Thinking Beyond Science

Even if the animals can reproduce throughout their lives, by far the majority of reproduction is going to occur at younger ages simply because most animals will die before they reach a grand old one. Because most reproduction happens in youth, changes that affect an animal’s reproductive chances at older ages won’t make much difference.

An animal with some modification that doubled its baby-making capacity at 50 years old wouldn’t have an evolutionary edge because it likely wouldn’t survive long enough to put its baby-doubling powers to use. By contrast, an animal which got a baby bonus aged three would probably still be alive after three years, raring to reproduce, and this trait would mean far more offspring – conferring a significant evolutionary advantage.

Huntington’s is a brain disease which is caused by a mistake in a single gene, typically giving rise to symptoms sometime between the ages of 30 and 50, and is usually deadly around 15 to 20 years after diagnosis. As we’ve seen, human life expectancy in prehistory was somewhere around 30 or 35 years so, from an evolutionary perspective at least, coming down with Huntington’s at age 40 and dying of it at 55 doesn’t much matter. A ‘wild’ human was already likely to have had several children, and their remaining reproductive lifespan was short. Even in modern times, it’s quite possible for a Huntington’s sufferer to have children before succumbing to the disease. Thus, in spite of its deadliness, Huntington’s persists, albeit rarely, in the human population.

Huntington’s is a clear example of an accidentally accumulated mutation, where a single gene causes something unambiguously and severely bad at post-reproductive ages. But, while deadly single-gene conditions provide clear examples, the bigger deal when it comes to normal aging is the cumulative effect of many different genes, working alone or in combination, conspiring to slightly erode our chances as we enter our post-reproductive years. Deadly mutations float around in the gene pool, to evolution’s studied indifference, as long as they kill us slowly enough to allow us to reproduce first. Taken together, these imperfect genes ignored by evolution are behind some of the processes which cause us to age.

So how does evolution make this deal with death and bring on animals’ decline in exchange for reproductive success? The answer is that genes often have multiple personalities. Modern genetics tells us that genes don’t exist in splendid isolation, coding for a single characteristic. They have multiple functions at different times and in different parts of the body, and interact with one another in complex networks. You should also raise an eyebrow if you ever hear someone speak of a ‘gene for’ a complex characteristic. Even traits as simple as eye colour are under the control of many different genes, and those genes also have multiple functions, playing a role in hair and skin colouration, and they may well moonlight in other processes in ways we are yet to uncover. The multifunctionality of single genes is known in biology as ‘pleiotropy’.

‘antagonistic pleiotropy’ – the idea of genes which have multiple effects allowing them to conspire to aid reproduction in early life, but go on to cause problems as the animal gets older. Imagine a mutation in our island-dwelling animals which increases the risk of an animal dying over the age of 30, but which allows it to reach reproductive maturity a year sooner than otherwise. Carriers of this mutation will rapidly expand in numbers compared to those without – the disadvantage to the few per cent of animals left alive post-30 is dwarfed by the enormous reproductive advantage which will accrue to young animals who now have an extra year to reproduce at a time when most are still alive.

Immortality, in biology as in mythology, always comes at a price. In biology, it’s not an ironic forfeit to amuse the gods, but the necessity to maintain your body for an indefinite period of time. Maintenance costs energy – energy which could be used growing muscles to outrun predators, developing an immune system to fend off disease, or becoming reproductively mature more quickly and producing offspring before something kills you.

Source : Ageless: The New Science of Getting Older Without Getting Old by Andrew Steele

Goodreads : https://www.goodreads.com/book/show/52954648-ageless

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