An Example of the Evolution of Life Span

Life span in a species is an evolved trait: if longer lives provide a competitive advantage over shorter-lived peers, then a species will tend to become longer lived over time. We humans are long-lived for our size in comparison to other mammals, and the current thinking on that is that it may have to do with our intelligence and social nature - there is a selection effect based on advantages to survival provided by the presence of post-reproductive elders in a collaborative environment.

Salmon provide another example of the impact of evolution on aging, with their unusual aging process driven by levels of predation. The environment in which a species lives has a strong effect on life span. Here is an open access paper that considers another collection of fish species in which life spans evolved to adapt to differing mortality rates caused by environmental factors:

Early evolutionary theories of aging predict that populations which experience low extrinsic mortality evolve a retarded onset of senescence. [Here], we study annual fish of the genus Nothobranchius whose maximum lifespan is dictated by the duration of the water bodies they inhabit. Different populations of annual fish do not experience different strengths of extrinsic mortality throughout their life span, but are subject to differential timing (and predictability) of a sudden habitat cessation. In this respect, our study allows testing how aging evolves in natural environments when populations vary in the prospect of survival, but condition-dependent survival has a limited effect. We use 10 Nothobranchius populations from seasonal pools that differ in their duration to test how this parameter affects longevity and aging in two independent clades of these annual fishes.

We found that replicated populations from a dry region showed markedly shorter captive lifespan than populations from a humid region. Shorter lifespan correlated with accelerated accumulation of lipofuscin (an established age marker) in both clades. Analysis of wild individuals confirmed that fish from drier habitats accumulate lipofuscin faster also under natural conditions. This indicates faster physiological deterioration in shorter-lived populations. [The] characterization of pairs of closely related species with different longevities should provide a powerful paradigm for the identification of genetic variations responsible for evolution of senescence in natural populations.



We know almost nothing about the social structure of human ancestors, but authoritative statements and even treatises about them far outrun the almost non-existent data. Let me have a crack at my own adaptationist storytelling: the presence of cannibalism is one of the few knowns about the "social system" of our ancestors, so that should be a good theme for our story. Our little band of ancestors is in crisis! They have no way to store food in anticipation of lean times to come, or do they (dun dun dun)? The natural selection fairy godmother has just the perfect solution for them: storing available food energy in the form of post-reproductive grandparents! By lengthening their lifespan without rejuvenation and providing them the opportunity to become ever more frail, they're moulded into walking, talking larders for their stronger and more agile descendants. That's just the ticket for escaping those trying fallow seasons so that the grandchildren can survive and thrive.

This hypothesis has many advantages:
• It relies on known behaviour of ancient hominids rather than unsubstantiated speculations
• There are analogues in other species, for instance arthropods that eat their mates to acquire the food energy for producing more eggs, as opposed to no analogy for social learning from grandparents
• We don't know how educational those grandparents may have been, but we can have a pretty good idea how edible they were
• We can rope in other hot topics to join in our far-reaching explanatory structure (for instance, this "explains" why people are more prone to obesity as they get older — gotta fatten up those cattle)

Despite the evident superiority of this adventitious hypothesis I just concocted, I have little hope for its adoption. Why? It does not flatter our modern sensibilities, which is what all the ancestor talk is really about. Have you ever noticed that phrenology provides a perfect microcosm of the 19th century world-view? They wrote their own cultural predilections into the very fabric of humanity, and the people who tell these ancestor stories are doing the same thing for the 21st century. Ancestor storytelling is modern phrenology.

Posted by: José at April 9th, 2013 10:59 PM

Post a comment; thoughtful, considered opinions are valued. New comments can be edited for a few minutes following submission. Comments incorporating ad hominem attacks, advertising, and other forms of inappropriate behavior are likely to be deleted.

Note that there is a comment feed for those who like to keep up with conversations.