Why we all get older at about the same rate is still not well understood.
Humans keep living longer and longer lives but the simple fixes like washing our hands or having decent indoor plumbing are behind us. To keep making gains, we are going to have to do some hard work and figure out why we all age at a pretty similar rate.
If we can figure that out, then we can work on either short circuiting the things that make us old and/or stalling the harmful effects of aging. This is what makes a new study out in the journal Physical Review Letters so interesting.
In this study, Justin Werfel, of the New England Complex Systems Institute, and his coworkers use mathematical modeling to show that nature selects for a species’ average life expectancy. This is exciting because one possible conclusion from this work is that we are programmed genetically to live a set number of years.
In other words, at some point a genetic program is switched on that causes aging to accelerate. The body gives up repairing itself and starts to break down. If we can find some way to delay or even turn off the switch, then maybe we can live for a significantly longer amount of time.
OK let’s get to work finding these genetic programs right? Not so fast. It turns out that we don't really have a good handle on why we age at the rate we do. Right now there are three main competing theories all with plenty of evidence to back each of them up.
Three Ways to Age
It is perhaps a bit surprising to realize that scientists still don't know why we age when we do. We know what aging is—the build up of DNA damage over time that makes us grow more frail and sickly as we get older.
What we don't know is why it happens at around the same rate in all of us. As I said, right now there are at least three big theories on why we all get old like we do. Which theory is right will determine how best to win the war on aging. (Or at least win a few more battles.)
The first of these models is the most appealing because it gives us the best chance to live a really long time. In this one, we are programmed to live a certain number of years. If we can tweak this programming, we might live for a very, very long time.
With the other two models we will have to keep making small incremental improvements in our life expectancy. One says that we all age at about the same rate because we all build up mutations at the same rate. The other one says that we all have some genes that are really helpful when we are young but hammer us when we get old.
Neither of these has the magic bullet appeal of the programmed aging model. And neither pushes the boundaries of human life to the several century mark.
With both of these we need to figure out lots of little tweaks throughout out our cells and bodies to add more years to our lives. These would definitely involve a long slog!
So we need to figure out why we get old to figure out how to slow it down. The current study can support the idea that we are programmed to die but there is still a lot of work to do.
The new study, as I mentioned, used mathematical models to show that nature selects for species with a certain life span depending on the resources available. One way this is a cool result is that it confirms yet again that there is more to natural selection than what is best for the individual. If only the individual was important, then we would all be immortal (or as close as possible).
The authors of the study propose that this longevity selection might have to do with competing for resources with your descendants. If you live too long, you will eat up all the food that should have gone to your kids and their kids.
Because your DNA will be more successful if your descendants are more successful, there is a balance between living longer and dying at the right time. Each species finds a different balance point.
What this study doesn’t necessarily do is say how a species gets to that balance point. It is possible that there is a program that turns on at a certain time that results in us all dying at around the same time but there are certainly other possible explanations.
It might be that there is a gene that is so great for us when we are young and fertile that we all have it. But that same gene causes us to have problems at around the same time later in life meaning we all start to break down around the same time.
Or it could be that a species reaches that perfect life span by how well it corrects its DNA damage. Consistent with this, there is some evidence that longer lived species are better able to fix any genetic errors more easily.
As you can see, we still have a lot to learn about why we age. And once we figure it out, maybe we can do something to live a whole lot longer. I am keeping my fingers crossed.