At first glance, the fact that we grow old and die seems like one massive evolutionary mistake. After all, if evolution is all about survival of the fittest, wouldn't the fittest individuals be those that can stay healthy and keep reproducing forever?
That's a bit of a simplification - and no, evolution isn't all about survival of the fittest, but it's a useful shorthand - but the fact is that there's no clear reason why aging evolved in the first place. Biologists have generally assumed that it's a side effect of other, more important adaptations. But there might be a more direct evolutionary benefit for aging.
Currently, the most common hypothesis to explain aging is that, in order to give the members of a population the best chance of surviving to reproductive age, certain genes need to be selected that also cause bodies to break down later in life and, ultimately, die off. There's some good evidence to support the basic theory - the genes that control aging are found throughout the planet's organisms, indicating whatever caused aging to evolve happened very early on, and the basic mechanisms work so well that they've barely changed in half a billion years and in millions of different species.
But none of this speaks to the possibility of direct benefits from aging. Now, André Martins of Brazil's University of Sao Paolo has created a computer simulation that he argues demonstrates just why aging and dying can benefit populations. In his model, he created two groups, one "mortal" and the other "immortal", and placed them on a grid that was subject to repeated environmental changes. He then ran a series of simulations to see how the two groups fared.
On a strictly individual level, you'd assume that the immortals would do better than the mortals. After all, they remain healthy indefinitely, and they don't have to worry about anyone else growing so sick or feeble that they become a burden. But Martins ran the simulation over many generations, and this meant the mortals had a chance to keep breeding. After all, the mortals had to keep reproducing, or else their population would die out.
As the environment changed, some of the mortal offspring were mutated or otherwise became better adapted to their surroundings. The mortals eventually became dominant over the immortals, who were increasingly ill-adapted to their altered environment. Worse, because most immortals still weren't dying off, they had no room to reproduce and replace the now ill-adapted members with younger, potentially fitter children.
Obviously, since this is just a computer simulation, it can't be considered "proof" of why and how aging evolved. But it complicates a theoretical framework that has generally considered aging strictly an unwanted side effect of other, more advantageous adaptations. It's also a reminder that evolution really does operate on a species-wide, multi-generational scale, and that natural selection is more than willing to sacrifice the fittest individuals — after all, what could seem more fit than an immortal? — in the name of a healthier species overall.