Entropy arguments for the primacy of aging are fairly silly. For one, there are actually species that appear to be essentially immortal. Secondly, the germline. Cellular systems are clearly capable in principle of indefinitely maintaining themselves against entropy by either asymmetric division in which molecular waste is partitioned into one child cell (single cell life) or leveraging asymmetric division to replace worn parts with good parts (multicellular life). To explain why aging exists and is so prevalent in biology one has to look at the level above physics, and that debate largely takes place in the overlap between evolutionary biology and molecular biochemistry.
The germline and hydra must be brought forward every time someone says that aging is an inevitable, universal consequence of thermodynamics:
The common perception that the bodies of all living beings age, is wrong. This has now been proved by a long-term experiment with the freshwater polyp Hydra, a microscopic animal. In a unique long-term experiment researchers created artificial conditions for the tiny water animals with their flimsy tentacles, which were free of fatal natural threats like predators. For almost ten years they have cared for of about 1,800 of the Hydras. Overall, the team has counted 3.9 million observation days of individual Hydra. The number of natural deaths per year, however, can be counted on one hand. On average there have been only five. When a Hydra passed away it was mostly due to laboratory accidents, such as a polyp sticking to the lid of its bowl and then drying up or simply having been dropped on the floor. From of the few natural deaths that remained researchers calculated Hydra's mortality. It is so low that even several lifetimes of researchers would not suffice to observe the end of the lifecycle of the polyps. Even after 500 years five percent of a cohort will still be alive. For two out of twelve of the Hydra cohorts under investigation, the risk of death was actually so small, that it will take 3,000 years until only five percent of the polyps remained. "Hydra apparently manages to keep its body young because it does not senesce by accumulating damages and mutations, as most other living beings do. Hydra are probably able to follow a special self-preservation strategy, as its body and cellular processes are rather simple." For instance, Hydra are capable of completely replacing parts of the body that are damaged or are somehow lost. It can even fully regenerate if its body is destroyed almost completely thanks to a high number of stem cells. Stem cells are capable of developing into any part of the body at any time. Additionally, as Hydra replaces all of their cells within only four weeks, it regularly and quickly expels all cells that have been changed genetically by mutations. Thus, damages have little chance to accumulate.
Current thinking on why near all species fail to behave like hydra looks somewhat like this:
Understanding why we age is a long-lived open problem in evolutionary biology. Aging is prejudicial to the individual and evolutionary forces should prevent it, but many species show signs of senescence as individuals age. Here, I will propose a model for aging based on assumptions that are compatible with evolutionary theory: i) competition is between individuals; ii) there is some degree of locality, so quite often competition will between parents and their progeny; iii) optimal conditions are not stationary, mutation helps each species to keep competitive. When conditions change, a senescent species can drive immortal competitors to extinction. This counter-intuitive result arises from the pruning caused by the death of elder individuals. When there is change and mutation, each generation is slightly better adapted to the new conditions, but some older individuals survive by random chance. Senescence can eliminate those from the genetic pool. Even though individual selection forces always win over group selection ones, it is not exactly the individual that is selected, but its lineage. While senescence damages the individuals and has an evolutionary cost, it has a benefit of its own. It allows each lineage to adapt faster to changing conditions. We age because the world changes.
If all of a hydra's cells are replaced every four weeks, I would say that the hydra has a life span of about four weeks. Ship of Theseus and all that.
But even if you don't subscribe to that particular view of what constitutes a consistent identity, the issue is still fundamentally thermodynamic. It is not a task of just maintaining a particular dynamic system indefinitely, but maintaining one with memory. If the hydra is storing absolutely no information about its past states, then it isn't evolving, and it is completely at the mercy of its environment. This makes it more akin to fire or a piece of iron rusting than life: just a consumptive chemical process. This harmonizes exactly with your last paragraph, things need to store some amount of memory of past failure if they are to adapt. This storing process is exactly aging, but how it manifests in different organisms can obviously be very different. Landauer's principle, then, tells us that since information is thermodynamic, so too must be aging. However, I don't really think the author of the original article was arguing at that deep of a level. In humans, many of the processes that we consider to be the detrimental effects of aging do occur because the components that make us do have memory effects. One particular example would be the cross-linking of elastin, causing degradation of vascular system efficiency.
