We Age Because the World Changes
Aging is an inevitability, or so we have to assume: the processes of evolution blindly but efficiently explore the space of possible living creatures, and have been doing so for a very, very long time. We might think that surely a very long-lived or ageless species would have a great advantage in evolutionary competition, its individual members able to produce descendants for far longer than competitors in more short-lived species. Yet virtually all species - with only a very few exceptions - age in easily measured ways. The species that age are also the species that have won in evolutionary terms, and therefore prospered and spread. Why is this?
A recent open access paper (in PDF format) explores one of the approaches used to answer this question, and does so in a very readable fashion:
Living organisms shouldn't age, at least if that could be helped (many of use would certainly like that, but our wishes are not a valid argument). Evolution works in a way that any species whose representatives have any distinct disadvantage will be driven to extinction. It makes sense then to assume that, if aging could be avoided, species that showed senescence as the individuals grow older should be replaced by others where aging does not happen (or happens at a much slower rate). Senescence increases mortality and an individual who dies of old age will leave, in average, a smaller number of descendants than another individual that does not age and manages to live and reproduce for a longer time. And yet many known living organisms show senescence. The time it takes for an individual to show signs of old age varies greatly among species, but aging seems so natural that many people fail to realize there is an apparent contradiction between senescence and evolution....
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.
And there is illustrated one of the present competing viewpoints on the origins of aging.
I posit that aging is unnatural, period. It serves no purpose, evolutionary or otherwise. It is nothing more than the consequence of living far beyond our "design specifications". Few will see a connection between teenage pregnancy and old age, but I posit that they are biologically linked. In the eyes of nature, we are "supposed" to reproduce in our teens, and die by predation, starvation, disease, trauma or hypothermia in our mid to late twenties. Indeed, studies have found that cognitive, immunological, and reproductive decline all begin at the same age...27 years! Bear in mind that civilization as we know it has existed for only 10,000 years. That is 0.5% of the 2.5 million year existence of the genus Homo. In other words, for 99.5% of human history parenthood before age 20 and death before age 30 was the order of the day. We are cavedwellers living in the modern world, eight-tracks in the MP3 universe.
Right, the article: "Old age begins at 27: Scientists reveal new research into ageing" http://www.dailymail.co.uk/sciencetech/article-1162052 from 15th March 2009 informs: The research at the University of Virginia, reported in the academic journal Neurobiology Of Aging, found that the first age of humans at which performance was significantly lower than the peak scores was 27. Nevertheless, aging in plants and animals is not unnatural. Aging in the current population of humans, however, is not natural either. Humans are doing all sorts of unnatural stuff with their brains. Designing software and writing with computers, for instance, most of it is unnatural. Yet, any normal human being is doing much unnatural stuff, because of the larger human brain. Thus, an aging human being is unnatural, because of a larger brain size that demands too much time for the process of senescence until it has eliminated the outdated individual from the genetic pool. Someone should come and fix this.
Our species is the most adaptable form of macroscopic of life on Earth, through the employment of our very own engine of behavioural adaptation - the human brain. Even if this model of aging is correct, it shows that aging is little more than a redundant, inferior and outmoded feature that should be done away with as quickly as possible.
So the variables here are
- time (senescence)
- number of offspring
- mutation rate.
I guess as humans we can't complain, being on the leading edge of nature's exploration space with respect to time. Could an increased mutation rate offset its penalty?
It's all in the numbers - someone please post a Mathematica notebook.
So, as it is with humans, the time factor (short life) already won out over an increased mutation rate?
Adam: Uh no, it's not that simple