Thursday, August 11, 2005

The Biology of Death The Biology of Death

I have a funeral to attend tomorrow, of the father of one of my best friends. But that isn't the purpose of this post.

I am in the midst of reading "The Biology of Death: The Origins of Mortality" by Andre Klarsfeld and Frederic Revah (translated from the French by Lydia Brady - which means I left the accents off of the authors' names).

So, why don't we live forever? Or, more accurately, why do we age? The answer is really fairly simple, but not intuitively obvious. The technical answer is because our fertility does not increase as we age. And indeed, with single births (typically), it can't.

It turns out that in every species that does not age, whether it be fauna or flora, the organisms increase their fertility as they get older. This is the case for Sequoia, for fish, for crabs, and, even bees. Surprisingly, in the later case, it is only the bred queens who don't age. The unbred queens do, though at a much lower rate than the workers do.

At first, natural selection would seem to be neutral here - it seems that there shouldn't be natural selection in favor of aging if fertility were level. But it turns out that you have to factor death from other causes into the equation. Because members of a species cohort die from other causes, their cumulative reproductive value decreases even if their personal fertility stays the same. It is only in species where fertility rises at the same time that members of their age cohort are dying that do not age. And, also, in many of these cases, the death rate outside of old age flattens out and is relatively flat. You can see this with Sequoia trees - about the only things that will kill them are awfully big forest fires or chainsaws (which are new enough not to affect this). But queen bees fall into this category too - as they are extremely well protected, as compared to the workers they produce.

The bad news is that it takes a lot of generations before natural selection will select for decreased aging. But the good news is that we have moved quite a ways down that path.

It turns out that, with one exception, there is a fairly constant relationship between calories consumed during a lifetime per lb (or kg in the book) and life expectancy for mammels. Ditto for number of total heartbeats. The one exception? Humans, who consume approximately 4 times this mamalian average of calories per lb during their expected lifespan, and their hearts beat approximately 3 times average.

What to make of this? My suggestion is that we have been undergoing natural selection for quite awhile in favor of longer life, and that is why our expected lifespans are probably three to four times what they should be, given the mamalian averages for calories or heartbeats.

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