While cancer mortality has plummeted in recent years, survival rates have doubled in the last 40. Yet actual cancer rates have not really fallen overall, and there has long been a subset of people who claim that is because of our "modern" lifestyle; things like food and trace chemicals in the environment.
Their solution is to be as ancient as possible; eat what they call a "paleo" diet and try to avoid chemicals that are not natural.
That's environmental science. Real science knows that the biggest risk for cancer, as it is with just about every disease, is age. The longer we live, the greater the chances that we will have a mutation or fault in our genes that will manifest in cancer. One hundred years ago, there was less cancer because the average life expectancy was only 42.
But a short lifespan was no guarantee. As evidence that cancer may have been common for our ancestors also, we have proof of osteosarcoma from a foot bone found at the Swartkrans cave dig site that dates back ~1,700,000 years. Fossilization of any kind is quite rare but we have other evidence of cancers, even 120,000 years ago. When you have multiple examples of cancers from fossils across different time periods and locations, it is reasonable to infer that they may have been more common than realized.
Credit Edward Odes (Wits)
That fossil had a bone tumor so it does not mean lifestyle is irrelevant in all cancers. The big reason cancer rates do not seem to decline is because a lot of smokers are still alive, and so are a lot of people who spent time sun-bathing. That sounds garish but it is accurate. Cigarette smoking, with hundreds of toxic chemicals, is second to living long enough as the risk for cancer. And obesity comes in behind that. While enjoying the sun is fine, the weight of evidence has shown too much can be harmful.
Smoking has plummeted, and the American Council on Science and Health is proud to take part of the credit for that, so its cancer impact will be substantially lessened in the future, while obesity is simply a cultural maturity issue; our ancestors did not easily have enough food unless they were rich and today, thanks to science and technology, even the poorest people in America can afford to be fat. Over time we won't all act like there won't be enough food tomorrow and binge today.
Still, two-thirds of any "increase" in cancer we see today is just because we live longer, not because of wheat or corn syrup or sugar. Instead, the modern world has prevented a lot of cancer. Stomach cancer is way down, for example. H. pylori, a stomach bacterium which promotes increased cell mutation, wasn't even discovered until 1982, so our modern lifestyle did not cause it, but our modern lifestyle has reduced its infections and therefore cancers.
Rather than being caused by the modern environment, a lot of cancers are still happening because of personal behavior risk factors rather than science or corporations selling products. You can make behavior exculpatory and try to blame wheat for your belly, but the wheat itself is not causing cancer any more than a fork is. It's what you are doing with both that might need to change.