Sunday, 5 June 2011

Why We Age

“Of all the wonders that I yet have heard,
It seems to me most strange that men should fear;
Seeing that death, a necessary end,
Will come when it will come.”

"Don't wanna waste no more time
Time's what we don't have
Everywhere I look someone dies
Wonder when it's my turn"
('Living is a Problem Because Everything Dies', Biffy Clyro)

Nick Lane's Life Ascending is a truly impressive text. It takes the 'ten greatest inventions of evolution' and pulls apart each one apart in depth, whilst maintaining a degree of accessibility to the lay reader.

The reviews in the link above should give you a broad enough overview of this ambitious book, but what I'd like to cover in more detail here is the content of the final chapter, Death.

Mindful that if I copy from one text I am guilty of plagiarism, but if I copy from more than one source then this post will be considered 'well referenced', let's proceed to break copyright law!

Lane quickly cuts to the quick in asking 'why do organisms grow old and die?'. Historically we are told that the answer was believed to be to do with 'the greater cause of the species'.

By the old and worn-out members dying, newer members of the species have more resource available to them. But as Lane points out, such reasoning is circular. Why not 'not age'?

The answer lies in Darwinianism/Darwinism.  It was first identified by Peter Medawar in the early 1950s and has everything to do with statistically probability!  Medawar's idea boils down to the fact that if a species front-loads its sex life, having sex early in its lifespan, it is likely to have more offspring than a specimen which backloads its sexual activity as the latter carries a risk.

You might reproduce only once you reach 100, but if you are hit by a bus or are predated on your 100th birthday, it is the end of the line for you, unlike those that started procreating in their 20s:

  • "Each species, according to Medawar, has a statistically probably lifespan, depending on the size of the individuals, their metabolic rate, their natural predators, physical attributes like wings, and so on. It that statistical lifespan is, say twenty years, then individuals who complete their reproductive cycle within that period will normally leave behind more offspring than those who don't.....genes that happen to cause heart disease after we are statistically dead will accumulate in the genome"
Thus these genes whether they lead to illness, disease or degeneration are 'beyond the reach of natural selection'. Quite a profound concept.

By way of example, Lane gives us the example of Huntingdon's chorea which is a 'relentless mental and physical degenerative disease' which only manifests after reproductive sexual maturity. Incredibly there is some evidence that the victims of this disease are 'more likely to be sexually successful earlier in life'.

Through this lens we can obviously appreciate the importance of procreation on who and what we are, but that is not all there is to it.  There are ways we can extend our healthspan...sexual maturity is but one factor to consider with procreation.

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