But aging truly is an inevitable consequence of thermodynamics, it's just that since everything is a consequence of thermodynamics, it's not a particularly illuminating argument.
> If all of a hydra's cells are replaced every four weeks, I would say that the hydra has a life span of about four weeks. Ship of Theseus and all that.
Most of our cells in our bodies are renewed in a few months or something. Yet people don't assimilate this to death.
Now, you may object that neurons don't follow this rule, so our subjective identity is not concerned by this. To this I will say that when people are concerned about ageing, they are indeed concerned about what happens to their brain (that is, neuro-degenerative diseases), but certainly not only that. The state of all the other tissues is at stake : the heart, the bones, the skin and so on. Yet all of these are renewed on a regular basis. That renewal is not perfect though, and it's that imperfection they blame on ageing.
The germline and hydra must be brought forward every time someone says that aging is an inevitable, universal consequence of thermodynamics:
http://www.demogr.mpg.de/en/news_press/press_releases_1916/f...
The common perception that the bodies of all living beings age, is wrong. This has now been proved by a long-term experiment with the freshwater polyp Hydra, a microscopic animal. In a unique long-term experiment researchers created artificial conditions for the tiny water animals with their flimsy tentacles, which were free of fatal natural threats like predators. For almost ten years they have cared for of about 1,800 of the Hydras. Overall, the team has counted 3.9 million observation days of individual Hydra. The number of natural deaths per year, however, can be counted on one hand. On average there have been only five. When a Hydra passed away it was mostly due to laboratory accidents, such as a polyp sticking to the lid of its bowl and then drying up or simply having been dropped on the floor. From of the few natural deaths that remained researchers calculated Hydra's mortality. It is so low that even several lifetimes of researchers would not suffice to observe the end of the lifecycle of the polyps. Even after 500 years five percent of a cohort will still be alive. For two out of twelve of the Hydra cohorts under investigation, the risk of death was actually so small, that it will take 3,000 years until only five percent of the polyps remained. "Hydra apparently manages to keep its body young because it does not senesce by accumulating damages and mutations, as most other living beings do. Hydra are probably able to follow a special self-preservation strategy, as its body and cellular processes are rather simple." For instance, Hydra are capable of completely replacing parts of the body that are damaged or are somehow lost. It can even fully regenerate if its body is destroyed almost completely thanks to a high number of stem cells. Stem cells are capable of developing into any part of the body at any time. Additionally, as Hydra replaces all of their cells within only four weeks, it regularly and quickly expels all cells that have been changed genetically by mutations. Thus, damages have little chance to accumulate.
Current thinking on why near all species fail to behave like hydra looks somewhat like this:
http://arxiv.org/abs/1103.4649
Understanding why we age is a long-lived open problem in evolutionary biology. Aging is prejudicial to the individual and evolutionary forces should prevent it, but many species show signs of senescence as individuals age. Here, I will propose a model for aging based on assumptions that are compatible with evolutionary theory: i) competition is between individuals; ii) there is some degree of locality, so quite often competition will between parents and their progeny; iii) optimal conditions are not stationary, mutation helps each species to keep competitive. When conditions change, a senescent species can drive immortal competitors to extinction. This counter-intuitive result arises from the pruning caused by the death of elder individuals. When there is change and mutation, each generation is slightly better adapted to the new conditions, but some older individuals survive by random chance. Senescence can eliminate those from the genetic pool. Even though individual selection forces always win over group selection ones, it is not exactly the individual that is selected, but its lineage. While senescence damages the individuals and has an evolutionary cost, it has a benefit of its own. It allows each lineage to adapt faster to changing conditions. We age because the world changes.