That idea was sketched in the 1940s by a British immunologist, Sir Peter Medawar. A decade later, the American evolutionist George Williams added a second step. Mutations often have multiple effects, and these can differ in their timing. Consider a mutation that has good effects early in life and bad effects late. If the bad effects come late, after the organism has most likely perished because of external threats, then these bad effects will have less importance than the early benefits. This is a “buy now, pay later” principle, with payments coming due only after you have probably left the scene anyway. So mutations with that combination of effects - helpful early, harmful late - will be beneficial over all and will accumulate in the population. Then if an individual survives all the external threats and reaches old age, it will be hit with the bill.
The Medawar effect and the Williams effect work together. Once each process gets started, it reinforces itself and also magnifies the other. As some mutations are established that lead to age-related decline, they make it even less likely that individuals will live past the age at which those mutation act. This means there is even less selection against mutations that have bad effects only at that advanced age. As a result, that age becomes harder and harder to exceed.
In the light of all this, I think it is becoming clearer how octopuses and other cephalopods came to have their peculiarly poignant combination of features. Like their mollusk relatives, early cephalopods had protective outer shells, which they carried along as they prowled the oceans. Then, in some animals, the shells were abandoned. This had several interlocking effects. First, it gave rise to their unique, outlandish bodies - in the octopus, a body that can take on any shape at will. This created an opportunity for evolution of finer behavioral control and large nervous systems. But the loss of the shell had another effect：It made the animals vulnerable to predators, especially fish.
That put a premium on the evolution of octopus wiles and camouflage. But there are only so many time those tricks will save the animal. Octopuses can't expect to survive long. This makes them ideal candidates for the Medawar and Williams effects to compress their natural life spans. As a result, octopuses have ended up with their unusual combination：a large brain and a short life.
This view is supported by the recent discovery of an exception to the usual octopus pattern, an exception that illuminates the rule. The octopuses I've been talking about tend to live in shallow water. But in 2014 researchers at the Monterey Bay Aquarium Research Institute released some remarkable images of a deep-sea octopus they had watched with remote-controlled submarines. This one octopus brooded its eggs for over four years. Even allowing for the fact that everything tends to happen slowly at these depths, that's a very long time. The total life span of this octopus might have been as long as 16 years.
The Medaway-Williams theory predicts that predation risks should be much less severe for this species than they are for shallow-water octopuses with shorter life spans. And the images taken by the Monterey researchers contain a strong clue that this is so：They show an octopus sitting out in the open with its eggs for years on end. It did not find itself a den. This suggests that this species has less to fear from predators than other octopuses do. As a result, evolution has turned the life span of this species differently.
Putting these things together, we can see how many features of the octopus could have stemmed from the abandoning of the shell all those year ago. This move set octopuses on a path of mobility, dexterity and nervous complexity, and it also led to live-fast-die-young lifestyle, an existence always exposed the predators around them.
If octopuses could somehow gain the upper hand against those predators, their natural life spans should increase, though it's hard to see them making it to our 115 years - and when one contemplates the thought of a century-old octopus, perhaps that's just as well.
