Re: What causes us to age

Date: Sun Jan 10 13:32:51 1999
Posted By: John Carlson, Medical student, MD/PhD (parasitology) , Tulane University, School of Medicine
Area of science: Medicine
ID: 911968722.Me

Message:

Dear Cody,

I don't think you will ever be satisfied with a physiological answer to your question. One type of medical research has emerged which has been called "Darwinian Medicine". People who research in this area use evolutionary theory to discover incredible new things not only about how the human body functions, but perhaps more interestingly, why it functions the way it does.

Humans have a really low rate of death until old age. Once old age comes, the death rate for humans increases dramatically. Asking why we get old is the same as asking why we die. Why has evolution selected for this life pattern?

Ultimately, the reason people grow old is because of trade-offs. Organisms tend to trade increased health when they're younger for decreased health when they're older. In other words, your genetics have programed you to be as fit as possible when you are young at the expense of health in old age. Why has this happened?

Let's take an imaginary organism and do some math.

First let's assume that the organism doesn't get old. Still, there is always a chance of death. (A disease might come and kill it or a predator might eat it.) Let's say that every year of the organism's life there is a 25% chance that the organism will die and a 75% chance it will live. Another way of writing this is that the probability of staying alive is .75 (out of 1). (This notation makes the math easier.) So after the first year of the organism's life there is a 75% chance it will still be alive. To get the probability of the organism still being alive after the second year, we have to calculate the 75% chance it will live through the first year, and then also figure in the probability that it will live through the second year. This comes out to be .75 X .75, which is .56, or a 56% chance the organism will still be alive. For the third year the probability of living is .75 X .75 X .75 = .42 or 42%.

Year of life:                     1    2    3    4    5    6

Probability of living this year: .75  .75  .75  .75  .75  .75

Probability of still being alive:.75  .56  .42  .32  .24  .18

Let's say, for the sake of saving time, that it's impossible for this organism to live longer than 6 years.

Now, evolutionary selection is concerned with only one thing: the passing on of genes. In this case, passing on genes is only possible by having offspring. Let's say our organism can have two children a year, starting on the second year. (We have to give it one year to mature!) So now we have:

Year of life:                     1    2    3    4    5    6

Probability of living this year: .75  .75  .75  .75  .75  .75

Probability of still being alive:.75  .56  .42  .32  .24  .18

# of children born if still alive:0    2    2    2    2    2

Now we can calculate the number of children this organism will have over its lifetime. To do this, for every year of life, we multiply the probability of it still being alive by the number of children born that year if the organism is alive. Then we add up the number of children over the six years, and that is the total number of children born for one organism during its lifetime. Now it's important to realize we're making calculations for an AVERAGE organism like this. So when the number of offspring is not a whole number, that's okay. Some organisms of this type will have more offspring, and some will have less, we're calculating the mean average.

Year of life:                     1    2     3    4    5    6

Probability of living this year: .75  .75   .75  .75  .75  .75

Probability of still being alive:.75  .56   .42  .32  .24  .18

# of children born if still alive:0    2     2    2    2    2

# of offspring:                   0   1.12  .84  .64  .48  .36

Total = 0+1.12+.48+.64+.48+.36 = 3.44 offspring.

So the average number of offspring for this organism is 3.44.

Now let's change things up a little bit. Let's change from a constant rate of death like we have for the organism above (.75) to a death rate that is lower when the organism is aged 1,2 and 3 years old, and higher when the organism is aged 4,5 and 6 years old. We'll add .10 to the probability of living through each of the first three years and subtract .10 from each of the last three. So now we have:

Year of Life:                     1     2     3     4    5    6

Probability of living this year: .85   .85   .85   .65  .65  .65

Probability of still being alive:.85   .72   .61   .40  .26  .17

# of offspring born if alive:     0     2     2     2    2    2

# of offspring:                   0   1.44   1.22  .80  .52  .34

Total = 0+1.44+1.22+.80+.52+.34 = 4.32 offspring.

Notice that on year six, under the constant death rate, the organism has an 18% chance of being alive. For the organism that traded health in youth for illness in the future, only 17% of the population is still alive. But the organism that is ill in old age had more children. And that's all that selection really cares about... passing on genes. Over time, selection would favor those that made the trade-off, and select against those that didn't make the trade-off. That's what has happened to humans.

Your systems are extra strong and healthy when you are younger to give you a better chance to pass on your genes. But the cost is that when older, your systems begin to wear out and fail. Many people get sad thinking about growing older and really hate the idea of death. One thing that always cheers me up a little is realizing that it is a trade. If I wanted to be super healthy when older, I wouldn't have been as strong and fast and healthy when I was younger.

It's great that you're asking these probing questions. I encourage you to read up on how exactly evolution works. It is an amazing process that is widely misunderstood or ignored. Check out the evolution links from the Mad Scientist Network's Library. If you have more questions, or something is unclear, please submit another question to MadSci Network. Also, I highly recommend reading Why We Get Sick: The New Science of Darwinian Medicine by Randolf Nesse and George Williams. It's a fascinating book which explores evolution, and how it relates to human health.

Thank you for your question!

John Carlson, MAD Scientist

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