We can say for sure that malignant tumours are a group of diseases that have always existed and always will exist. Despite the belief that many of today’s illnesses are a by-product of our lifestyles, stress or pollution, tumours have existed for a long time. Remnants of tumours have been discovered in dinosaur fossils dating back 70 – 80 million years. The oldest known malignant tumour found in a human is in the 3.9 – 4.2 million-year-old skull of an ancient Australopithecus specimen discovered by Luis Leaky in 1932. Cancer cells were also discovered in the bones of Egyptian and Peruvian mummies, which date back to 3,000 BCE. Ancient papyri discovered by Edwin Smith and Georg Ebers detail not only the symptoms of cancer, but also the treatment methods used thousands of years ago. Therefore, we know that cancer has existed in humans for at least one million years. Why hasn’t nature noticed its mistake and attempted to correct it via evolution?
Malignant tumours can be found in most, if not all, vertebrates and most often in mammals. Tumours have been discovered in pets and domesticated animals (dogs, cats, goats, etc.) as well as wild animals, whose illnesses are much more difficult to study. Detailed accounts include those of Virginia opossums, coyotes, red foxes, Bengali tigers, African lions, snow leopards, Asian elephants, a variety of whales and many others. However, it’s suspected that the incidence of cancer in wild animals is much lower than in humans. But there are exceptions. One of these is the DTFD, or devil facial tumour disease, of the Tasmanian devil. Morphologically speaking, the disease is similar to sarcomas of neuroendocrine origin and in some areas devil facial tumour disease can affect up to 80% of these unusual creatures. This has been linked to the behaviour of this species and the vicious wounds they inflict on one another with their sharp teeth. The disease is passed from one victim to the next via blood or saliva. Malignant tumours of the urogenital system (urethra and penis) are found somewhat frequently in one population of California sea lions and this is believed to be the result of local pollution. On the other hand, nearly 27% of all deaths of beluga whales in Canada’s St. Lawrence River estuary are the result of intestinal cancer. And although a commonly held myth claims that sharks are immune to malignant tumours (which is why their cartilage is often recommended to cancer patients), the fact remains that they sometimes suffer from one of the most aggressive types of skin tumours – melanoma.
However, there are exceptions – animals in which tumours have yet to be found, such as the naked mole-rat. Can this help us understand why humans and some animals have tumours? Could we use this information to research potential prevention methods?
The mechanisms that stimulate or suppress the development of tumours among mammals are supposedly similar, yet differ among the various orders of this class. Over 40 years ago this difference was named “Peto’s paradox” (in honour of the English scientist Richard Peto). This means, that although it may seem that the larger the individual’s number of cells (the larger the animal), the more the tumour’s development must be proportional to its host body’s size, in fact this isn’t true. Regardless of size, both small creatures and large mammals alike develop tumours and environmental factors seem to have a larger effect on this than an animal’s size. Only 5% of elephants die from malignant tumours, but this statistic is one in five for humans. It turns out that the elephant genome contains many tumour-suppressing genes (p53). This has also been found in many other mammals. Elephants have roughly 20 times more copies of the p53 gene than humans. This is why human cells become malignant much easier than those of elephants or whales. This p53 gene has the ability to slow the proliferation process to gain time to correct errors, but if this isn’t possible then the p53 gene signals the cells to commit suicide, which scientists call apoptosis. Elephants and whales are not only large mammals, but they also live long lives giving cells ample opportunity to make mistakes during the division process, which could lead to the creation of cancer clones, but this doesn’t happen. It turns out that the beluga whale genome, on the other hand, has mutations that protect its DNA from potential damage. It would appear that we could solve this problem by artificially enriching people with the correct genes. However, this may not be as simple or safe as it may seem, because everything in nature has its own specific order.
According to data from the American Cancer Society, the risk of developing one of many malignant tumours is 46% for men and 38% for women. Data for the United Kingdom is even more disheartening – 54% for men and 48% for women. At the end of 2016, there were 73,983 registered cancer patients in Latvia, but 49,272 in the year 2000. The number of patients has increased one and a half times. Why has cancer become such a common occurrence? There are, of course, some logical explanations – an increase in population, an increase in life expectancy, better diagnostics and tracking. It appears that cancer is the unfortunate by-product of man’s evolution.
Once a spermatozoon encounters an egg, an embryo is formed within a few days, which contains a few hundred cells. By the time maturity has been reached at age 18 these cells have divided so many times that we can’t even say for sure how many cells are contained in a human body. Cell multiplication is strictly controlled by our bodies. While an embryo develops, some cells of the palm are subjected to apoptosis, or programmed death, to form gaps between the fingers. A tumour is also the result of cell division, but it develops by circumventing or ignoring the body’s control mechanisms.