Depends on one's perspective.
The Earth's natural radioactivity heats its internals giving us a liquid outer core.
That core is responsible for generating the planet's magnetic field.
That magnetic field blocks solar and cosmic radiation which would otherwise be very intense on the Earth's surface.
So intense that life likely wouldn't have self-assembled and started evolving under the barrage.
(We'll return to our original question in a bit.)
Evolution has led to us.
Evolution is the process by which genes change over time yielding new species. Genes which provide species the attributes to survive persist (as does the species), those which don't perish.
If genes were "fixed" (non-mutable) they couldn't evolve to form new species. Changing environmental conditions would have likely wiped out the earliest life long ago, even with our magnetically shielded planet. Life's continuity requires "changeable" (mutable) genes.
It is important to point out that your genes don't care about YOU particularly. They care about THEIR proliferation. YOU are just a particular body type devised by your genes to help THEM proliferate. If your genes can keep YOU intact long enough to get THEM passed along to another generation, then they've been successful.
Your success will wane, unfortunately.
Why is cancer the second leading cause of U.S. deaths?
In 1900, cancer was number seven on the list, but by the 1940's it had become the number two (behind heart disease) leading cause of death, which is where things stand today.
What happened over those forty years?
We learned to conquer the other major causes of deaths like tuberculosis and pneumonia (then the number one and two leading causes respectively).
Those conquests allowed us to live even longer...our life expectancy increased by about 26 years!
Since cancer strikes most people at age 60+ it became more prevalent in our society.
Cancer tends to strike older people because our genes have programmed themselves to largely maintain their fidelity through our reproductive age (see preceding section). It doesn't take one mutation for cancer to emerge, it takes at least two and usually many more depending on the cancer. And genes are very good at repairing many mutations, though not perfect....sometimes the mutations aren't repaired or they may be mis-repaired.
Radiation (cosmic, solar, terrestial) is one of the agents of damage. It is a carcinogen ("cancer causing") and a mutagen ("mutation causing"). By causing mutations in genes in sex (sperm and egg) cells, it can promote gene diversification and survival (mutated genes which provide a beneficial factor to offspring). This enhances the process of evolution.
But in somatic (body, non-sex) cells these mutations can lead to cancer (now might be a good time to recall the first question asked on this page).
Over time, individual somatic cells accumulate more and more damage which escapes repair mechanisms or are mis-repaired. Most of these cells will die, but some of these cells can develop attributes which allow them a survival advantage in their micro-environment.
They EVOLVE and outperform your normal tissue!
This is cancer! It is the evolution of some of your somatic cells in the micro-environment of your body. And real success occurs when some of these cells leave their original site and invade and conquer other tissues, a process called metastasis.
Cancer is a fundamental property of mutable genes which themselves are fundamental to the propagation and evolution of life.
Though we started waging the War On Cancer in 1971 (when cancer had already been the second leading cause of death for 30 years) under President Nixon, cancer still remains the second leading cause of death even some 40 years later.
Cancer Isn't The Plan - Senescence (Aging) Is
Through most of our evolution, genes succeeded if the person lived to around 30 years of age and had offspring. The parent could then expect his/her demise, be it by disease, as animal food, homicide, or whatever.
Human genes evolved to maintain their fidelity up to about this age, and then it becomes inefficient for them to maintain it. Specific tumor suppressor genes play a key role. If a tumor suppressor gene is mutated, that is a key step towards cancer.
But the normal tumor suppressor mechanisms also affect normal cells. These effects lead to the overall body changes we observe over time which we called senescence. There is no need for your genes to maintain your youthfulness and vigor when thwarting cancer after the reproductive age your genes have determined for you.
So cancer is the outcome of many random mutations ...some people get lung cancer, others colon cancer, etc. Which cancer one gets partially depends on the environmental conditions in which your body (and your parents and their parents, etc.) finds itself, over which your genes have little control (until recently, through your intelligence).
Your genes have to fight the carcinogen onslaught continually over time, while promoting your greatest physical attributes as a young adult, making your body the most attractive to the body of someone else (sexual selection).
The process isn't perfect...some mutations may have occurred. Cancer may have even started. But it can only progress fully in very few people, otherwise our species wouldn't survive. After our individual reproductive peaks are reached, senescence overcomes our youthfulness as the fight against carcinogens continue.
Senescence is the plan and cancer the result of fortuitious victory by forces working against the plan.
Both are integral to life and human evolution.
And radiation has played and continues to play a key role.
This article or this one may be helpful and this NIH video is very good, but very technical (skip to 10:30 to avoid introductions):