All higher living organisms are composed of cells. Each cell runs biological DNA-based programs. These programs are very different compared to programs on a computer (it's much more about biochemistry than electrical circuits). These programs are supposed to make cells perform their tasks correctly. But over time, the DNA can be affected by chemicals or faulty copy processes. The DNA sequence changes and the program "code" of the DNA is affected as well. The cells with altered DNA will behave slightly differently and in many cases nothing bad happens. But sometimes the changes affect functionality that is important to control the behaviour of the cell.

The thing is, a single cancer cell is not a problem. The problems really start, once the cancer cells creates a great number of copies of itself and disrupts the operations of the other normal cells.

What makes cancer tricky, is that cancer cells still are derived from normal cells and they might look too similar to normal cells so that it is difficult for the immune system to spot and kill them.

What is now the root cause? The root cause is errors in the DNA sequence that accumulate over time. This is a natural process. The cells have some DNA repair mechanisms, but they are not perfect. To fully cure cancer, you would need to correct all DNA sequence errors which is not practical. However, in practise, this is not what is needed. Instead, it suffices if the cancer cells are being killed. And our immune system is pretty good at this. But at some point, some cancer will have a faulty DNA seuence, with new program code, that helps it hide from the immune system and then we need the help of doctors to get rid of the cancer.

AIUI, it isn't really a single disease with a specific cause. It's more like, our body is composed of trillions of individual cells. Each cell is theoretically an independent life form capable of doing its own thing. They normally each have a program, encoded in their DNA, that keeps them operating in service of the overall organism of our body - replicating only when needed, dying when not needed, doing things useful to the body while they're alive, etc. There's various environmental factors, radiation and chemicals and such, constantly attacking that DNA. The DNA has self-repair mechanisms, plus the body's defense mechanisms to kill badly behaving cells, so most mutations are either repaired, ignored, or immediately fatal to the cell, kind of like changing a random character in your program.

Over all of those trillions of cells and dozens of years, once in a while, a particular cell gets just the right batch of random mutations that make it possible to stay alive while breaking free of its programming and acting as its own organism, independent of the body's plan. It may then proceed to reproduce on its own, potentially disrupting the body's functions and/or consuming all of its resources if it grows quickly enough.

So each individual cancer case is a uniquely evolved life form. There's a few common threads that many tend to share, but no telling how many each particular case may have. Thus, the broad brush approaches tend to be the most successful - surgery, chemotherapy, and radiation. More directed genetic approaches are tougher because of the uniqueness of each case.

Evolutionarily, it's a tricky balance. Too few mechanisms to keep all of your cells under control all the time, and your body dissolves into a mess of cancers too soon, like before you can reproduce. Too many mechanisms wastes resources, can potentially go haywire and damage your body themselves, like autoimmune diseases, or make it too hard for your body to evolve new and useful adaptations to the environment. We already have good enough controls that getting cancer before normal reproduction age is very rare. But now we're living a lot longer, much longer than our bodies are evolutionarily adapted to keep us alive for, so we see a lot more of these kinds of problems.

I agree with most of what you say.

If living beyond the age we were designed for is the cause, I wonder why the American Cancer Society says:

By 2030 the incidence rates among people ages 20 – 34 years will increase by 90% for colon cancer and by 124.2% for rectal cancer.

Just evolution whispering “kill, consume, multiply, conquer” to the cells in your body in the normal way that evolution does. It's pretty common for small mutations to occur in your cells from time to time. Cancer requires several mutations to line up in ways that allows cells to start multiplying without constraint - evading normal growth controls and apoptosis and the immune system and such. The cells where the mutations don't align and which start multiplying with everything else you never even notice.

It's not that there's a fault in evolution's design so much as that evolution doesn't really care that much that people die of cancer. Whether or not it could be possible to create a non-senescent humanoid species by evolution evolution has a fixed budget of complexity it can work with given germline mutation rates and the selective pressure our species is under. And reducing germline mutation would mean we would evolve more slowly in the face of new selective pressures like new diseases.

People are working on ways to end cancer. Aubrey de Grey has a plan[1] which, unlike his others for fighting aging, I think is totally crazy but hey at least someone is working on it. But its certainly not something that could evolve in nature.

[1]https://en.wikipedia.org/wiki/Strategies_for_Engineered_Negl...

Reminds me of the suicide gland of some cephalopods with mating. It is one thing to starve to death while guarding their eggs constantly but an actual gland to kill them?

Likely a result of letting their niche be taken by descendants made them more adaptable than ones which lived until other constraints or more or less bad luck (cummulative fatality risks over time) killed them.