The reason why humans get some forms of cancer – while other animals don’t – could simply be down to an evolutionary accident. This is one theory being suggested in a new study published in Biological Reviews, which looks at what animals get which cancers.

Cancer occurs when cells mutate and grow out of control. As such, they are not unique to humans and are found across almost all multicellular organisms on the planet, including animals – and even plants. If the disease was caused by entirely random mutations, then you would expect all species to be equally affected by different types and incidence rates. But this is not what we see.

In order to delve into why humans are more likely to get particular types, a team of researchers from the UK's University of Liverpool and Brazil's Escola Superior de Ciências da Saúde set out to conduct the largest survey of animal cancer data to date. They hoped that by looking in detail at what species get which cancers and when they could try and tease out the disease's evolutionary underpinnings in humans.

They were able to show that a whole variety of animals have been found to develop tumors, from polar bears to leopard frogs, jellyfish to insects. All can get cancer to varying degrees, but fascinatingly they found significant differences when comparing them to humans.

It seems that some types of the disease are widespread across most species regardless of where they fall on the evolutionary tree, such as lymphomas and leukemia. But then there are others that appear to be limited just to humans, such as lung, prostate, and testicular cancer.

The question now is why humans are more likely to get these specific forms of the disease. “Perhaps unique mutations during the evolution of the human lineage contribute to the disproportionately high incidence of some cancers in our species when compared to all other studied species,” suggested lead researcher Dr Joao Pedro De Magalhaes. In other words, maybe they were simply evolutionary “accidents” that cropped up in our own lineage after we split from chimps.

But that might not be the only potential cause. Among animals, humans are pretty unique due to our longevity, compared to our body size, something that's only expected to increase. The researchers suggest that by living longer we might be experiencing cancers that our ancestors would not have. It might also have something to do with the way in which our lives have changed both technologically and culturally.

“Our work highlights the different evolutionary pressures acting on cancer early in life (with a high prevalence of blood cancers presumably driven by the need to fight pathogens) and cancer late in life that escapes natural selection, including human-specific cancers that may be evolutionary accidents or related to a mismatch with the modern environment and lifestyle,” added Dr De Magalhaes.