Around 2008, while snorkeling and scuba diving in my free time, I began watching the unusual group of animals known as cephalopods, the group that includes octopuses, cuttlefish and squid. The first ones I encountered were giant cuttlefish, large animals whose skin changes color so quickly and completely that swimming after them can be like following an aquatic, multi-armed television. Then I began watching octopuses. Despite being mollusks, like clams and oysters, these animals have very large brains and exhibit a curious, enigmatic intelligence.
I followed them through the sea, and also began reading about them, and one of the first things I learned came as a shock：They have extremely short lives - just one or two years.
I was already puzzled by the evolution of large brains in cephalopods, and this discovery made the questions more acute. What is the point of building a complex brain like that if your life is over in a year or two? Why invest in a process of learning about the world if there is no time to put that information to use? An octopus's or cuttlefish's life is rich in experience, but it is incredibly compressed.
The particular puzzle of octopus life span opens up a more general one. Why do animals age? And why do they age so differently? A scruffy-looking fish that inhabits the same patch of sea as my cephalopods has relatives who live to 200 years of age. This seems extraordinarily unfair：A dull-looking fish lives for centuries while the cuttlefish, in their chromatic splendor, and the octopuses, in their inquisitive intelligence, are dead before they are 2? There are monkeys the size of a mouse that can live for 15 years, and hummingbirds that can live for over 10. Nautiluses (who are also cephalopods) can live for 20 years. A recent Nature paper reported that despite continuing medical advances, humans appear to have reached a rough plateau at around 115 years, though a few people will edge beyond it. The life spans of animals seem to lack all rhyme or reason.
We tend to think about aging as a matter of bodies wearing out, as automobiles do. But the analogy is not a good one. An automobile's original parts will indeed wear out, but an adult human is not operating with his or her original parts. Like all animals, we are made of cells that are continually taking in nutrients and dividing, replacing old parts with new ones. If you keep replacing the parts of an automobile with new ones, there is no reason it should ever stop running.
At least in principle, the puzzle of aging has been largely resolved, through some elegant pieces of evolutionary reasoning. Imagine some kind of animal with no tendency to decline in old age. It just keeps going, and keeps reproducing, until some accident or predator get hold of it. In such a species, like any other, genetic mutations continually arise. Sometimes (very rarely) a mutation occurs that makes organisms better able to survive and reproduce；more often mutations are harmful and are filtered out by natural selection. But in some cases, a mutation arises that acts so late in an organism's life that its effects are usually irrelevant, since the organism has already died for another reason, such as being eaten. Natural selection will have little effect on that mutation, so it will become either more common in the population, or less common, purely by chance.
Eventually, some mutations of this kind will become common, and everyone will be carrying them around. Then when some lucky individual does succeed in living a long time without being eaten, it will run into the (usually harmful) effects of the late-acting mutations. It will appear to have been “programmed to decline,” because the effects of those lurking mutations will appear on a schedule. The population has now evolved a natural life span.
