequence of the evolution of their life histories and as a side effect of the cessation of growth in sexually mature adults. The main mechanisms leading to the loss of this ability for growth were different across clades and occurred at distinct periods of time. The appearance of senescence in various unrelated clades during various periods of animal evolution suggests convergent evolution of senescence, and hence, a lack of homology.
Evidence that the loss of ability of indeterminate growth in terrestrial mammals results in senescence, is provided by secondarily aquatic mammals. In addition to lower rate of senescence, tetrapods that have undergone secondary aquatic adaptation include the longest-living mammals, such as the bowhead whale (Balaena mysticetus), all of which live for over a hundred years. Researchers have examined reproductive materials from mature female bowheads but did not see positive evidence of senescence. Similarly, the maximum and average lifespan of aquatic and semi-aquatic reptiles, which also secondarily returned to the water, exceed those of their terrestrial relatives.
Our analysis suggests that senescence may have emerged as a side effect of the evolution of adaptive features that allowed the colonisation of land. Perhaps specialisation and adaptation of animals to life on land was accompanied by senescent phenotypes, as a side effect of evolution. Thus, senescence in mammals (including humans) may be a trade-off compromise between land colonisation and longevity. In our relatively short synthesis, we presented adaptations that involve animals best suited to life in terrestrial environments. We emphasised that senescence occurred in parallel with highly adaptive traits. Examples of secondary aquatic mammals indicate that it is evolutionarily possible to delay the onset of senescence symptoms. The aquatic environment, which offers conditions that allow animals to grow larger, due to greater density of fluid medium (water) and facilitates the maintenance of appropriate body temperatures, seems to favour negligible senescent phenotypes or an extremely delayed appearance of the signs of senescence, shortly before the death of the individual (as seen in fish, octopus, and whales). Similarly, the functioning of soaring birds seems to be less costly in terms of energy compared to their relatives with passerine-type flights. Hence, we may conclude that an energy-efficient life in a stable environment can delay the symptoms of senescence and promote a longer life.