Ageless Animals, the Sea Urchin Edition
Many species of animal age very slowly, and some so slowly that researchers have not yet been able to pin down either a rate of aging or a life span absent predation and accident. Some of those species might not age at all, but we'll have to wait for researchers to establish whether or not this is the case. Take the lobster, for example. Despite all the eating and farming that takes place, there's less funding for basic research into lobster biology than you might expect. As a consequence:
To date, there is no proven method to determine the exact age of a lobster. ... as best scientists can tell, lobsters age so gracefully they show no measurable signs of aging: no loss of appetite, no change in metabolism, no loss of reproductive urge or ability, no decline in strength or health.Lobsters, when they die, seem to die from external causes. They get fished by humans, eaten by seals, wasted by parasites, but they don't seem to die from within. Of course, no one really knows how the average lobster dies. There are no definitive studies.
Turning to another common sea creature, species of sea urchin appear to be in the same boat as the lobster: if they're aging, they're doing it very slowly.
The red sea urchin Strongylocentrotus franciscanus is a long-lived species and may live in excess of 100 years based on tagging studies in the field and corroboration from radiocarbon analyses as reported in the literature. Size-specific survival estimates reported here show no change in annual survival probability across the 6 largest 0.5cm size classes from 14.6 to 18.1cm. In addition to no change in survival probability there is no reduction in reproductive capacity with size. Red sea urchins show no evidence of senescence and so do not fit well within the context of the disposable soma theory of the evolution of longevity.
You'll recall that a baseline definition for aging is an increase in mortality rate with time, which doesn't happen for these urchins, or at least to any degree that can be detected via study methods at the present level of funding. This presents a challenge for the classic interpretation of the evolution of aging, which suggests that putting more resources towards tissue repair and the upkeep of biological systems that can support extended life spans is a losing proposition. Evolution favors the quick win, a fast and efficient organism that can reproduce quickly - and rapidly fall apart afterwards.
Or so the thinking goes. Clearly, extremely long-lived and possibly ageless species did evolve, so theory must be extended. The latest thinking on the matter theorizes that crowding can lead to a runaway evolutionary competition for longer lives, an arms-race for the biology that lets you wait out the competition for limited space:
adults live in crowded but stable conditions in which new opportunities for maturation arise rarely. In such situations, it behooves an individual organism to outlive its neighbors, so that when they die its seedlings or larvae have a place to dig in and grow up.
Does the humble urchin have any relevance to the future of human aging and longevity medicine? Not directly, I'd imagine, in the sense of applying learned science. But as more people understand the range of what is possible in living organisms, support for engineering a better healthy life span in our species will grow, whether or not that goal is achieved by mining nature for new metabolic biotechnology.
I think these animals are more important than we think. If you look at the various different theories of aging they apply at a low-level and would apply to all multi-cellular life. There's not really anything that would be specific to humans or even mammals. de Grey's seven causes of aging, for example, all cover things you'd expect to find in a lobster. I feel like a lot of stuff is being glossed over and taken for granted when we look at other animals and casually dismiss differences in lifespan.
Regarding what you said here:
"Clearly, extremely long-lived and possibly ageless species did evolve, so theory must be extended."
Why the theory must be extended ?, the theory doesn't rule out the possibility of negligble senesence.
The theory basically says 3 things:
1. In nature, an organism will not die of aging, rather from external death factors.
2. The better the organism can cope with external death factors, the longer it will live in nature.
3. If a certain organism lives an average lifespan of X years in nature, then when moved to a protected environment, it will start to show external signs of aging, soon after the age of X.
Now, I'm not exactly following why lobsters need a special treatment, and can't fit in this regular, non-extened theory. We just need to accept the fact that lobsters evolved to deal with their external death factors, much much better than we did, before the modern age.
"Theory", not "the theory." Meaning the collection of thinking and theorizing about evolution and how it explains aspects of the world we see.
When I said "the theory", I meant the classic thought about why and when organisms age.
I'll now adress what you said below, in order to be clearer:
"You'll recall that a baseline definition for aging is an increase in mortality rate with time"
Agreed.
"which doesn't happen for these urchins, or at least to any degree that can be detected via study methods at the present level of funding."
Agreed.
"This presents a challenge for the classic interpretation of the evolution of aging..."
Here is where I don't agree. The reason is we simply don't know what is the average lifespan of the lobster/urchins in nature (without human predation).
It could be that for these animals, "X" is 300 years (see my #3 point in earlier post), so aging phenotypes will be hard to discover (especially without proper funding) for those younger than 300 years. Ideally we need to get them to the lab, to a controlled and protected environment, and let them live for 500 years. It could be that after 300-400 years or so, we'll start to see significant signs of aging. Of course this is not practical.
What we're doing now with the lobsters/urchins, might be similar to taking an 18 year old human person, inspect him, and say that because he does not show signs of aging, humans don't age.
Of course if we take a 50 year old human, we'll not say this, but, the equivalent for a 50 year old human, in lobsters/urchins, might be a 300-400 year old lobster/urchin, and we simply don't have them to inspect (since in nature they die before such age, from external death factors, and also, it's not practical to grow them at the protected environment of the lab to prove that they do age, because they live too long).
To conclude, my entire point here is that in order to explain why we can't detect aging in the lobsters/urchins we can put our hands on currently, there might be no need to extend the classic interpretation of the evolution of aging, because it might be at work here, just on a "larger timescale".