Not the person you're replying to, but the theory I've heard is the following.
The background for this theory is that evolution is gene-centric, not individual-centric. What is fitter is what will better propagate the genes, which is not necessarily what's best for the individual.
So it can build "designs" that have a better chance of propagating genes earlier on, where features that help that (I'm making this example up, but say something like faster growth) may later have a deleterious affect on the individual. And since evolution is gene focused, there's not as much evolutionary pressure for a longer and healthier life for the individual.
The background for this theory is that evolution is gene-centric, not individual-centric. What is fitter is what will better propagate the genes, which is not necessarily what's best for the individual.
So it can build "designs" that have a better chance of propagating genes earlier on, where features that help that (I'm making this example up, but say something like faster growth) may later have a deleterious affect on the individual. And since evolution is gene focused, there's not as much evolutionary pressure for a longer and healthier life for the individual.