Peto’s Paradox is the observation that the incidence of cancer does not correlate to the number of cells present in an organism, meaning, on average, the larger the organism, the less likely the organism is to get cancer. This is considered to be paradoxical as if each cell was given an equally likely chance to become cancerous, then with a larger organism, given the greater number of cells there is an expectation for the chance of an organism’s cancer incidence to significantly increase. However, this is not the case.
The Paradox was first formulated in 1977 by Richard Peto who was an English epidemiologist who studied at the University of Oxford.  The scientist realised that humans were vastly less cancer susceptible than mice on a cell by cell basis. Within a species of organism, the size of the organism
has a positive correlation with cancer incidence, however, this is inversed across species; the larger the organisms of a species, the lower the cancer incidence on a cellular basis compared to anotherspecies. This tends to result in a similar prevalence or cancer in all organisms.
There are three different theories for why this is the case. One of these is the hyper tumour theory, which is the idea that as tumours grow within the organism, it diverts the blood supply to the tumour to help it grow. However, a hyper tumour can then grow which cuts off the blood supply to
the tumour causing it to die off and the bigger the animal, the larger the tumour has to be for it to become a problem and so the higher the chance for a hyper tumour to grow. Another is the fact that smaller animals tend to have a higher metabolic rate, which means that more reactions happen in a
space of time for smaller animals hence the chance for cancerous cells to develop is higher. Finally, there is an idea that the larger the animal, the more Tumour Suppressor genes there are, which is the idea I will be exploring.
Cancer is caused by the mutation of Proto-Oncogenes, which can enable the cancerous cell to hide from the body, call for resources, lose its ability to perform apoptosis and multiply quickly. The mutation of these genes is restricted by Tumour Suppressor genes which prevent these mutations of
the Proto-Oncogenes from happening or can cause the cell to undergo apoptosis, controlled cell death, if it is unable to repair the DNA sequence. The more Tumour Suppressor genes, the more proteins are made to prevent the mutation of the Proto-Oncogenes. Scientists have discovered that the larger the organism, the more Tumour Suppressor genes the organism has and so the lower the incidence of cancer.
The increased number of Tumour Suppressor genes in a larger organism is a direct example of evolution in motion. The larger the organism the more cells it has. The more cells it has the higher the risk of cancer, which means the selection pressure of cancer is much higher in larger organisms.
The higher the risk of cancer, the more organisms in that species die to cancer. Due to random mutation, an organism in that species has more Tumour Suppressor genes and is therefore more successful as the incidence of cancer in it is less. This is then passed on and through natural selection
becomes common within the population, hence resulting in the larger organism having more Tumour Suppressor genes than a smaller organism and so the lower the chance of a cell becoming cancerous.
This would account for the fact that the larger the organism within a species, the higher the incidence for cancer since organisms within a species will have the same or very similar numbers of Tumour Suppressor genes, but the number of cells will vary depending on the size of the organism.
What does this mean for humanity? In the UK, the number of cancer deaths is 163.5 deaths per 100,000 men and women per year (2011-2015 deaths). This results in 109,545 deaths due to cancer in the UK per year. Selection pressure is the driving force of evolution and the greater
selection pressure the faster the rate of extinction or evolution. This pressure is defined as any cause that reduces reproductive success in a portion of a population, driving natural selection. Is cancer a great enough of a selection pressure for humans to evolve to have more Tumour Suppressor genes and for the prevalence of cancer to be lowered? This answer can be referred to the definition of selection pressure. It is highly likely that the selection pressure of cancer is not enough to result in any substantial evolutionary advantage for having more Tumour Suppressor genes and the fact that the average age to have a baby in 2017 in the UK was 28.8 years old , cancer has little to no impact on the reproductive success in a proportion of a population.
In conclusion, cancer is an incredibly small selection pressure for the human population with negligible effect on the reproductive success of a human and so the increase in number of Tumour Suppressor genes in a human is highly unlikely to occur and therefore humanity will not become
more cancer resistant.
- Nunney, Richard (January 2013). “The real war on cancer: the evolutionary dynamics of cancer suppression” Last accessed: 21 September 2020: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3567467/
- National Cancer institute (April 2018). “Cancer statistics” Last accessed: 19 September 2020: https://www.cancer.gov/about-cancer/understanding
- UCMP. “Understanding evolution” Last accessed: 19 September 2020: https://evolution.berkeley.edu/evolibrary/article/evo_25
- Office for National Statistics (January 2019) “Birth characteristic in England and Wales: 2017” Last accessed 20 September 2020:https://www.ons.gov.uk/peoplepopulationandcommunity/birthsdeathsandmarriages/livebirths/bulletins/birthcharacteristicsinenglandandwales/2017#:~:text=The%20average%20age%20of%20first,or%20subsequent%20births%20in%202017.