Since the late 19th century, evolutionary biologists have assumed that natural selection favors individuals with long lifespans. It makes some intuitive sense: the longer you live, the more time you have to get busy making babies, maximizing your reproductive potential.
As for what determines an individual organism's lifespan in the first place, scientists have largely concluded that this is a result of a mix of extrinsic factors (such as predation, disease, or accidents) and intrinsic factors (the biological decay that eventually results in death).
"Lifespans are selected for and genetically programmed."
But according to new research published earlier this year in PLOS One, these theories are wrong: it turns out it's natural selection may have pushed organisms to have an internal time for how long they're supposed to life. We are, in essence, genetically programmed to self-destruct.
"We somehow have this idea that lifespans are kind of inevitable in some fundamental sense," Yaneer Bar-Yam, the president of New England Complex Systems Institute, told me on the phone. "What we're saying is that this isn't true, that lifespans are selected for and genetically programmed."
At first glance, the idea seems paradoxical. Traditional evolutionary theory is based on a "selfish" model of individual organisms where lifespans are always maximized. This seems fairly intuitive: a lineage with a gene that made individuals die well before their 'natural' expiration date wouldn't exactly last very long. But a number of examples in nature seem to directly contradict this theory.
Take, for example, semelparous species such as octopus that live until they give birth and then abruptly die. A number of theories have been advanced to explain away this contradiction, but they still can't account for contrary evidence, such as how an octopus is able to continue living and mating if a certain hormonal gland is surgically removed directly after the octopus gives birth. If this gland isn't removed, the octopus will die. This curious trait has the hallmark of genetic programming, not biological decay.
Bar-Yam and his colleagues are arguing that natural selection actually favors traits that self-limit consumption and reproduction, not selfish maximalism, including lifespan limiting mortality. In other words, organisms may be able to have longer lifespans than they presently do, but natural selection has actually favored individuals that clock themselves out early.
To reach this conclusion, Bar-Yam and two colleagues at Harvard's bioengineering institute started out with a basic question: is there an environmental context in which genes will self-limit lifespans?
"Not only does the answer turn out to be yes, but in some sense it's generically true," said Bar-Yam. "It's more than possible—it turns out that it's almost always the case."
So how could evolutionary scientists have the link between mortality and natural selection so wrong? Aside from August Weismann—who in 1882 did actually argue that death was programmed—it's because when they considered the effect of evolutionary selection, they were taking averages across organisms and their environments instead of considering each individual organism in its local context. By removing the individual from its particular place or location within a given population, this average ignores the complex relationship between that individual and its environment.
To get a more dynamic and accurate view of evolutionary mortality, Bar-Yam and his colleagues applied a technique for researching complex systems called spatial modeling. By looking at how an individual's local context affects their fitness, Bar-Yam and his colleagues were able to show that traits which may be an advantage in the short-term (such as an individual's longevity or 'selfish' resource consumption) can actually be a significant disadvantage in the long term, and vice versa.
According to Bar-Yam and his colleagues, the long term success of this self-limiting paradigm is not just limited to resource consumption and procreation. It also governs lifespans within a population, which are optimized and limited based on the species local environment.
"If you overexploit your resources, you're going to be in trouble."
That natural selection favors organisms that self-limit their lifespans has some pretty profound implications. In the first place, it helps shed some light on the current problems faced by human species which are increasingly global in scope, yet still hyper local when considered in a galactic context. Our economic systems hardly function on a paradigm of self-limitation—global finance capitalism is expansive by definition and thrives on selfish behavior. While this may work out well for the most selfish individuals in the short term, if Bar-Yam and his colleagues are correct it could be cataclysmic for our species in the long run.
"What people do affects their environment and that affects their ability to survive," said Bar-Yam. "This is something we're all well aware of today. If you overexploit your resources, you're going to be in trouble."
But everything is not all gloom and doom. As Bar-Yam points out, if death is genetically programmed, that also means it can probably be hacked.
"Humans aren't subject to the same environment we were millions of years ago when our lifespan was established," said Bar-Yam. "This suggest we could adjust that mechanism and have people that live a lot longer."