What a great question - and one that has been asked since the time of recorded human history!

Because people can die of a variety of things (infection, trauma, etc...) let's stick with a specific question of natural AGING here - the natural, time related deterioration of the physiological functions necessary for survival and fertility (this is how a life scientists defines "aging"). I'll also keep it short, but know that there is a lot more out there on this topic than what I will provide here.

Natural, time-related aging characteristics - as opposed to those from diseases of aging such as cancer and heart disease - affect all individuals of a species.Why must a biological system deteriorate (or "senesce") over time

It takes a tremendous amount of energy for a biological system to resist entropy. Eventually, the organs, tissues, cells and genes simply decay physically. This is the so-called ”wear and tear" model of aging.

There is a current, more sophisticated hypothesis that specific life spans in different species are determined by genes and environment that affect a trade-off between the energy used for early growth and reproduction versus the energy allocated for maintenance and repair. It's a complicated interplay. But regardless of the details, I think your question is really getting at why biological systems don't go on forever.

Here are two examples to get you thinking about how we can probe the question you ask:

First, let's think about what happens in your body - lots of things. Right? One of the most fundamental is cell division - new cells replace old, throughout your lifetime. In those cells, the DNA must be replicated faithfully each round of division, and the genes must be expressed correctly for the cell to carry out its function as part of a tissue or organ. Sometimes, mistakes are made in DNA replication - mistakes are "mutations" which could negatively affect that cell's function. In a young, healthy person, those mistakes are detected and often fixed via a process called DNA repair. Over time, the efficiency of DNA repair goes down - small traumas and mistakes build up and eventually the "system" collapses due to accumulated problems. So, investigating basic mechanism of DNA replication and cell division (in many species) - and especially stem cell biology - are likely to continue to give us new insight into normal aging. Having a healthy lifestyle that supports your body's natural state potentially delays the senescence, but not forever. One common example is anti-oxidants. You might see ads from these supplements or hear about certain foods that are rich in anti-oxidants. This is because they reduce something called Reactive Oxygen Species (ROS), which cause DNA damage in your cells. So, keeping ROS low, in theory, slows aging by slowing DNA damage.

Second, nature has done some experimenting for us. There is a human genetic disease called PROGERIA (you can look it up if you are interested in more information). This is a genetic disorder that results in premature aging. Some forms of progeria are extremely fast - in one called Hutchinson-Gilford progeria, an 8 year old child typically has a phenotype similar to that of an 80 year old! In the past decade, new DNA sequencing technologies have revealed that these children carry a mutation in a gene that encodes a nuclear protein involved in regulating DNA repair. So, even as a baby, they cannot correct mistakes - which accumulate quickly and cause that premature aging.

Finally, this question provides an opportunity for us to think outside the box - there are exceptions in Nature to aging. For example, many turtles have very long life spans. BUT, unlike other long lived species, mortality rate and reproductive rate does not increase with age, and in the few studies where scientists have looked, a very old turtle does not appear to exhibit any obvious signs of senescence. One measure of this is the "quality" of the ends of the chromosomes (the telomeres). In most organisms, the telomeres shorten over time, leading to DNA damage that can cause the cell to die or function improperly. But even in very old turtles, the telomeres shorten extremely slowly, if at all. No one knows why. Interesting, turtles also are very good at dealing with ROS. It might be related to how they deal with oxygen deprivation - the same enzymes that allow a turtle's organs to deal with low oxygen also protect against ROS.

So, like many questions about life, science does not yet know the answer to this very old question - we continue to gather evidence and re-evaluate models, seeking to understand. Thanks for joining us, simply by being curious!

One way to understand why all people eventually die is due to parts “wearing out.” One example is how when cells divide, they sometimes lose a portion of their DNA called a telomere. Due to this telomere loss, a cell can usually divide only about 40 times before it has to stop. If it divided beyond that point, it would start losing important DNA and could become cancerous. When cells don’t divide anymore, they get old and don’t work as well leading to disease.

Another aspect leading to death is that DNA is constantly being mutated at a low rate throughout life. Many times, the accumulation of these mutations can lead to cancer. The longer someone has lived, the more mutations they will have and the more likely they will get cancer. A third important cause of eventual death is due to the accumulation of damaged biological molecules like proteins. One way these proteins are damaged is that in the process by which we take in oxygen to make energy, sometimes oxygen can escape and react with other molecules thereby damaging them.

The combination of all these factors means that sooner or later a person is going to die. Unlike a car, we can’t switch out parts that get worn out because each person’s DNA is unique. Also some damage is throughout the entire person and there is therefore no way to fix all of the damage.

That being said, there is no universal law saying that people can’t live forever. It’s just a really hard thing to accomplish when there are so many things that can go wrong. Given enough time, everything in a human being will break down. To really make humans live a very long time, it would likely require electronic enhancements or serious genetic engineering, both of which would be highly controversial. Also, it’s better for the species in general if people don’t live forever. By constantly reproducing and dying off, we make it so that humans can have the best set of genes for the current time. Not to mention the other enormous issues humanity would face if anyone lived forever.

Humans have genetic traits that are good for surviving to childbearing age but bad for surviving to old age. It's a trade-off, but natural selection only cares about survival to reproduction, so these bad long-term traits persist.

Things wear out. It takes a lot of energy to keep things working well. A simple example is that your room gets messy unless you work to keep it neat and clean. Our bodies wear out, too. Scientists are working hard to understand aging. Science Magazine has a special issue this week about people aging [Dec 4 2015]. It's interesting.